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Field of view question
Quick question about field of view....

I want a long telephoto. I just don't know how long I need to do what I want to do..

I have a 50-200 tele..

If I take a picture at 200mm, is there a way to extrapolate what the field of view would have been with a longer lens?.

Here's what I was thinking. Since a 400mm is twice as long as my 200mm, if I take the picture with my 200, and halve it vertically and horizontally, that would be the field of view that a 400 would give me. If I halve THAT in both directions, that would show me an 800mm's FOV..

Is that valid? Is an 800's FOV *really* 1/16th of a 200's FOV?.

Thanks for any info......

Comments (20)

Yes, you are correct. Doubling the focal length doubles the length of an image on the sensor, and also doubles the width... i.e. it is 4x bigger, so 1/4 the field of view..

You can test this with your 50-200 and a brick wall!.

Best wishesMike..

Comment #1

Mike703 wrote:.

You can test this with your 50-200 and a brick wall!.

Now that's a good point, and one of those "why didn't I think of that" moments. .

Thanks for the info.....

Comment #2

You also need to be aware of Barnack:.

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

Charlie DavisNikon 5700, Sony R1, Nikon D300HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/'Experience: Discovering that a claw hammer will bend nails.Epiphany: Discovering that a claw hammer is two tools...'..

Comment #3

That is a great tool to learn. I am going to spend some time there...

Comment #4

Skydvr wrote:.

If I take a picture at 200mm, is there a way to extrapolate what thefield of view would have been with a longer lens?.

Yes.

Here's what I was thinking. Since a 400mm is twice as long as my200mm, if I take the picture with my 200, and halve it vertically andhorizontally, that would be the field of view that a 400 would giveme. If I halve THAT in both directions, that would show me an800mm's FOV..

Yes..

Is that valid? Is an 800's FOV *really* 1/16th of a 200's FOV?.

Yes..

That is why on P&S digicams, they use the zoom factor. To extrapolate, imagine a 200-800mm lens, which would be a 4x zoom lens (i.e. a lens that would let you get an image that is 4 times bigger at 800mm than at 200mm)...

Comment #5

Skydvr wrote:.

Is that valid? Is an 800's FOV *really* 1/16th of a 200's FOV?.

Thanks for any info.....

Yes. There's nothing whatsoever wrong with your calculations. However, behind your enquiry lies a question unasked... and that is, "Just how good is the image quality resultant from such radical cropping?"..... and the answer is, not very good at all..

Not only will you be working with only 1/4 or 1/16th of the precious pixels you paid for when the camera was bought, but you will also be magnifying any faults in the image (blur from camera shake, subject movement etc.) and any shortcomings in the lens.... which shortcomings will be there when enlarged enough to see them! .

The degree of magnification that reveals these faults is in linear proportion to the enlargement, so a crop to 1/4 the width will make any blur 4 times as noticeable as in the uncropped image..

Conclusion: Use a lens that frames the image as intended if you can. Cropping to gain magnification is a last resort strategy, and a certain route to poorer image quality.Regards,Baz..

Comment #6

Barrie Davis wrote:.

Conclusion: Use a lens that frames the image as intended if you can.Cropping to gain magnification is a last resort strategy, and acertain route to poorer image quality..

Isn't this what happens when we use a full frame lens on a cropped-sensor camera? We are only cropping the original image to gain apparent magnification?.

Ralph..

Comment #7

Is there a formula to determine how many pixels on a computer screen your image will take with different lenses - assuming you use the same camera with the same resolution?.

There are times I would like to see in advance what a scene might look like with a wider angle lens that I don't yet own..

For example, a picture taken with my 36mm lens just takes in a meadow with two trees on either side. That image is 2816 pixels wide on my computer monitor. If I use a 27mm lens, that same scene would be a smaller percentage of the resulting 2816 pixel image..

Ralph..

Comment #8

Skydvr:.

Quick question about field of view....

I want a long telephoto. I just don't know how long I need to dowhat I want to do..

I have a 50-200 tele..

If I take a picture at 200mm, is there a way to extrapolate what thefield of view would have been with a longer lens?.

Here's what I was thinking. Since a 400mm is twice as long as my200mm, if I take the picture with my 200, and halve it vertically andhorizontally, that would be the field of view that a 400 would giveme. If I halve THAT in both directions, that would show me an800mm's FOV..

