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focal length/distorsion Q
In 35mm terms, it is said that a camera lens set between 50-70mm focal length apprximates to the human eye view. With ultrazoom compact cameras, is it fair to say that the focal length which more or less corresponds to natural vision results in the least amount of distorsion ? Or does the least-distorsion point vary with size of zoom ? .

AH..

Comments (29)

Photophile wrote:.

In 35mm terms, it is said that a camera lens set between 50-70mmfocal length apprximates to the human eye view..

I think most would say the figure is 45-50 mm. But there is no single 'correct' figure, because our field of view isn't a simple rectangular frame and we don't see with a fixed gaze the way a camera does..

With ultrazoomcompact cameras, is it fair to say that the focal length which moreor less corresponds to natural vision results in the least amount ofdistorsion ? Or does the least-distorsion point vary with size ofzoom ? .

It's wrong to call it distortion, it is simply a perspective effect. The world really does look like that if your field of view is wide enough. And the foreshortening you see with a long lens or a pair of binoculars is exactly what you see if you crop your vision by looking through a cut-out in a sheet of card (try it!). Somewhere in between is a field of view which looks fairly similar to the unaided eye..

The size of the camera makes no difference to this. If the field of view is (plucking a figure out of the air) say 60 degrees, the image geometry will be the same regardless of whether it is taken with a 35 mm camera or a smaller sensor camera with a proportionately shorter lens...

Comment #1

Photophile wrote:.

In 35mm terms, it is said that a camera lens set between 50-70mmfocal length apprximates to the human eye view..

No, it doesn't. Human vision is around 150 degrees wide by 90 degrees high. You can easily check this for yourself using only your outstretched arms to approximate the angles..

What a normal lens does do is give you no magnification. If you walk around with your eye to the viewfinder, around 50mm (on a 35mm camera) will feel natural. Longer or shorter focal lengths will feel strange, because the scenery will change quicker or slower than you expect. You'll bump into and trip over things..

With ultrazoomcompact cameras, is it fair to say that the focal length which moreor less corresponds to natural vision results in the least amount ofdistorsion ? .

You are talking about perspective. That has to do with camera-to-subject distance, not focal length. Focal length is a side-effect if you want an object to fill the frame from a certain distance (i.e. perspective), you are forced into using a certain focal length..

Here's an example that shows that distance, not focal length, is what counts with perspective:.

Http://www.dpreview.com/learn/?/Glossary/Optical/Perspective_01.htm.

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #2

Nickleback wrote:.

What a normal lens does do is give you no magnification. If you walkaround with your eye to the viewfinder, around 50mm (on a 35mmcamera) will feel natural..

This is not how the normal FOV is defined. It is supposed to give the same retinal image as the original scene (no magnification or reduction) when viewing a *print* under standard conditions. Viewfinder magnification depends on the camera, and the normal focal length depends on the format, but the normal FOV does not..

(FOV means "field of view", commonly expressed as "35mm equivalent focal length". Differences in *actual* focal length at the same FOV are irrelevant for distortion.).

With ultrazoomcompact cameras, is it fair to say that the focal length which moreor less corresponds to natural vision results in the least amount ofdistorsion ? .

You are talking about perspective. That has to do withcamera-to-subject distance, not focal length..

I disagree. Most of what we call "wide-angle distortion" is not perspective, but rectilinear distortion. It occurs when the line between camera and subject is not perpendicular to the sensor - i.e., it is responsible for the "stretched edges" in wide-FOV photos. This has nothing to do with camera-to-subject distance..

Notably, the most common mistake in wide-angle people photos is to put the people near the edge of the frame. The perspective effects from trying to fill the frame with someone's face might be just as bad, but that's a lot more obvious to the photographer!.

Focal length is a side-effect if you want an object to fill the framefrom a certain distance (i.e. perspective), you are forced into usinga certain focal length..

Here's an example that shows that distance, not focal length, is whatcounts with perspective:.

Http://www.dpreview.com/learn/?/Glossary/Optical/Perspective_01.htm.

This experiment does show that perspective effects as such (i.e., relative sizes and positions of objects in the photo) depend on distance - but it cannot show whether perceived distortion is caused by distance or FOV, because these two variables are related by framing as you say above..

It may be the *difference* between the camera's perspective on the subject and the viewer's perspective on the photo that causes the appearance of distortion. (Mere perspective effects are something we see all the time with our unaided eyes.) And that difference is a function of FOV..

I have never seen a convincing demonstration of distortion caused by distance - that is, a single photo that I would look at and say "This is distorted" - at a normal FOV and in a realistic photographic situation..

Incidentally, a similar experiment would demonstrate rectilinear distortion - just take the crop and the 80mm photo from the *corner* of the wide-angle photo..

Alan Martin..

Comment #3

Steve Balcombe wrote:.

Photophile wrote:.

In 35mm terms, it is said that a camera lens set between 50-70mmfocal length apprximates to the human eye view..

I think most would say the figure is 45-50 mm. But there is no single'correct' figure, because our field of view isn't a simple rectangularframe and we don't see with a fixed gaze the way a camera does..

It's not supposed to approximate the human visual field - it's based on how we view prints. (But it's reasonable to suppose that this says something about the way we *use* our visual field.).

With ultrazoomcompact cameras, is it fair to say that the focal length which moreor less corresponds to natural vision results in the least amount ofdistorsion ? Or does the least-distorsion point vary with size ofzoom ? .

It's wrong to call it distortion, it is simply a perspective effect.The world really does look like that if your field of view is wideenough..

Then why does wide-angle distortion so often come as a surprise to the photographer?.

