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definition of focal length
The dpreview glossary definition of focal length is "the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the subject (at infinity) is 'in focus'. Why this 'at infinity'?-garrisonhttp://www.umefotographie.com..

Comments (26)

Well, if you chose some other subject distance, such as a tree at 10 meters, then the focal distance would have to be calculated differently for each subject..

To achieve focus on closer subjects, the lens assembly adjusts the position of one or more of it's elements. This may affect the aforementioned distance, or the field of view, or both. Thus, it's important in the definition of the focal distance..

If you took your prime 50mm lens and calculated the whole range at all focal distances, it would likely be something around a 46mm-52mm zoom. With a simpler definition, you find it's really only at focus on infinity at 50mm..

[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..

Comment #1

Gking wrote:.

The dpreview glossary definition of focal length is "the distance inmm from the optical center of the lens to the focal point, which islocated on the sensor or film if the subject (at infinity) is 'infocus'. Why this 'at infinity'?.

It is at infinity because....

Light rays approaching the lens from infinity are parallel with each other, and are not converging or diverging. Naturally this is the obvious standard from which to measure the converging power of the lens....

...(or the diverging power for that matter, although diverging lenses cannot form real images by themselves so are not used as objectives in cameras)....

Note: High "converging power" equals short f-length. Low "converging power" equals long f-length..

Note also: Focused at infinity is where the lens is closest to the image plane.. it being necessary to progressively move the lens away from the image plane, and towards the subject plane, in order to focus closer distances.Regards,Baz..

Comment #2

At what distance from the camera does infinity begin then? Is this lens related? The infinity symbol on my 60mm begins after 2 m. Thanks Ed & Baz.-garrisonhttp://www.umefotographie.com..

Comment #3

Gking wrote:.

At what distance from the camera does infinity begin then? Is thislens related? The infinity symbol on my 60mm begins after 2 m. ThanksEd & Baz..

How far is it to infinity... ??Huh! That's quite a question. Akin to, 'how long is "for ever" ? .

Perhaps you should ask Buzz Lightyear. I understand he has been.. "to infinity and beyond." .

Wherever it is, we know that infinity is w-a-y beyond the furthest star... .

So, joking apart and in practical terms, the lens's version of infinity is far enough away for light rays to be *close enough to parallel* that any errors arising in determining the "resultants" of focal length......

....like scaling of f/numbers, and calculating Depth of Field......

.....are too small to worry about..

Which distance the lens manufacturers' actually settle on is very much determined by the practicalities of differentiating distance on the focusing scale of the lens... that's if the lens has even got a scale in the conventional sense many lenses these days do not..

With the compressed focusing scale of short f-lengths, it may be practical to suggest it is just beyond a couple of meters.. as you have noticed. With longer lenses, that have shallower DoF, the scale will be more spread and the lens engraving suggest infinity is beyond a "massive" 20 metres...

....where neither is even remotely correct, of course. Regards,Baz..

Comment #4

Correct, infinity is 'forever' away. If you traveled at the speed of light from the beginning of the universe until now, you would not be at infinity. You would have traveled 15 billion light years, but it still wouldn't be infinity. Just to give you an idea how far a light year is, the closest star is 4 light years away, and it would take us 17,884 years to get there on the fastest man-made object ever created - the Helios spacecraft..

However, with a normal error tolerance, infinity can be about 30 feet away or so.....

Comment #5

Gking wrote:.

The dpreview glossary definition of focal length is "the distance inmm from the optical center of the lens to the focal point, which islocated on the sensor or film if the subject (at infinity) is 'in focus'..

Unfortunately, that definition is almost irrelevant to most photography. It applies to a "thin lens" which is positioned at an appropriate distance from the imaging plane in order to achieve focus (typically with a bellows). Modern medium format and smaller cameras use complex lens systems containing multiple elements, and the lens is located at a fixed mount distance from the imaging plane..

The type of photographic lenses that we deal with are actually rated for "effective focal length". This is a measurement of the magnification power of the lens system (when focused at infinity). It is the size of the image cast by an object of size "x" at distance "x"..

For example, if an object 200 feet long is imaged from a distance of 200 feet, and the projected image of the object is 50mm (at the designed imaging distance), then the lens has an effective focal length of 50mm...

