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09-25-2008, 01:42 PM   #1
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Perspective vs. FOV

I was a while wondering about FOV change and crop factors (which I am absolutely understand both as perception and physics), why is it so important... FOV reduction is just like cropping an image and nothing more. We would like to take photos, most likely as we see it (beside the case when for example we really want to change the perspective or doing effects). For 35mm format we use 50mm so often, because it has both the right perspective and FOV.

But which is more important for making good photos? Perspective or FOV?

In the APS-C sense we use the crop factor for FOV, but the perspective remains the same for the focal length. So I am curious which is to be found more important for field works. For example I do enjoy the perspective of wide angles for macro works (moving all the surroundings far away and/or out of focus), but it is horrible for portraits.

My 50mm feels a good lens, but I cannot use mine, as it has a broken aperture which stays always wide open. Soon I'll change it for a working one.

Honestly, the question has some financial background also, as I'm thinking about buying something like FA35/f2 as a "normal" lens but I'm afraid of the perspective, as it is far from the perceptual one.


Last edited by Chaoron; 09-25-2008 at 02:01 PM.
09-25-2008, 02:10 PM   #2
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Perspective depends *only* on the camera-to-subject distance, and the distance at which the photograph is viewed. It is not relative to the focal length.

This Wikipedia article contains more details: Perspective distortion (photography - Wikipedia, the free encyclopedia)

To get the same subject size with the FA35 on APS-C as you would with the FA50 on film, you'll stand at roughly the same distance (because of the crop-factor/FOV change) and thus the perspective will be roughly the same (assuming the same print size & viewing conditions).
09-25-2008, 02:49 PM   #3
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QuoteOriginally posted by RBellavance Quote
Perspective depends *only* on the camera-to-subject distance, and the distance at which the photograph is viewed. It is not relative to the focal length.

This Wikipedia article contains more details: Perspective distortion (photography - Wikipedia, the free encyclopedia)

To get the same subject size with the FA35 on APS-C as you would with the FA50 on film, you'll stand at roughly the same distance (because of the crop-factor/FOV change) and thus the perspective will be roughly the same (assuming the same print size & viewing conditions).
Sorry, still not understand. Will I experience the same perspective (roughly), or just the distorsion of the further/nearer objects in the scene? I think my moving changes the perspective, but the 'how much' is still based on the focal length. I am just asking about the difference between the APS-C and FF in the perspective (and not FOV).

Btw, please forget the print size... we live in a digital world where there are only resolutions.

Last edited by Chaoron; 09-25-2008 at 03:00 PM.
09-25-2008, 03:51 PM   #4
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QuoteOriginally posted by Chaoron Quote
Sorry, still not understand. Will I experience the same perspective (roughly), or just the distorsion of the further/nearer objects in the scene? I think my moving changes the perspective, but the 'how much' is still based on the focal length. I am just asking about the difference between the APS-C and FF in the perspective (and not FOV).

Btw, please forget the print size... we live in a digital world where there are only resolutions.
I know it is difficult, but here are the basic facts:

Perspective is dependent on distance only.
A picture taken at the same distance with two different lenses will have the same perspective. I posted an example picture some time ago and will see if I can find it.

Edit: Found the images...See new post below...
FOV is dependent on focal length, but also on sensor size.
Image circle plus crop factor
Focal length, by itself, does not determine either perspective or FOV.
The reason why you typically see different perspective with photos taken with different focal lengths is because the distance varies for a given subject size on the finished image.

Likewise, a 35mm lens is normal for APS-C, wide-angle for 35mm, and an ultra-wide for medium format.
Steve


Last edited by stevebrot; 09-25-2008 at 04:18 PM.
09-25-2008, 04:13 PM   #5
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Here are the two images. Same distance, different focal lengths, both cropped to give the same composition. Notice the converging lines of the cutting board. They are identical in both images.





You can tell which one was taken with the longer focal length by comparing the DOF, but that is another confusing discussion

Steve
09-25-2008, 04:54 PM   #6
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QuoteOriginally posted by Chaoron Quote
Will I experience the same perspective (roughly), or just the distorsion of the further/nearer objects in the scene?
Steve has it nailed, and hopefully his images will help. Perspective has *absolutely nothing* to do with focal length. You can take a picture of a person with a 35mm or 50mm or 70mm and the perspective is *exactly* the same. Cropping the 35mm image would produce an image indistinguishable from the 50 or 70 (assuming you got DOF to match).

QuoteQuote:
I think my moving changes the perspective, but the 'how much' is still based on the focal length.
Again, no way can focal length be related. Take two steps forward, repeat the above experiment - again, the images will be *completely* indistinguishable once cropped. Your moving is what changed the perspective, and the degree to which it changed has *nothing* to do with focal length.

QuoteQuote:
I am just asking about the difference between the APS-C and FF in the perspective (and not FOV).
There are none. The only differences are in FOV. A 28mm lens produces a narrow FOV on APS-C than on FF, but perspective is *identical*, and both of these are *identical* to the perspective you'd get with an 18mm lens, 24, 50, 70, or 135.

Now, *because* of the FOV differences between APS-C and FF, or between different focal lengths, that encourages you to change position accordingly. that is, taking a picture of a person with a 35mm lens on FF, you might try to stand real close to get them to fill the frame. Whereas with APS-C, you might take a step or two backward. that will of course alter perspective. But it's not the focal length or the size of the sensor that did it - it was your feet.
09-25-2008, 05:03 PM   #7
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QuoteOriginally posted by Chaoron Quote
...For 35mm format we use 50mm so often, because it has both the right perspective and FOV....
As noted above, the perspective is a side-affect of focal length. What focal length does affect is the FOV and the magnification. A 50mm lens has about the same magnification as the human eye. As a result, when we raise the camera to our eye, the subject looks about the same as what we saw without the camera.