And the foreshortening you see with a long lens or a pair ofbinoculars is exactly what you see if you crop your vision by lookingthrough a cut-out in a sheet of card (try it!)..

No, that's just the loss of depth perception from viewing with one eye. Try comparing the view through the cut-out with what you see when you close or cover one eye - now there isn't such a difference, is there?.

The size of the camera makes no difference to this. If the field of viewis (plucking a figure out of the air) say 60 degrees, the image geometrywill be the same regardless of whether it is taken with a 35 mm cameraor a smaller sensor camera with a proportionately shorter lens..

*This* part is absolutely correct, and refutes a common misconception..

As for the rest... well, take a look at my other post:http://forums.dpreview.com/...forums/read.asp?forum=1002&message=25628900.

Alan Martin..

Comment #4

Alan Martin wrote:.

I have never seen a convincing demonstration of distortion caused bydistance - that is, a single photo that I would look at and say "Thisis distorted" - at a normal FOV and in a realistic photographicsituation..

I have seen very noticeable perspective distortion, and I do mean distortion caused by distance. It was shown to me in college, where examples had been shot specially. The pictures had the whole class falling about with laughter. .

The demo images showed a seated man reading a book, with his legs and feet stretching somewhat towards camera.....

Shot 1 showed the effects of "too long" a lens 500mm on 35mm or something like. Naturally the feet were minuscule and the legs super-short compared with the body of the man. The effect was quite amusing..

Shot 2 showed the effects of "too short" a lens (say, 28mm in this case) but shot close enough for the body of the sitter to be equal sized with the first example. Here the feet were VAST, like clown's feet..

Together the images were very funny to behold.....

... and very instructive about perspective effects generally, and also what can happen when taken to extreme.Regards,Baz..

Comment #5

Alan Martin wrote:.

Then why does wide-angle distortion so often come as a surprise tothe photographer?.

Perspective distortion comes as a surprise because it is frozen. In real life, when we change perspective, we see the change evolve. In a print, it's just there, without the context of how it got there..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #6

Alan Martin wrote:.

It's not supposed to approximate the human visual field - it's basedon how we view prints. (But it's reasonable to suppose that thissays something about the way we *use* our visual field.).

You are right. It would be "reasonable" to suppose it says something about how we *use* our visual field, but the idea that the 50mm (equivalent) FoV somehow approximates human vision really is completely wrong..

Human vision is a sophisticated combination of many different phenomena, some of which are psychological and still being investigated. However, as far as FoV is concerned we can state that....

1) The eye accepts a very WIDE field horizontally, and a pretty wide one vertically.... but this wide field image is NOT very sharp at all. In fact it is 'distinctly' blurred.... if you will forgive the contradiction in terms! .

2) Sharpness is provided locally and dynamically by an extremely high definition "lens" of very NARROW angle (call it 600mm equiv) that scans about within the blurred wide field..

It is this scanning mechanism that sequentially builds an impression of what is before on the cortex of the brain, using what is a unique combination of very different "lenses" from the extreme ends of the f-length ranges available to cameras..

Now, the best that can be said for the 50mm (equiv) is that it is in the *middle* of the range of the pair of widely separated f-lengths we do use to see with ... and, like a lot of averages....

(anybody here got 2.3 children?).

... it is a very poor representation of reality.Regards,Baz..

Comment #7

Alan Martin wrote:.

Nickleback wrote:.

What a normal lens does do is give you no magnification. If you walkaround with your eye to the viewfinder, around 50mm (on a 35mmcamera) will feel natural..

This is not how the normal FOV is defined..

I'm well aware of that. Now check viewfinder specs for 35mm cameras....

You are talking about perspective. That has to do withcamera-to-subject distance, not focal length..

I disagree. Most of what we call "wide-angle distortion" is notperspective, but rectilinear distortion..

"recilinear distortion" is perspective..

It occurs when the linebetween camera and subject is not perpendicular to the sensor - i.e.,it is responsible for the "stretched edges" in wide-FOV photos. Thishas nothing to do with camera-to-subject distance..

It has everything to do with camera to subject distance. Parts of the subject are further away than other parts of the subject. You can correct this distortion by aligning the lens to the subject using a perspective correction lens..

Note that it isn't called a rectilinear correction lens....

Notably, the most common mistake in wide-angle people photos is toput the people near the edge of the frame..

Yes, as people near the edge are further from the camera. As you shoot closer and wider, the difference in distance to camera of a subject in the middle and a subject on the edge becomes larger..

This experiment does show that perspective effects as such (i.e.,relative sizes and positions of objects in the photo) depend ondistance - but it cannot show whether perceived distortion is causedby distance or FOV, because these two variables are related byframing as you say above..

It shows it perfectly by showing that distance is the cause of perspective effects..

Look at B and C. Different focal lengths, same subject distance, same perspective..

Now look at B and D. Same focal length, different subject distance, different perspective..

I have never seen a convincing demonstration of distortion caused bydistance - that is, a single photo that I would look at and say "Thisis distorted" - at a normal FOV and in a realistic photographicsituation..

Because at a normal FOV and a realistic photographic situation you are shooting with a perspective that is pretty close to how you normally look at things..

Incidentally, a similar experiment would demonstrate rectilineardistortion - just take the crop and the 80mm photo from the *corner*of the wide-angle photo..

Compare it with an 80mm photo of the same area, swing around it's nodal point to point at the same spot..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #8

Barrie Davis wrote:.

Alan Martin wrote:.

I have never seen a convincing demonstration of distortion caused bydistance - that is, a single photo that I would look at and say "Thisis distorted" - at a normal FOV and in a realistic photographicsituation..