Comment #6

Doug Pardee wrote:.

Gking wrote:.

The dpreview glossary definition of focal length is "the distance inmm from the optical center of the lens to the focal point, which islocated on the sensor or film if the subject (at infinity) is 'in focus'..

Unfortunately, that definition is almost irrelevant to mostphotography. It applies to a "thin lens" which is positioned at anappropriate distance from the imaging plane in order to achieve focus(typically with a bellows). Modern medium format and smaller camerasuse complex lens systems containing multiple elements, and the lensis located at a fixed mount distance from the imaging plane..

However complex the lens, there is a point which is in effect the optical centre (*see below) - finding it is not entirely straightforward though! I would argue that the *definition* of focal length - as opposed to a practical way of *measuring* it, is exactly as given in the glossary..

The type of photographic lenses that we deal with are actually ratedfor "effective focal length". This is a measurement of themagnification power of the lens system (when focused at infinity). Itis the size of the image cast by an object of size "x" at distance"x"..

For example, if an object 200 feet long is imaged from a distance of200 feet, and the projected image of the object is 50mm (at thedesigned imaging distance), then the lens has an effective focallength of 50mm..

I'm going to put my neck on the block here, by responding to this without first looking up the term "effective focal length", which I haven't heard before....

You can't have an object of size x at a distance x if the distance is infinity. You could slightly alter the definition to specify a subject which subtends an angle of 60 degrees, but were discussing definitions here and that is not what you said..

Then we run into another problem. Not all lenses have a field of view of at least sixty degrees. I don't just mean a usable photographic image that wide, I mean any image. And you can't just take a fraction of the 60 degree angle because the relationship between focal length and angle of view is not linear. Then we have the problem of rectilinear vs. fisheye lenses..

To be continued....

* Edit - I lazily copied the term 'optical centre' - the correct term would be secondary nodal point...

Comment #7

Steve Balcombe wrote:.

I would argue that the *definition* of focal length - as opposed to apractical way of *measuring* it, is exactly as given in the glossary..

Indeed, and I hope that I didn't say otherwise. My concern was for the practical aspect: that the "focal length" that is marked on ordinary photographic lenses is *not* the focal length of the definition, but rather the effective focal length..

The true focal lengths of photographic lenses are of no real interest to the average photographer working with non-bellows equipment. We simply assume that the lens is properly designed for the mount distance (the "register") and thus focuses at that distance..

What we care about is the effective focal length, which is the number that is marked on the lens and the number that is used for f-number calculation. That numberthe effective focal lengthis purely an indication of the magnification of the lens system..

I'm going to put my neck on the block here, by responding to thiswithout first looking up the term "effective focal length", which Ihaven't heard before....

You've probably heard the term a lot, misused to mean "adjusted for crop factor" or some such thing. Searching the Web for "effective focal length" turns up a lot more of those misuses than the correct usage. .

You could slightly alter the definition to specify a subject whichsubtends an angle of 60 degrees, but were discussing definitionshere and that is not what you said..

I wasn't trying to provide a precise definitionno mention was made of the primary principal point, for instanceand maybe I should have been more clear about that. I was merely trying to provide an easily-grasped understanding of what the "focal length" markings on a common photographic lens mean..

Then we run into another problem. Not all lenses have a field of viewof at least sixty degrees. I don't just mean a usable photographicimage that wide, I mean any image. And you can't just take a fractionof the 60 degree angle because the relationship between focal lengthand angle of view is not linear..

Again, I was just giving a description that I hoped would be easily understood. The sizes and fractions all scale linearly (forget about angles). If a 20-foot object at 200 feet casts an image 50mm long, then the lens has an effective focal length of 500mm..

In actuality, the effective focal length is computed by the lens manufacturer based on the prescription and design of the lens system. Effective focal length is not measured, so the angle of view of the lens system is not really relevant..

Then we have the problem of rectilinear vs. fisheye lenses..

Indeed, fisheyes are a different matter...

Comment #8

The trouble is that the definition should have said the point where parallel rays of light converge etc etc..

As for infinity, it all depends but a pair of binoculars ought to prove the point: go outside on a clear night with the binoculars and focus on the moon (which is a long, long way away) then try finding and focusing on Jupiter or Mars and the things will have to be refocused slightly, then re-focus again on a star and then (if you know where it is) try refocusing on the Andromeda Galaxy which is large enough to see with the naked eye but outside our galaxy. BTW each of these is further and further away..