Is that valid? Is an 800's FOV *really* 1/16th of a 200's FOV?.

Thanks for any info......

Comment #9

Kd6vm wrote:.

Barrie Davis wrote:.

Conclusion: Use a lens that frames the image as intended if you can.Cropping to gain magnification is a last resort strategy, and acertain route to poorer image quality..

Isn't this what happens when we use a full frame lens on acropped-sensor camera? We are only cropping the original image togain apparent magnification?.

Ralph.

Ralph:.

Once again, the magnification is a function only of the ratio of the size of the image on the sensor to the subject; there is nothing "apparent" about it. It is what it is..

The answer to your quest is NO. As Barrie stated, cropping a full frame picture would throw out pixels. The image captured on a crop sensor has all the pixels it started with; whatever that sensor provides. Barrie's point was that image quality would suffer as a result of cropping a a full-frame image, because fewer pixels are used to depict the image. That is NOT the case where all of the senor's pixels are used to record the image..

Dave.

Http://www.pbase.com/dsjtecserv..

Comment #10

Dsjtecserv wrote:.

Once again, the magnification is a function only of the ratio of thesize of the image on the sensor to the subject; there is nothing"apparent" about it. It is what it is..

The answer to your quest is NO. As Barrie stated, cropping a fullframe picture would throw out pixels. The image captured on a cropsensor has all the pixels it started with; whatever that sensorprovides. Barrie's point was that image quality would suffer as aresult of cropping a a full-frame image, because fewer pixels areused to depict the image. That is NOT the case where all of thesenor's pixels are used to record the image..

Dave.

Thanks, Dave for clarifying Barrie's comment. I was unsure of what he meant. As far as the other issue is concerned, I am not trying to belabor the point. When I have more experience under my belt, I am sure it will become clear to me..

Thanks again,Ralph..

Comment #11

Kd6vm wrote:.

Is there a formula to determine how many pixels on a computer screenyour image will take with different lenses - assuming you use thesame camera with the same resolution?.

Yes..

For example, a picture taken with my 36mm lens just takes in a meadowwith two trees on either side. That image is 2816 pixels wide on mycomputer monitor. If I use a 27mm lens, that same scene would be asmaller percentage of the resulting 2816 pixel image..

It would be (2816)*(27/36) pixels wide (2112 pixels)...

Comment #12

Dave_s93 wrote:.

Kd6vm wrote:.

Is there a formula to determine how many pixels on a computer screen your >>image will take with different lenses - assuming you use the same camera with >>the same resolution? For example, a picture taken with my 36mm lens just >>takes in a meadow with two trees on either side. That image is 2816 pixels >>wide on my computer monitor. If I use a 27mm lens, that same scene would >>be a smaller percentage of the resulting 2816 pixel image..

Yes.It would be (2816)*(27/36) pixels wide (2112 pixels)..

Thank you so much. Now I wonder why I didn't figure that one out. I am going to do some calculations with that equation..

Ralph..

Comment #13

Dave_s93 wrote:.

Kd6vm wrote:.

Is there a formula to determine how many pixels on a computer screenyour image will take with different lenses - assuming you use thesame camera with the same resolution?.

Yes..

For example, a picture taken with my 36mm lens just takes in a meadowwith two trees on either side. That image is 2816 pixels wide on mycomputer monitor. If I use a 27mm lens, that same scene would be asmaller percentage of the resulting 2816 pixel image..

It would be (2816)*(27/36) pixels wide (2112 pixels)..

Actually I think it would be more like 2080. The width of the flat field encompassed by the angle of view varies with the tangent of the angle, so changing the angle a given percent doesn't change the width by an equivalent percent. But as an approximation for relatively small changes in angle, that approach would work well enough..

Dave.

Http://www.pbase.com/dsjtecserv..

Comment #14

Dsjtecserv wrote:.

Dave_s93 wrote:.

It would be (2816)*(27/36) pixels wide (2112 pixels)..

Actually I think it would be more like 2080. The width of the flatfield encompassed by the angle of view varies with the tangent of theangle, so changing the angle a given percent doesn't change the widthby an equivalent percent. But as an approximation for relativelysmall changes in angle, that approach would work well enough..

Dave.

Hi Dave, that is probably accurate. I started to figure the problem using angle-of-view charts that came with my lens, but I had a little trouble formulating an equation to suffice for my purposes. Besides, math was never my strongest point..