I have seen very noticeable perspective distortion, and I do meandistortion caused by distance. It was shown to me in college, whereexamples had been shot specially. The pictures had the whole classfalling about with laughter. .

The demo images showed a seated man reading a book, with his legs andfeet stretching somewhat towards camera.....

Shot 1 showed the effects of "too long" a lens 500mm on 35mm orsomething like. Naturally the feet were minuscule and the legssuper-short compared with the body of the man. The effect was quiteamusing..

Shot 2 showed the effects of "too short" a lens (say, 28mm in thiscase) but shot close enough for the body of the sitter to be equalsized with the first example. Here the feet were VAST, like clown'sfeet..

This does not refute my claim. Have you ever seen similar distortion in a photo with a normal field of view?.

Of course there are measurable perspective effects, i.e., changes in the relative sizes and positions of objects, and these are caused by distance. But I am not convinced that these effects are perceived as distortion without a wide or narrow FOV..

Alan Martin..

Comment #9

Nickleback wrote:.

Alan Martin wrote:.

I disagree. Most of what we call "wide-angle distortion" is notperspective, but rectilinear distortion..

"recilinear distortion" is perspective..

No, no. nickleback. In this regard Alan is quite right..

What is usually known as 'rectilinear distortion' occurs at the edges of super wide angle shots, even if the image elements falling there are actually the same distance or further away than the central ones..

It occurs when the linebetween camera and subject is not perpendicular to the sensor - i.e.,it is responsible for the "stretched edges" in wide-FOV photos. Thishas nothing to do with camera-to-subject distance..

Yes. Quite so. It is a kind of edge s-p-r-e-a-d-i-n-g that is only cancelled by having the image plane wrap around th lens in a curve... which some wide angle film cameras do, of course. .

The usual diagrammatic example of this kind of lateral image spread shows a row of vertical columns at 90 degrees to the lens axis. Then, by geometry, it is shown how the ones at the edges are widened because they do not fall at 90 degrees on to the film or sensor..

Note: These columns are actually further away than central ones... but they are still bigger/wider in the image... and, this happens with a lens that DOES have rectilinear drawing... it being the rectilinear drawing that does it!!.

[I will try to find an example of this onhttp://www, and post a link here.]Regards,Baz..

Comment #10

Alan Martin wrote:.

Barrie Davis wrote:.

Alan Martin wrote:.

I have never seen a convincing demonstration of distortion caused bydistance - that is, a single photo that I would look at and say "Thisis distorted" - at a normal FOV and in a realistic photographicsituation..

Baz's comments snipped..

This does not refute my claim. Have you ever seen similar distortionin a photo with a normal field of view?.

I didn't say that it did, Alan. I just detailed the circumstances under which I had seen serious perspective distortion... where it had been exagerated to make the point..

These things are all a matter of degree, and the degree to which we tolerate these "distortions" does not remain constant, either. I notice that perpective effects of all kinds, including converging verticals, are now much less disturbing to people generally than ever they were when I was first shown the man with funny legs. .

Regards,Baz..

Comment #11

Alan Martin wrote:.

Steve Balcombe wrote:.

Photophile wrote:.

In 35mm terms, it is said that a camera lens set between 50-70mmfocal length apprximates to the human eye view..

I think most would say the figure is 45-50 mm. But there is no single'correct' figure, because our field of view isn't a simple rectangularframe and we don't see with a fixed gaze the way a camera does..

It's not supposed to approximate the human visual field - it's basedon how we view prints. (But it's reasonable to suppose that thissays something about the way we *use* our visual field.).

That's what I said - "our field of view isn't a simple rectangular frame and we don't see with a fixed gaze the way a camera does." What are you disagreeing with exactly?.

With ultrazoomcompact cameras, is it fair to say that the focal length which moreor less corresponds to natural vision results in the least amount ofdistorsion ? Or does the least-distorsion point vary with size ofzoom ? .

It's wrong to call it distortion, it is simply a perspective effect.The world really does look like that if your field of view is wideenough..

Then why does wide-angle distortion so often come as a surprise tothe photographer?.

For the reason that nickleback has already given, and for another reason - do you really need me to spell it out?.

And the foreshortening you see with a long lens or a pair ofbinoculars is exactly what you see if you crop your vision by lookingthrough a cut-out in a sheet of card (try it!)..

No, that's just the loss of depth perception from viewing with oneeye..

No it isn't. *All* photography (except stereo photography, obviously) is monocular. Your supposed explanation doesn't address the difference between photography with a 50 mm lens and a 500 mm lens. And in any case, the foreshortening effect is seen equally with a monocular telescope and binoculars. You are completely wrong..

The size of the camera makes no difference to this. If the field of viewis (plucking a figure out of the air) say 60 degrees, the image geometrywill be the same regardless of whether it is taken with a 35 mm cameraor a smaller sensor camera with a proportionately shorter lens..

*This* part is absolutely correct, and refutes a common misconception..

As for the rest... well, take a look at my other post:http://forums.dpreview.com/...forums/read.asp?forum=1002&message=25628900.

I see that others have already tackled the wrong stuff in that post, but perhaps I'll add a comment or two of my own...

Comment #12

Alan Martin wrote:.

Nickleback wrote:.

What a normal lens does do is give you no magnification. If you walkaround with your eye to the viewfinder, around 50mm (on a 35mmcamera) will feel natural..

This is not how the normal FOV is defined. It is supposed to givethe same retinal image as the original scene (no magnification orreduction) when viewing a *print* under standard conditions.Viewfinder magnification depends on the camera, and the normal focallength depends on the format, but the normal FOV does not..

(FOV means "field of view", commonly expressed as "35mm equivalentfocal length". Differences in *actual* focal length at the same FOVare irrelevant for distortion.).