In each case the object would appear to be at infinity for all practical purposes but the binoculars will need slight adjustment to the focus to cope. Point made?.

What a shame. btw, they no longer mark the focal plane on cameras nor do they say precisely what they mean by "from the camera". Do they mean the focal plane, the nodal point of the lens, the front element or what..

And (continuing the rant) why don't they put the tripod bush on the lens' axis and (better still) at the nodal point for a stated focal length. Sorting it out for a panorama is a nightmare (Leica Digilux 2 excepted)..

OK, rant over: I'll take a tablet and lie down for a bit..

Regards, David..

Comment #9

David Hughes wrote:.

What a shame. btw, they no longer mark the focal plane on cameras.

My 400D does have a focal plane mark. At last my lowly 400D scores a point!! .

Nordo they say precisely what they mean by "from the camera". Do theymean the focal plane, the nodal point of the lens, the front elementor what..

The firmly established convention is that focusing distance is measured from the film plane. This may seem a bit odd, being out of step with the way optical calculations work, but it's universally used in photography. (Universally AFAIK - if anybody knows of any exception please take the floor!).

And (continuing the rant) why don't they put the tripod bush on thelens' axis.

Many do of course. I have three cameras here - film SLR, DSLR and digital compact (a larger one - Fuji S602Z) and all three do..

And (better still) at the nodal point for a stated focallength. Sorting it out for a panorama is a nightmare (Leica Digilux 2excepted)..

The secondary nodal point is where FL is measured from. The axis of rotation for panoramas is the entrance pupil - which is somewhere in the lens so it wouldn't be possible to provide a tripod bush. It would be nice if it was marked though - I don't think it ever is..

OK, rant over: I'll take a tablet and lie down for a bit..

Taken mine already...

Comment #10

Doug Pardee wrote:.

Gking wrote:.

The dpreview glossary definition of focal length is "the distance inmm from the optical center of the lens to the focal point, which islocated on the sensor or film if the subject (at infinity) is 'in focus'..

Unfortunately, that definition is almost irrelevant to mostphotography. It applies to a "thin lens" which is positioned at anappropriate distance from the imaging plane in order to achieve focus(typically with a bellows). Modern medium format and smaller camerasuse complex lens systems containing multiple elements, and the lensis located at a fixed mount distance from the imaging plane..

The key phrase in that dpr definition is "optical center". With a "thick" lens, it is the rear principal point (there are other names for this, which are essentially identical). Like many "definitions" of physical things, it's not intended to be relevant to every field. The stated definition IS correct and simple..

The type of photographic lenses that we deal with are actually ratedfor "effective focal length". This is a measurement of themagnification power of the lens system (when focused at infinity). Itis the size of the image cast by an object of size "x" at distance"x"..

I have never heard of this definition. There seems to be a problem, as a lens focused at infinity will have difficulty producing a sharp image of an object at a finite distance..

For example, if an object 200 feet long is imaged from a distance of200 feet, and the projected image of the object is 50mm (at thedesigned imaging distance), then the lens has an effective focallength of 50mm..

Hmmm... I tried to do this mathematically and still haven't been able to make YOUR system jibe with classical physics..

But even if it's wrong (perhaps only "effectively" correct), I contend that with digital cameras it's difficult for most people to "measure" the size of the image. And equally difficult for most people to measure the distance from the image plane to the rear principal point. These definitions are not intended to be easy for lay people to use..

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #11

Chuxter wrote:.

The key phrase in that dpr definition is "optical center". With a"thick" lens, it is the rear principal point (there are other namesfor this, which are essentially identical). Like many "definitions"of physical things, it's not intended to be relevant to every field.The stated definition IS correct and simple..

It is indeed. But it is also irrelevant to the working photographer, who usually has no idea what the back and front focal lengths of his/her lenses are, nor where the lenses' principal points are. The back and front focal lengths and the principal points are irrelevant to practical photography. All that matters to the photographer is that the lens is designed to focus at the correct distance from the mountthat the Flange Focal Length is equal to the lens register of the camera. (The above does not apply to bellows cameras such as large-format.).