Many thanks,Ralph..

Comment #15

Dsjtecserv wrote:.

Dave_s93 wrote:.

For example, a picture taken with my 36mm lens just takes in a meadowwith two trees on either side. That image is 2816 pixels wide on mycomputer monitor. If I use a 27mm lens, that same scene would be asmaller percentage of the resulting 2816 pixel image..

It would be (2816)*(27/36) pixels wide (2112 pixels)..

Actually I think it would be more like 2080. The width of the flatfield encompassed by the angle of view varies with the tangent of theangle, so changing the angle a given percent doesn't change the widthby an equivalent percent. But as an approximation for relativelysmall changes in angle, that approach would work well enough..

True. What I gave is only an approximation. It does not work out too well for wide angles, but is a better approximation with telephoto lenses..

If you really want to be accurate, you would need the crop factor to calculate the true FOV, and correspond that to the object size..

I created a spreadsheet with the formulas, so here they are:.

36mm where the width of the camera is 2816 pixels and the object occupies the full 2816 pixels..

For FF camera:.

In order for the object to be that size, the object width to object distance ratio needs to be 1:1.1456 (i.e. 1 ft wide, 1.1456ft away).36mm = 2816 pixels27mm = 2220 pixels.

For 1.5x crop camera:Object size to distance ratio would need to be 1:1.5936mm = 2816 pixels27mm = 2166 pixels.

For 1.6x crop camera:Object size to distance ratio would need to be 1:1.683536mm = 2816 pixels27mm = 2160 pixels.

Note, the formula I used to calculate the FoV is:2*ATAN(sensor size/(2*focal))*180/PI..

Comment #16

Dave_s93 wrote:.

If you really want to be accurate, you would need the crop factor tocalculate the true FOV, and correspond that to the object size..

I created a spreadsheet with the formulas, so here they are:.

36mm where the width of the camera is 2816 pixels and the objectoccupies the full 2816 pixels..

For FF camera:In order for the object to be that size, the object width to objectdistance ratio needs to be 1:1.1456 (i.e. 1 ft wide, 1.1456ft away).36mm = 2816 pixels27mm = 2220 pixels.

For 1.5x crop camera:Object size to distance ratio would need to be 1:1.5936mm = 2816 pixels27mm = 2166 pixels.

For 1.6x crop camera:Object size to distance ratio would need to be 1:1.683536mm = 2816 pixels27mm = 2160 pixels.

Note, the formula I used to calculate the FoV is:2*ATAN(sensor size/(2*focal))*180/PI.

Very helpful formula, Dave. I will be using it quite regularly in contemplating future lens acquisitions, as well as planning for photos..

Ralph..

Comment #17

Hi,.

Here's a simple way of seeing what you'll get but I'm only giving figures for a 35mm camera, or so-called full frame or what you'd see in 35mm equivalent: OK?.

Get a piece of card and cut a hole in it that's one inch by one and a half inches. Or, if you want to be precise a rectangle 24mm by 36mm..

Hold the card with the hole in front of you and look through the hole. When th card is 300mm away you are seeing the FoV of a 300mm lens on a 35mm camera. And so on for any distance you care to name. Try it at 1000 and 100 and 35mm f'instance..

You want it in other aspect ratios? Well, the diagonal of the rectangle you cut out is about 43mm and if you cut a rectangle that's 35mm x 26mm it will have the same diagonal but will be 4:3 aspect ratio (more or less). And the diagonal will still be 43mm, so you've the same field of view. So hold it 300mm away and look through it and you'll see the picture you'd get with a 300mm lens (in 35mm film terms) and a 4:3 aspect ratio. Which is what your monitor, TV and a lot of other things have including Academy Standard 35 mm film shots. "Academy Standard" is what most 1930' and 40's movies were shot in on 35mm cine film. In medium format it was called the "Ideal Standard" and was a nominal 45mm x 60mm on the negative..

Hope this helps..

Regards, David..

Comment #18

That's neat! Not sure if my arms are a meter long though Best wishesMike..

Comment #19

Mike703 wrote:.

That's neat! Not sure if my arms are a meter long though .

Hi,.

Carry a dSLR and a few lens etc around for a while and one arm will slowly extend to whatever length suits you....

Regards, David..

Comment #20

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