I agree that this should be with reference to a print, not the image in a viewfinder..

But neither nickleback nor the OP used the term field of view. The question referred to focal length on a 35 mm camera, and nickleback answered in the same terms. But if you insist on defining field of view, at least get it right. It is not "commonly expressed as a 35 mm equivalent focal length". Actual physical focal length is the thing that is commonly expressed as a 35 mm equivalent. Field of view is usually (in photography) given in degrees.



With ultrazoomcompact cameras, is it fair to say that the focal length which moreor less corresponds to natural vision results in the least amount ofdistorsion ? .

You are talking about perspective. That has to do withcamera-to-subject distance, not focal length..

I disagree. Most of what we call "wide-angle distortion" is notperspective, but rectilinear distortion..

Perspective is *solely* a function of distance..

Yes, the thing that you are calling wide-angle distortion is a consequence of projecting a wide field onto a flat surface. But it only occurs because the subject is large enough and close enough to project that way. Move it further away and the problem goes away - *regardless of whether you switch to a longer lens or let it occupy a small centre part of the field of the wide angle lens*. The perspective has change because, and only because, the distance has changed..

It occurs when the linebetween camera and subject is not perpendicular to the sensor.

The only place where this line is perpendicular is the dead centre of the field of view..

- i.e.,it is responsible for the "stretched edges" in wide-FOV photos. Thishas nothing to do with camera-to-subject distance..

For a given subject (which is what a real-world examination of this issue requires), the angle of incidence of the widest points on the subject varies with distance, and only distance. The geometry of the projection varies with distance and only distance..

Notably, the most common mistake in wide-angle people photos is toput the people near the edge of the frame..

Yes. And the second most common mistake is not realising that the same 'distortion' effect occurs at the edge of the field of view of the unaided eye. But ordinarily we don't look at things that way, we move our eyes to look directly at them..

The perspective effectsfrom trying to fill the frame with someone's face might be just asbad, but that's a lot more obvious to the photographer!.

Try looking at somebody from *really* close up. It makes their nose look bigger and their ears move around the back. You don't need a camera to get this effect..

I've snipped the rest of your post, it just goes on in much the same way. You need to start looking more carefully at things *without* a camera..

The ancient Greeks understood all this stuff. They made their columns wider at the top so they would appear straight...

Comment #13

I apologise if my original question was a bit vague - the responses it has generated nevertheless make very interesting reading - particularly regarding perspective and rectilinear/barrel distorsion. If I may use a couple of scenrios (bear in mind I'm a chemist by training rather than an optics physicist!).

Scenario 1. I'm sure we're all familiar with this simple situation when using a compact digicam - I want to take a (portrait) picture of a friend standing infront of me, facing me. Suppose on my compact I go the widest angle possible and I stand just 1m away from my subject and click. There is noticable barrel distorsion and my friend's facial features suffer a little as a result..

Scenario 2. Now suppose I move back a few meters, then zoom in to fit the subject in the frame by approx the same amount, but the focal is such that it corresponds to the human eye vision. I can accomplish this by using one eye to look through the EVF and the other looking at my friend, until the two images coincide and appear as one. Will this second picture provide a more 'natural' look, without any distorsion than compared to scenario 1 ? .

I ask this because with x10, x12 and x18 optical zooms readily available, I presume there is a cost in terms of image distorsion (at wider angles) considering the cameras are meant to be used mainly at the telephoto end..

Regards..

AH..

Comment #14

The closer to the subject, the greater the distortion (if we are talking about the distortion typically attributed to wide angle lenses)..

Getting close enough will distort regardless of focal length. The reason for the myth of focal length distortion is that photographers typically move closer with short focal lengths in order to fill the frame. The shorter distance to subject is what causes the distortion..

Back up enough with a wide-angle, and you won't have distortion, but you might have a hard time seeing the main part of your subject. .

Photophile wrote:.

In 35mm terms, it is said that a camera lens set between 50-70mmfocal length apprximates to the human eye view. With ultrazoomcompact cameras, is it fair to say that the focal length which moreor less corresponds to natural vision results in the least amount ofdistorsion ? Or does the least-distorsion point vary with size ofzoom ? .

AH.

Galleries: http://www.dheller.net.

Many folks on dpreview.com list their equipment here, but don't list any links to their images. Do they collect equipment? Or take pictures?..

Comment #15

Photophile wrote:.

I ask this because with x10, x12 and x18 optical zooms readilyavailable, I presume there is a cost in terms of image distorsion (atwider angles) considering the cameras are meant to be used mainly atthe telephoto end..

There are two sources of distortion here..

One is perspective, rectilinear distortion, etc. as we have been discussing in the other replies. This kind of distortion can be present even if straight lines appear straight. And to the extent that the lens is involved, the only thing that matters is the 35mm equivalent focal length (i.e., the FOV) that you take the photo at. (Even if you blame it on distance, FOV relates framing to distance.).

The other kind is "geometric distortion" (barrel or pincushion distortion), which causes straight lines to appear curved, but is not as noticeable in the absence of straight lines. This depends on the lens, and it could very well be worse on a higher zoom ratio lens. You can find measurements of it in dpreview's reviews, but only at the extremes of the zoom range..

Alan Martin..

Comment #16

Photophile wrote:snip.

Scenario 1. I'm sure we're all familiar with this simple situationwhen using a compact digicam - we want to take a portrait picture ofa friend standing infront of me, facing me. Suppose on my compact Igo the widest angle of 6.3mm and I stand just 1m infront of mysubject and click. There is noticable barrel distorsion and myfriend's facial features suffer a little as a result..