I have never heard of this definition. There seems to be a problem,as a lens focused at infinity will have difficulty producing a sharpimage of an object at a finite distance..

See my response to Steve Balcombe. This is not a definition, it's a description designed to be easily grasped. The definition involves considerably more and is not relevant to the Beginners Questions forum..

Hmmm... I tried to do this mathematically and still haven't been ableto make YOUR system jibe with classical physics..

First, it's not "MY system". It's the way that photographic lenses designed for fixed mounts are universally marked. The focal length markings are EFL and reflect exactly one aspect of the lens: it's magnification when focused at infinity..

EFL is also the only figure that is relevant for aperture calculations (f-numbers) and for depth of field calculations, because both are actually based on magnification..

And as for mathematics and classical physics, I don't know where you're having difficulty. Mathematically it's a simple magnification; all that you need are multiplication and division. In optics it's still a simple magnification, and determines the position of the second principal surface relative to the image plane..

I contend that with digital cameras it's difficult for most people to"measure" the size of the image..

Why would they want to? The point is that EFL is purely a measure of magnification and is unrelated to the physical distance from the lens to the imaging plane (a figure which is relatively constant for all lenses on a fixed lens mount)..

But it's not really that hard to measure anyway. The size of the film or the imaging sensor is a known quantity. On my camera the sensor is 14.8mm high. If an object produces an image 14.8mm high, it will completely fill the frame vertically..

One practical usage is this: suppose that I'm going to be photographing a stage presentation from a distance of 50 feet. I want to be able to capture a 3-foot high close-up of one of the performers in portrait mode. My sensor is 22.2mm wide, which will be the height in portrait mode. I can readily calculate that this will require a lens with an EFL of 22.2*50/3& 370mm..

One instructional usage is that it becomes easier to understand about focal lengths and "crop factor" if one understands that the lens's marked focal length simply determines how large a given object will be when projected onto the imaging plane, and that the sensor size then determines how much of the projected image will be captured..

Some additional information here:http://www.mellesgriot.com/products/optics/fo_5.htm.

As I cautioned in my response to Steve Balcombe: the term "effective focal length" has been widely hijacked to mean "adjusted for crop". In lens specifications, it has long been replaced by simply "focal length", which to an extent hijacked *that* term from the optical sciences. This is my big point: the definition of focal length in this site's glossary is the optical definition, but is not the (virtually universal) usage of the term in photography...

Comment #12

Doug Pardee wrote:.

...Theback and front focal lengths and the principal points are irrelevantto practical photography..

Yes, but they only need to be found or referenced when measuring FL..

All that matters to the photographer isthat the lens is designed to focus at the correct distance from themountthat the Flange Focal Length is equal to the lens register ofthe camera..

I think photographers only need to know that the lens is capable of focusing on his subject(s). The Flange Focal Length and the Lens Register (whatever they are) are uninteresting..

Hmmm... I tried to do this mathematically and still haven't been ableto make YOUR system jibe with classical physics..

First, it's not "MY system". It's the way that photographic lensesdesigned for fixed mounts are universally marked. The focal lengthmarkings are EFL and reflect exactly one aspect of the lens: itsmagnification when focused at infinity..

Can you offer evidence that all photographic lenses are marked with EFL. My detector is going off!.

EFL is also the only figure that is relevant for aperturecalculations (f-numbers) and for depth of field calculations, becauseboth are actually based on magnification..

EFL is identical to FL..

I contend that with digital cameras it's difficult for most people to"measure" the size of the image..

Why would they want to?.

Because "your" formula required measuring the image size in order to compute FL! I was saying that it's difficult to measure..

The point is that EFL is purely a measure ofmagnification and is unrelated to the physical distance from the lensto the imaging plane (a figure which is relatively constant for alllenses on a fixed lens mount)..

I can't agree with any of that...simply not true. Or perhaps we are miscommunicating?.

To start, if you said that "the physical distance from the lens to the imaging plane is relatively constant for all lenses on a fixed lens mount", this is not true. The location of the second principal point relative to the mounting flange of the lens moves as the lens is zoomed..

Next, EFL is clearly NOT a measure of magnification, else it would be called something like Effective Magnification..