[Look here, for the sake of my pedantic sensibilities, could we agree to spell it 'distortion', please, and not 'distor-s-ion'. Thanks. ].

What you are seeing is not barrel distortion. Barrel distortion is something else..

Barrel distortion is the failure of the lens to draw the subject's straight lines in a rectilinear and straight fashion. Most often seen in wide angle lenses, and at the edges of the field. Barrel distortion is NOT a perspective effect perspective is not involved with barrel distortion..

The wide angle effect that you ARE seeing comes about from enlargement of the nearer central zones of the image. This is because close subjects (in the middle or not) are closer to the lens by a large multiple of the average of distances to all subject elements, and close-to-the-subject use of a wide angle lens DOES emphasise the perspective effects of that close use..

Indeed, anything in the image that is twice as close (or HALF the distance) as anything else will always appear exactly TWICE as large in frame as that other item. Obviously, wide angles used close to the main subject make those conditions happen much more often than long lenses.. it is inevitable!! But it can't happen if your subject is flat and square-on, like a brick wall, say..

It is also important to note that this same "close perspective distortion" would be apparent from that same (close) distance, even if the camera was REMOVED FROM YOUR EYE..

In effect, it isn't really distortion at all, because it is entirely natural and not something to do with either cameras or lenses, only viewpoints and distances. Indeed, if you were to look at the final print itself from sufficiently close a viewpoint, that apparent "distortion" (as we obliged to call it) would disappear completely..

Furthermore, and for your information... even a fish eye lens image, [the sort that's circular and so bent it looks as if reflected in a chromium globe] looks perfectly natural when viewed from close enough.Try it. Use one eye from about 4" away and over the exact centre of the picture..

Scenario 2. Now suppose I move back a few meters, then zoom in to fitthe subject in the frame by approx the same amount, but the focal issuch that it corresponds to the human eye vision..

Oh dear! I already went into that. There is no one focal length that "corresponds to human vision." Don't believe anybody that says there is..

I can accomplishthis by using one eye to look through the EVF and the other lookingat my friend. Will this second picture provide a more 'natural' look,without any distorsion than compared to scenario 1 ? .

Standing back will reduce close viewing perspective effects, and zooming in to fill frame will ensure good pixel density and that gives good image quality. If this more distant perspective effect is the one you prefer, then use it..

In fact, why don't you actually do a test and produce a whole set of shots, using all the distances the different f-lengths your lens provides adequate framing for, and see what YOUR personal preferences in an "elegant perspective shooting distance" happens to be?.

I ask this because with x10, x12 and x18 optical zooms readilyavailable, I presume there is a cost in terms of image distorsion (atwider angles) considering the cameras are meant to be used mainly atthe telephoto end..

No. Not at all..

There is often some extra barrel distortion at the wide end of super-wide-range zooms, but it ain't anything you would ever see in portrait photography. It would only show up if you deliberately shot straight lines and gridded test pattens, like the ones DPReview uses in it's reviews. Barrel distortion and long-end "pin-cushion" distortion can be safely ignored for portraits and landscape photography.Regards,Baz..

Comment #17

Barrie Davis wrote:.

Scenario 2. Now suppose I move back a few meters, then zoom in to fitthe subject in the frame by approx the same amount, but the focal issuch that it corresponds to the human eye vision..

Oh dear! I already went into that. There is no one focal length that"corresponds to human vision." Don't believe anybody that says thereis..

In one very important sense this is true - the field of view of the human eye (as we perceive it) and of a photograph are so different in the way they work that they can't be directly compared..

But on the other hand there is a small range of focal lengths that produce an image which, when printed at a moderate size and viewed at a moderate distance, looks the same, geometrically, as the way we saw the original scene. That range of focal lengths is around 45-50 mm on a 35 mm system..

It's not that there is no 'normal' focal length, but that we have to properly understand what we mean by normal...

Comment #18

Noticeable barrel or pincushion distortion is typical for either low-quality lenses or lond range zooms, otherwise it shouldn't be there at all. Maybe you meant perspective?.

Http://lordofthelens.smugmug.com/..

Comment #19

Steve Balcombe wrote:.

But on the other hand there is a small range of focal lengths thatproduce an image which, when printed at a moderate size and viewed ata moderate distance, looks the same, geometrically, as the way we sawthe original scene. That range of focal lengths is around 45-50 mm ona 35 mm system..

Steve,.

Hmmm... I appreciate that you are taking pains to be moderate in your use of language, and I respect your efforts to be non-confrontational.... .

But I still can't go along with the idea that there is anything 'normal' about standard lenses (those lenses where f-length equals image diagonal or thereabouts) other than that they are NOT extreme focal lengths like super teles or super wides..

It isn't anything about the 50mm length as a lens, per se, but about the fact that we do not use our EYES even remotely like we use cameras, and vice versa..

It is an important difference as far as I'm concerned. I feel the lenses we use should be selected for how the photograph *should be*, and not on the basis of some (ultimately spurious) belief in what our eye sees as normal..

It's not that there is no 'normal' focal length, but that we have toproperly understand what we mean by normal..

And I'd say there isn't any 'normal' length at all, just normal for the different photographic tasks... wides for landscapes and interiors so called "standard" f-lengths for groups short teles for portraits long teles for sports and wild life ....

Hmmm... on consideration I suppose long teles and super wides are special cases..

We rarely use them because we actually WANT the perspective of very distant or very near subjects, but, for one reason or another, we just can't get to ideal distances from those subjects, and the severely flattened/steepened perspectives are more often just something we are obliged to put up with... (shrugs)Regards,Baz..

Comment #20

Steve Balcombe wrote:.

But if you insist on defining field of view, at least get it right.It is not "commonly expressed as a 35 mm equivalent focal length".Actual physical focal length is the thing that is commonly expressedas a 35 mm equivalent..

What I meant is this: if you say that a camera has "a 35mm equivalent focal length of 50mm", that is the same as saying that it has "a diagonal angle of view of 47 degrees". (And every other 35mm equivalent FL has a corresponding diagonal AOV.).

I guess we disagree on the meaning of "expressed as" - and I wasn't really trying to *define* FOV..

Field of view is usually (in photography) given in degrees. In otherfields it is sometimes expressed as field width at a given distance("385 feet at 1000 yards" for example)..

I have no problem with that, indeed an angle in degrees is the most direct / 'proper' way to express it. I only disagree with the "usually" part, because 35mm equivalent focal lengths are much more commonly used..

Yes, the thing that you are calling wide-angle distortion is aconsequence of projecting a wide field onto a flat surface. But itonly occurs because the subject is large enough and close enough toproject that way. Move it further away and the problem goes away -*regardless of whether you switch to a longer lens or let it occupy asmall centre part of the field of the wide angle lens*..

A small CENTRE part of the field? What if you let it occupy a small corner part of the field, which is where the rectilinear distortion occurs?.

That makes no difference to camera-to-subject distance (if you just turn the camera in place), but it does produce obvious distortion of the subject..

How do you explain that, while maintaining that wide-angle distortion is caused entirely by camera-to-subject distance?.

It occurs when the linebetween camera and subject is not perpendicular to the sensor.

The only place where this line is perpendicular is the dead centre ofthe field of view..

Exactly - rectilinear distortion is zero only at that one point, and increases quadratically with the angle away from the perpendicular..

10 degrees = 1.5% distortion20 degrees = 6.4% distortion30 degrees = 15% distortion40 degrees = 31% distortion50 degrees = 56% distortion.

Of course, to take in a point 50 degrees off perpendicular, you need a lens with at least a 100 degree diagonal AOV, i.e., a 35mm equivalent FL of 18mm or less..

For a given subject (which is what a real-world examination of thisissue requires), the angle of incidence of the widest points on thesubject varies with distance, and only distance. The geometry of theprojection varies with distance and only distance..

Do you only put your subjects in the center of the frame? Why use a wide-angle lens if there is nothing at the edges of the frame?.

The *maximum* angle away from the perpendicular is half of the diagonal angle of view - regardless of camera-to-subject distance..

Notably, the most common mistake in wide-angle people photos is toput the people near the edge of the frame..

Yes. And the second most common mistake is not realising that thesame 'distortion' effect occurs at the edge of the field of view ofthe unaided eye. But ordinarily we don't look at things that way, wemove our eyes to look directly at them..

Have you tried it? Do spheres look like ellipses in your peripheral vision? They don't for me. (Use a big sphere or you won't see much at all.).

There might be distortion in the optical image on the retina, but there is a lot of subsequent processing so that we don't perceive it..

Try looking at somebody from *really* close up. It makes their noselook bigger and their ears move around the back. You don't need acamera to get this effect..

Indeed - this is a rare example where perspective effects, as seen with the unaided eyes, are perceived as distortion. Unlike virtually all of the perspectives that occur in photographs, it is not a view that we are accustomed to seeing..

That's why I had to add the qualifier "in a realistic photographic situation"..

Alan Martin..

Comment #21

Nickleback wrote:.

What a normal lens does do is give you no magnification. If you walkaround with your eye to the viewfinder, around 50mm (on a 35mmcamera) will feel natural..

This is not how the normal FOV is defined..

I'm well aware of that. Now check viewfinder specs for 35mm cameras....

How many currently available cameras have 100% viewfinder magnification with a normal lens?.

For reduced frame DSLRs, the viewfinder magnification isn't even specified for a normal lens - instead, it is specified for a 50mm lens, which isn't a normal lens for the format..

So explaining a "normal lens" in terms of the viewfinder tends to support the misconception that it's always the same actual focal length..

It occurs when the linebetween camera and subject is not perpendicular to the sensor - i.e.,it is responsible for the "stretched edges" in wide-FOV photos. Thishas nothing to do with camera-to-subject distance..

It has everything to do with camera to subject distance. Parts ofthe subject are further away than other parts of the subject. Youcan correct this distortion by aligning the lens to the subject usinga perspective correction lens..

This isn't the kind of distortion I was talking about. I meant the kind that turns spheres into ellipses. That happens near the edges of the frame in wide-angle photos, but it wouldn't happen for the same sphere at the same distance if the camera were pointed directly toward it..

A perspective correction lens (a.k.a. a shift lens) can put the undistorted area somewhere other than the center of the frame. You could do the same thing with an ordinary wider-angle lens, by cropping off-center afterward. It can also be simulated with software "perspective correction". But none of these things actually changes camera-to-subject distance..

Note that it isn't called a rectilinear correction lens....

No, because some people use a more inclusive definition of "perspective" that also includes projection..

But the oft-repeated claim that "perspective depends solely on camera-to-subject distance" applies only to a narrower definition..

This experiment does show that perspective effects as such (i.e.,relative sizes and positions of objects in the photo) depend ondistance - but it cannot show whether perceived distortion is causedby distance or FOV, because these two variables are related byframing as you say above..

It shows it perfectly by showing that distance is the cause ofperspective effects..

Look at B and C. Different focal lengths, same subject distance,same perspective..

Now look at B and D. Same focal length, different subject distance,different perspective..