But it's not really that hard to measure anyway. The size of the filmor the imaging sensor is a known quantity. On my camera the sensor is14.8mm high. If an object produces an image 14.8mm high, it willcompletely fill the frame vertically..

Yes, you and I can do it, but most beginners can't. Most beginners can't apply the Pythagorean Theorum to compute the diagonal of a sensor, given the height and width..

One practical usage is this: suppose that I'm going to bephotographing a stage presentation from a distance of 50 feet. I wantto be able to capture a 3-foot high close-up of one of the performersin portrait mode. My sensor is 22.2mm wide, which will be the heightin portrait mode. I can readily calculate that this will require alens with an EFL of 22.2*50/3& 370mm..

Hmmm...most beginners just grab their P&S camera and zoom until the performer looks correct in their viewfinder (if they have one), otherwise they use their LCD..

If they have a fixed FL lens, they just walk... .

One instructional usage is that it becomes easier to understand aboutfocal lengths and "crop factor" if one understands that the lens'smarked focal length simply determines how large a given object willbe when projected onto the imaging plane, and that the sensor sizethen determines how much of the projected image will be captured..

Nah...just look at the image captured in the sensor..

Some additional information here:http://www.mellesgriot.com/products/optics/fo_5.htm.

This MellesGriot stuff is very technical. Most beginners won't understand anything! I have used them as a handy reference to formulas and terms for 25 years..

For example, they define the EFL as, "...this is both the distance from the front focal point (F) to the primary principal point (H) and the distance from the secondary principal point (H" ) to the rear focal point (F" ). I contend that this doesn't help beginners..

But while we are here, the MellesGriot site defines EFL = f and then says, "Unfortunately, f is measured with reference to principal points which are usually inside the lens so the meaning of F is not immediately apparent when a lens is inspected." Note that they are speaking of a bare, unmounted lens...not a packaged lens system with a dozen lens elements. But it works with complex lens systems too. This is how I measure FL...by locating the principal points!.

But I found nowhere in this white paper, any reference to "your" version of FL, ie that FL was equal to the image width when the object subtended a 53.13 degree angle..

This is my big point: the definition of focal length inthis site's glossary is the optical definition, but is not the(virtually universal) usage of the term in photography..

I agree that it is the optical definition. I disagree that somehow it is not a useful parameter when using a camera..

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #13

We're getting WAY beyond the charter for the Beginners Questions forum. We're not here to argue the finer points of optics..

My point was, and remains, that defining the focal length as being the distance from the optical center of the lens to the imaging plane is misleading and basically worthless for fixed-mount photographic lenses. If someone doesn't know what a focal length is, they're sure not going to know what the "optical center" means. People are going to believe that it's a point somewhere around the middle of the physical lens, which you and I both know is not at all correct. And even if they did know what the optical center means, they're not going to know where the optical center of their lenses is..

Not only does that definition have no practical value, it has no value as far as understanding focal length and it's effects, either..

It's a thoroughly worthless definition. Please note that I never said that it was "incorrect". I said that it was "irrelevant" to the practicing photographer..

And I don't think that changing "optical center" to "second principal point" helps..

Chuxter wrote:.

"your" formula required measuring the image size in order to compute FL!.

I managed to get both you and Steve confused on what I was trying to say. I imagine that you're not the only ones whom I confused. I think maybe it was my use of the word "measurement". Whatever it was, let me clarify this for the third time:.

I wasn't saying that this was the definition of FL. I wasn't saying that this is how FL is determined in practice. I was just explaining the *effect* of FL in a way that I hoped would be easily understood by the beginners in this forum..

To start, if you said that "the physical distance from the lens tothe imaging plane is relatively constant for all lenses on a fixedlens mount", this is not true..

An example:.

The Canon EF lens mount flange is 44mm from the imaging plane. One of Canon's 14mm lenses is 94mm long, so it runs from 44 to 138 mm away from the imaging plane. One of Canon's 200mm lenses is 136mm long, so it runs from 44 to 180 mm away from the imaging plane. In my opinion, 44-138 vs. 44-180 constitutes little difference in "physical distance from the lens to the imaging plane", especially considering the dramatic difference in focal length..

The location of the second principal point relative to the mounting flangeof the lens moves as the lens is zoomed..