Sure - where "perspective" refers to the differences in relative sizes and positions. But it does not show that these changes, in themselves, cause a perception of distortion. If D looks distorted while B and C don't, that correlates with FOV just as well as it does with distance..

The only way you can separate the effects of FOV and distance is to not use the same subject with the same framing. Is photo A distorted? Hard to tell, it's not a good subject. But that's why I asked for a single normal-FOV photo that looks distorted, rather than a comparison..

I have never seen a convincing demonstration of distortion caused bydistance - that is, a single photo that I would look at and say "Thisis distorted" - at a normal FOV and in a realistic photographicsituation..

Because at a normal FOV and a realistic photographic situation youare shooting with a perspective that is pretty close to how younormally look at things..

If FOV determines how the perspective relates to "how you normally look at things", and that in turn determines perceived distortion, then where do we disagree?.

But the straightforward prediction of "wide-angle distortion depends on camera-to-subject distance" is that if you take a distorted wide-angle photo, then you should be able to take a similarly distorted normal-FOV photo from the same position..

I haven't seen an example of that - my practical experience is that perceived distortion correlates very well with FOV, and not with focal length or subject distance (at the same FOV)..

Incidentally, a similar experiment would demonstrate rectilineardistortion - just take the crop and the 80mm photo from the *corner*of the wide-angle photo..

Compare it with an 80mm photo of the same area, swing around it'snodal point to point at the same spot..

Sure, you can do that, if you *want* to match the wide-angle photo's rectilinear distortion. But you don't have to, so you can get distorted and undistorted photos from the same distance, which was my point..

Alan Martin..

Comment #22

Nickleback wrote:.

Alan Martin wrote:.

Then why does wide-angle distortion so often come as a surprise tothe photographer?.

Perspective distortion comes as a surprise because it is frozen. Inreal life, when we change perspective, we see the change evolve. Ina print, it's just there, without the context of how it got there..

This explanation just doesn't hold up for rectilinear distortion. When have you ever seen a sphere look like an ellipse with your unaided eyes?.

Even for other types of distortion, it seems unlikely. If you close your eyes and walk to a different position, you won't see distortion when you open your eyes and stand still - whether you scan the scene with your eyes or not..

Alan Martin..

Comment #23

Steve Balcombe wrote:.

That's what I said - "our field of view isn't a simple rectangularframe and we don't see with a fixed gaze the way a camera does." Whatare you disagreeing with exactly?.

The implication that a "normal lens" is defined directly in terms of (assumptions about) the human eye..

But I don't have a definitive reference for how it *is* defined, and I agree that it is somewhat arbitrary in any case..

Then why does wide-angle distortion so often come as a surprise tothe photographer?.

For the reason that nickleback has already given, and for anotherreason - do you really need me to spell it out?.

You mean inattention? .

But neither of those explanations holds up... I've never seen a sphere look like an ellipse with my unaided eyes..

And the foreshortening you see with a long lens or a pair ofbinoculars is exactly what you see if you crop your vision by lookingthrough a cut-out in a sheet of card (try it!)..

No, that's just the loss of depth perception from viewing with oneeye..

No it isn't. *All* photography (except stereo photography, obviously)is monocular..

I just meant that I don't see any distortion or foreshortening when looking through a cut-out, as compared with unobstructed monocular vision. I wasn't comparing with a viewfinder, a photo, or binoculars..

Your supposed explanation doesn't address the difference betweenphotography with a 50 mm lens and a 500 mm lens..

No, because rectilinear distortion is mostly negligible at focal lengths longer than normal. I did specify "wide-angle distortion"....

Still, if there is a "difference between photography with a 50 mm lens and a 500 mm lens", as practical experience confirms, isn't that a matter of FOV?.

I realize that the relative sizes and proportions of objects in a photo are determined by subject distance, and that this can be taken as a definition of "perspective"..

But I have yet to see any evidence that perceived distortion is controlled entirely by *relative* sizes and proportions, rather than also being influenced by the *absolute* size of the retinal image..

Here's an interesting paper that discusses wide-angle distortion, although it does not address telephoto compression:http://www.cnbc.cmu.edu/cns/papers/nn1553.pdf.

I found it in this post from News Discussion, which has another interesting link about projections for panoramic photos:http://forums.dpreview.com/...forums/read.asp?forum=1000&message=25612332.

Alan Martin..

Comment #24

Photophile wrote:.

In 35mm terms, it is said that a camera lens set between 50-70mmfocal length apprximates to the human eye view. With ultrazoomcompact cameras, is it fair to say that the focal length which moreor less corresponds to natural vision results in the least amount ofdistorsion ? Or does the least-distorsion point vary with size ofzoom ? .

Well, frankly speaking, I think the answer is simply NO. There is no direct link between the focal length and the amount of distortion of a lens..

The focal length and the amount of distortion per se have nothing to do with each other. You can have a 50mm lens displaying lots of chromatic aberration (distortion) and a 28 mm lens showing virtually no chromatic aberration (distortion) or vice versa. There are many different types of distortion such as chromatic aberration, spherical aberration, barrel distortion, vignetting, ....

Now speaking in practical terms, zoom lenses in general show the most amount of distortion at their largest and smalles focal length. This is not necessarily so but usually. Therefore, a zoom lense with a focal range from 50 to 200 mm usually has the most distortion at around 50 mm and 200 mm. The least distortion is goint to be somewhere in the middle, maybe 120 mm..

In ultrazoom compact cameras the 50 mm focal lenght is usually somewhere in the middle between the wide angle (e.g. 35mm) and tele (e.g. 200mm). However, the least amount of distortion is not necessarily at exactly 50mm it may well be at 70, 80 or 100mm. That depends on the zoom lens and there is no general rule for predicting the focus displaying the least amount of distortion in a zoom lens..

Does that answer your question?.

Best regards.

Kikl..

Comment #25

Alan Martin wrote:.

But the straightforward prediction of "wide-angle distortion dependson camera-to-subject distance" is that if you take a distortedwide-angle photo, then you should be able to take a similarlydistorted normal-FOV photo from the same position..

You can take a similarly distorted normal-FOV photo, but you usually don't. It would break your "realistic photo situation" rule. You don't normally take a shot of a large subject from 2 feet away with a normal lens, the way you would with a wide angle. You also don't take than same camera and lens and tilt it to get the edges/corners of a wide angle shot..

Incidentally, a similar experiment would demonstrate rectilineardistortion - just take the crop and the 80mm photo from the *corner*of the wide-angle photo..

Compare it with an 80mm photo of the same area, swing around it'snodal point to point at the same spot..

Sure, you can do that, if you *want* to match the wide-angle photo'srectilinear distortion..

So I see you agree with me..

But you don't have to.

Sure. To get the whole subject in with the longer lens you'd have to stand back a lot further, altering the perspective..

So you can getdistorted and undistorted photos from the same distance, which was mypoint..

The "undistorted" longer lens photo from the same distance is not of the same subject. It is of a small portion of the wider angle subject..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #26

Nickleback wrote:.

Alan Martin wrote:.

But the straightforward prediction of "wide-angle distortion dependson camera-to-subject distance" is that if you take a distortedwide-angle photo, then you should be able to take a similarlydistorted normal-FOV photo from the same position..

You can take a similarly distorted normal-FOV photo, but you usuallydon't. It would break your "realistic photo situation" rule. Youdon't normally take a shot of a large subject from 2 feet away with anormal lens, the way you would with a wide angle..

Sorry, but most of the wide-angle photos I have seen are not "of a large subject from 2 feet away". They still show distortion, and normal-FOV photos taken from the same locations don't..

You also don't take than same camera and lens and tilt it to get theedges/corners of a wide angle shot..

No, of course you don't. How does this support the claim that the distortion is caused by camera-to-subject distance?.

So you can getdistorted and undistorted photos from the same distance, which was mypoint..

The "undistorted" longer lens photo from the same distance is not ofthe same subject. It is of a small portion of the wider anglesubject..

Who said anything about comparing photos of the same subject? The camera-to-subject distance theory predicts that distortion should be seen in the normal FOV photo even if it doesn't have the same composition. And you can choose whatever small portion you want..

If you have to use a shift lens to reproduce the distortion, that would tend to disprove the theory as far as wide-angle distortion is concerned, wouldn't it?.

If you make "same content" a requirement, then you can no longer distinguish between effects of distance and effects of FOV..

Alan Martin..

Comment #27

Alan Martin wrote:.

Sorry, but most of the wide-angle photos I have seen are not "of alarge subject from 2 feet away". They still show distortion, andnormal-FOV photos taken from the same locations don't..

Normal FOV photos from the same location don't show the same subject..

You also don't take than same camera and lens and tilt it to get theedges/corners of a wide angle shot..

No, of course you don't. How does this support the claim that thedistortion is caused by camera-to-subject distance?.

Draw a line from your camera to the subject. How far away is it? If you give only one answer, you must be shooting a pinhead..

Subjects typically are 3 dimensional. Distance to subject depends on where on the subject you measure. Even if you look at 2-dimensional subjects that are lined up perfectly with the camera, the distance from the center to the camera is different then the distance from the edge to the camera..

So you can getdistorted and undistorted photos from the same distance, which was mypoint..

The "undistorted" longer lens photo from the same distance is not ofthe same subject. It is of a small portion of the wider anglesubject..

Who said anything about comparing photos of the same subject?.

What's the point of comparing a different subject?.

Thecamera-to-subject distance theory predicts that distortion should beseen in the normal FOV photo even if it doesn't have the samecomposition..

No, for the reasons given above. You aren't seeing as far to the sides and the top. The sides and the top, in the same plane as the center and aligned to the camera, are further away from the camera when your view is wider..

If you have to use a shift lens to reproduce the distortion.

Who said anything about using a shift lens? Just swing the camera around the lens nodel point to take an image of the parts of the subject you are missing by looking at a smaller FOV..

If you make "same content" a requirement, then you can no longerdistinguish between effects of distance and effects of FOV..

Sure you can. Different distance gives you different perspective. Different focal length gives you different FOV..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #28

Nickleback wrote:.

Alan Martin wrote:.

Sorry, but most of the wide-angle photos I have seen are not "of alarge subject from 2 feet away". They still show distortion, andnormal-FOV photos taken from the same locations don't..

Normal FOV photos from the same location don't show the same subject..

So if wide-angle distortion depends on camera-to-subject distance - BUT only for the same subject - then what explains which photos will have this distortion in a collection of photos of *different* subjects?.

Answer: it depends on field of view..

This is not inconsistent with your theory, because when you insist on the same framing, field of view and distance are perfectly correlated!.

And it is a much better and more practical answer, especially here in the Beginners' Questions Forum, than to start talking about perspective and camera-to-subject distance. (Which are certainly involved, but are not the whole story.).

I have no problem with refuting the misconception that actual focal length is what causes the distortion. Just don't contradict all practical experience by claiming that it isn't associated with field of view..

Did you read the Nature Neuroscience paper I linked to elsewhere in the thread?http://www.cnbc.cmu.edu/cns/papers/nn1553.pdf.

These people know a thing or two about human visual perception... and *they* don't seem to think that wide-angle distortion depends on camera-to-subject distance..

Alan Martin..

Comment #29

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