(As you know) the second principal point is an imaginary spot located on the optical axis, one FL ahead of the imaging plane. For telephoto lenses it can be ahead of the lensfor the 200mm lens above, it's located 20mm in front of the lens. For retrofocal lenses it can be behind the lensfor the 14mm lens noted above, it's located 30mm behind the lens, inside the mirror chamber of the camera..

The physical distance from the lens to the imaging plane, however, is fixed by the mount distance. The position of an imaginary point doesn't really factor in here..

But I found nowhere in this white paper, any reference to "your"version of FL, ie that FL was equal to the image width when theobject subtended a 53.13 degree angle..

It should go without saying. Draw a simple diagram, including the crossed lines from object to image. If an object of size z is distance x in front of the crossing point, and the imaging plane is distance y behind the crossing point, the size of the projected object is z*y/x. When the lens is focused at infinity, the crossing point is the second principal point, and hence y=FL (by definition). Thus we get the size of the projected object as FL*z/x. My example had z=x, so we get a projected size of FL...

Comment #14

Doug Pardee wrote:.

One practical usage is this: suppose that I'm going to bephotographing a stage presentation from a distance of 50 feet. I wantto be able to capture a 3-foot high close-up of one of the performersin portrait mode. My sensor is 22.2mm wide, which will be the heightin portrait mode. I can readily calculate that this will require alens with an EFL of 22.2*50/3& 370mm..

I can't seem to locate the EFL rating for my lens; please see link to mfg. site:.

Http://nikonimaging.com/...products/lens/af/micro/af_micro60mmf_28d/index.htmAlso, thank you for this link.

Http://www.mellesgriot.com/products/optics/fo_5.htm-garrison..

Comment #15

Gking wrote:.

Doug Pardee wrote:.

One practical usage is this: suppose that I'm going to bephotographing a stage presentation from a distance of 50 feet. I wantto be able to capture a 3-foot high close-up of one of the performersin portrait mode. My sensor is 22.2mm wide, which will be the heightin portrait mode. I can readily calculate that this will require alens with an EFL of 22.2*50/3& 370mm..

I can't seem to locate the EFL rating for my lens; please see link tomfg. site:.

Http://nikonimaging.com/...products/lens/af/micro/af_micro60mmf_28d/index.htm.

There is no EFL. Doug is confusing you..

The FL is stated on the top line: "AF Micro-Nikkor 60mm f/2.8D". Note that the Specifications also don't list the maximum aperture or minimum aperture. And the working distance is listed in the Main Features section, but not the Specifications. This is a rather sloppy document!.

To complicate matters, it's possible that this lens varies the FL as it is focussed! Some internal focusing lenses do. But don't worry about that....

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #16

Chuxter wrote:.

To complicate matters, it's possible that this lens varies the FL asit is focussed! Some internal focusing lenses do. But don't worryabout that....

I don't think the 60mm Micro-Nikkor has any floating elements..

If I recall correctly, it is a conventional fixed focal length lens of flat field, optimised for macro distances, and with built-in extended focusing movement..

Of course, even without any actual change in f-length caused by internal elements reconfiguring themselves for some reason....

(improved sharpness in close focusing, possibly?).

... fixed focal length lenses ALWAYS reduce their acceptance angle (the amount you 'get in the picture') as they are focused closer..

This effect is perfectly normal, and occurs because the sensor is effectively being moved "away" from the lens.... therefore, progressively more and more of the lens' circle of coverage (acceptance angle) is falling off the EDGES of the sensor!!.

The above effect is not inconsiderable with close up shooting..

For instance, at 1:1 reproduction......

[ where the lens is racked out to 2xf instead of the 1xf of infinity focus a condition also known as "same size reproduction" ].

... only 1/4 of the lens's useful coverage area is falling on the sensor, meaning 3/4 of the image thrown by the lens is going to waste! In effect the acceptance angle of the 60mm lens at infinity is halved both vertically and horizontally, and therefore "equals" a 120mm lens (that's infinity focused) when it is close focused at same size... (1:1 repro)..

This has another interesting effect.....

As a lens is focused closer, it automatically garners the ability to cover a sensor LARGER than the one covered at infinity. The closer you get, the bigger that sensor could be. This is how I was able to use a 50mm enlarging lens an optic designed strictly for 35mm printing in a darkroom as a close-up copying lens on a Mamiyaflex C330 roll-film camera of 2-1/4" square image size.Regards,Baz..

Comment #17

Barrie Davis wrote:.

Chuxter wrote:.

To complicate matters, it's possible that this lens varies the FL asit is focussed! Some internal focusing lenses do. But don't worryabout that....

I don't think the 60mm Micro-Nikkor has any floating elements.If I recall correctly, it is a conventional fixed focal length lensof flat field, optimised for macro distances, and with built-inextended focusing movement..

You may be right? Read here:.

Http://www.nikon-image.com/eng/LensGuide/opt_tech.html.

It says, in part: "Nikon is a pioneer in the development of Close-Range Correction (CRC) systems. Sometimes referred to as "Floating Element" designs, CRC enables selected Nikkor lenses to provide comparable performance at both near and far focusing distances.".

And in the link the OP provided:.

Http://nikonimaging.com/...../lens/af/micro/af_micro60mmf_28d/index.htm.

It says, in part, regarding the 60mm f/2.8 macro lens: "Close-Range Correction (CRC) system provides high performance at both near and far focusing distances.".

This implies that it DOES have floating elements?.

Of course, even without any actual change in f-length caused byinternal elements reconfiguring themselves for some reason....

(improved sharpness in close focusing, possibly?).

I think CRC allows them to correct abberations?.

I wish Nikon, Canon and others would tell us clearly what these fancy lenses really do to things like FL and FOV!.

... fixed focal length lenses ALWAYS reduce their acceptance angle(the amount you 'get in the picture') as they are focused closer..

This effect is perfectly normal, and occurs because the sensor iseffectively being moved "away" from the lens.... therefore,progressively more and more of the lens' circle of coverage(acceptance angle) is falling off the EDGES of the sensor!!.

The above effect is not inconsiderable with close up shooting..

For instance, at 1:1 reproduction......

[ where the lens is racked out to 2xf instead of the 1xf ofinfinity focus a condition also known as "same size reproduction"].

... only 1/4 of the lens's useful coverage area is falling on thesensor, meaning 3/4 of the image thrown by the lens is going towaste! In effect the acceptance angle of the 60mm lens at infinity ishalved both vertically and horizontally, and therefore "equals" a120mm lens (that's infinity focused) when it is close focused at samesize... (1:1 repro)..

This has another interesting effect.....

As a lens is focused closer, it automatically garners the ability tocover a sensor LARGER than the one covered at infinity. The closeryou get, the bigger that sensor could be. This is how I was able touse a 50mm enlarging lens an optic designed strictly for 35mmprinting in a darkroom as a close-up copying lens on a MamiyaflexC330 roll-film camera of 2-1/4" square image size..

Yes, there was a recent thread on this:.

Http://forums.dpreview.com/...forums/read.asp?forum=1002&message=24987617.

I recommend people use Barnack to quantify this effect, which you described quite well, BTW. It's available here:.

Http://www.stegmann.dk/mikkel/barnack/.

It only allows the focus distance to be reduced to 3.9" (0.1m), but for most macro lenses, that's enough..

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #18

Chuxter wrote:.

It says, in part, regarding the 60mm f/2.8 macro lens: "Close-RangeCorrection (CRC) system provides high performance at both near andfar focusing distances.".

This implies that it DOES have floating elements?.

Yes. It does imply floating elements, doesn't? I got that wrong. In truth, I wasn't entirely sure when I posted.....

However, you will realise my reason for posting was to draw attention to "normal" reductions of viewing angle that take place with ALL lenses at closest focus, and it happens regardless of whether any other mechanisms, say floating elements for focusing/zooming, also change viewing angle is some way..

Thanks, Charlie. Regards,Baz.

PS. How many esses are you using in "focused" ? (focussed) ?My spell checker is picking up double 's' as an error.... [??]..

Comment #19

Barrie Davis wrote:.

Chuxter wrote:.

To complicate matters, it's possible that this lens varies the FL asit is focussed! Some internal focusing lenses do. But don't worryabout that....

I don't think the 60mm Micro-Nikkor has any floating elements.If I recall correctly, it is a conventional fixed focal length lensof flat field, optimised for macro distances, and with built-inextended focusing movement..

Of course, even without any actual change in f-length caused byinternal elements reconfiguring themselves for some reason....

There's no need to speculate on this. At 1:1 this lens is focused at 220 mm ( see another recent thread started by the same OP as this one, at http://forums.dpreview.com/...forums/read.asp?forum=1002&message=25064961 ), and also at 1:1 the focal length is exactly 1/4 of the focusing distance (subject distance plus image distance in more conventional optics terms) - therefore 55 mm..

If we assume the 60 mm rating is correct at infinity focus, then the focal length has changed...

Comment #20

Barrie Davis wrote:.

Chuxter wrote:.

It says, in part, regarding the 60mm f/2.8 macro lens: "Close-RangeCorrection (CRC) system provides high performance at both near andfar focusing distances.".

This implies that it DOES have floating elements?.

Yes. It does imply floating elements, doesn't? I got that wrong. Intruth, I wasn't entirely sure when I posted.....

However, you will realise my reason for posting was to draw attentionto "normal" reductions of viewing angle that take place with ALLlenses at closest focus, and it happens regardless of whether anyother mechanisms, say floating elements for focusing/zooming, alsochange viewing angle is some way..

Yes, I think that is the more important concept for Beginners to grasp. Besides, I don't know if the subject lens zooms-while-focusing....

PS. How many esses are you using in "focused" ? (focussed) ?My spell checker is picking up double 's' as an error.... [??].

I'm old and shaky. Besides, sometimes I misspell words? .

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #21

PS. How many esses are you using in "focused" ? (focussed) ?My spell checker is picking up double 's' as an error.... [??].

I'm old and shaky. Besides, sometimes I misspell words? .

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/.

And I specialize in misapropisms (I mean malapropisms).Thanks for what was a tremendous thread.-Garrisonhttp://www.umefotographie.com..

Comment #22

Steve Balcombe wrote:.

Barrie Davis wrote:.

Chuxter wrote:.

To complicate matters, it's possible that this lens varies the FL asit is focussed! Some internal focusing lenses do. But don't worryabout that....

I don't think the 60mm Micro-Nikkor has any floating elements.If I recall correctly, it is a conventional fixed focal length lensof flat field, optimised for macro distances, and with built-inextended focusing movement..

Of course, even without any actual change in f-length caused byinternal elements reconfiguring themselves for some reason....

There's no need to speculate on this. At 1:1 this lens is focused at220 mm ( see another recent thread started by the same OP as thisone, at.

Http://forums.dpreview.com/...forums/read.asp?forum=1002&message=25064961 ), and also at 1:1 the focal length is exactly 1/4 of the focusing distance (subject distance plus image distance in more conventional optics terms) - therefore 55 mm..

If we assume the 60 mm rating is correct at infinity focus, then thefocal length has changed..

Unfortunately, it's not that simple. If the lens was a thin lens, then your calculations would be right. But the lens is not thin. And we don't have much information about it. But we are rather sure that the two principal points are NOT at the same place in the lens, as would be the case with a think lens..

The following diagram shows ONE possibility:.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

Note that the 1st principal point is behind the 2nd principal point. That's all it takes to keep the FL = 60mm....

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #23

Makes sense - but currently it is speculation of course. I am reaching the point with this discussion where I really crave some authoritative information. (No disrespect to the current contributors intended! - I am sure you know what I mean.).

I've been kept pretty busy with work these last few days, but when I get some time I'll spend some of it hunting for resources...

Comment #24

Steve Balcombe wrote:.

Makes sense - but currently it is speculation of course. I amreaching the point with this discussion where I really crave someauthoritative information. (No disrespect to the current contributorsintended! - I am sure you know what I mean.).

I do. I'm a facts and data kinda guy....

I've been kept pretty busy with work these last few days, but when Iget some time I'll spend some of it hunting for resources..

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #25

Focal length versus distance (magnification) data for the 60 mm f/2.8 Micro Nikkor are shown here.http://www.nikonians.org/dcforum/DCForumID63/16356.html#1.

The actual focal length at the near limit (1:1) is about 50 mm, so the spacing between object and image space principal points is about 20 mm (not negative 20 mm as implied in the diagram above). Measured data for magnification versus extension from which these results were obtained is shown here.http://gvk.smugmug.com/photos/48780447-O-1.gif.

Gerry..

Comment #26

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