QuoteQuote:
I'm thinking about buying something like FA35/f2 as a "normal" lens but I'm afraid of the perspective...
I purchased the FA35/2 early in the summer and had some of the same fears regarding perspective. Those fears were groundless. I have found that the FA35/2 is a great lens that behaves, for the most part, as if it were a 50mm lens on 35mm film.

Steve
09-25-2008, 07:21 PM   #8
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I think I understand these ideas, but reading the above doesn't elucidate them for me. Here's my attempt to say things in a way that makes sense to me...perhaps to show the world how wrong I am. :-)

Perspective has to do with how things in the frame relate to each other, whereas field of view has to do with what things are included in the frame. Perspective is determined solely by spatial relationships between the locations of the subjects and of the camera. Two lenses of different focal lengths in the same location pointed in the same direction will provide identical perspectives, as demonstrated by Steve's pictures. The difference will be that the shorter lens will include more subjects (by having a larger frame) and the longer lens will have more detail of the subjects that are in the frame (with a given sensor/film). If you want to use a 35mm lens to capture the same frame as a 50mm would, the camera needs to move closer to the subjects, which will change the perspective; the lenses did not change the perspective, but moving the camera relative to the subjects did.

As I understand Chaoron's thought, the perspective for a given lens remains the same despite the cropping, but if the user wants to maintain the same frame then the camera's location must be moved back and perspective will be changed by that move. This is correct. A 50mm lens will provide a different perspective when moved from a film body to a APS-C body if the same subject matter is captured, because the APS-C camera will need to be further away from the subjects. However, a 35mm lens almost perfectly corrects for that. The 35mm lens on an APS-C camera will provide identical perspective AND FOV as a 52.5mm lens on a film or FF camera.

I'm not sure what Chaoron means by the perspective being far from the perceptual one. If the "perceptual" perspective is what one sees looking through the viewfinder, and having that close to what the naked eye sees, then the viewfinder magnification comes into play. (On my GX-10, "normal" with the viewfinder is around 47mm.) If "perceptual" perspective is how a person sees the final print, then the size of the print and the viewer's distance from the print need to properly match the perspective of the image and so determine what focal length lens is best.

And as far as whether perspective or FOV are more important for making good photos, I can only say that they are merely two tools in the artist's kit, and while they can both be used well or poorly neither is more important.

09-25-2008, 07:34 PM   #9
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QuoteOriginally posted by JonPB Quote
I think I understand these ideas, but reading the above doesn't elucidate them for me. Here's my attempt to say things in a way that makes sense to me...perhaps to show the world how wrong I am. :-)

Perspective has to do with how things in the frame relate to each other, whereas field of view has to do with what things are included in the frame. Perspective is determined solely by spatial relationships between the locations of the subjects and of the camera. Two lenses of different focal lengths in the same location pointed in the same direction will provide identical perspectives, as demonstrated by Steve's pictures. The difference will be that the shorter lens will include more subjects (by having a larger frame) and the longer lens will have more detail of the subjects that are in the frame (with a given sensor/film). If you want to use a 35mm lens to capture the same frame as a 50mm would, the camera needs to move closer to the subjects, which will change the perspective; the lenses did not change the perspective, but moving the camera relative to the subjects did.

As I understand Chaoron's thought, the perspective for a given lens remains the same despite the cropping, but if the user wants to maintain the same frame then the camera's location must be moved back and perspective will be changed by that move. This is correct. A 50mm lens will provide a different perspective when moved from a film body to a APS-C body if the same subject matter is captured, because the APS-C camera will need to be further away from the subjects. However, a 35mm lens almost perfectly corrects for that. The 35mm lens on an APS-C camera will provide identical perspective AND FOV as a 52.5mm lens on a film or FF camera.

I'm not sure what Chaoron means by the perspective being far from the perceptual one. If the "perceptual" perspective is what one sees looking through the viewfinder, and having that close to what the naked eye sees, then the viewfinder magnification comes into play. (On my GX-10, "normal" with the viewfinder is around 47mm.) If "perceptual" perspective is how a person sees the final print, then the size of the print and the viewer's distance from the print need to properly match the perspective of the image and so determine what focal length lens is best.

And as far as whether perspective or FOV are more important for making good photos, I can only say that they are merely two tools in the artist's kit, and while they can both be used well or poorly neither is more important.
Sounds like you've got it! Go to the head of the class!

Steve
09-25-2008, 09:27 PM   #10
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QuoteOriginally posted by Marc Sabatella Quote
Again, no way can focal length be related. Take two steps forward, repeat the above experiment - again, the images will be *completely* indistinguishable once cropped. Your moving is what changed the perspective, and the degree to which it changed has *nothing* to do with focal length.
Ummmmm...........no.

As soon as you move, perspective changes. As you move farther, the near/far relationship of the camera to objects as various distance also changes.
A couple of steps with a long lens generally will show far less perspective shift than a very short one, also, providing there are objects at varying distance within the scene.
09-25-2008, 09:41 PM   #11
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a longer focal length will bring the background closer when the subject is kept the same size
check this out, very useful DOF2
09-25-2008, 10:12 PM   #12
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QuoteOriginally posted by k100d Quote
a longer focal length will bring the background closer when the subject is kept the same size
check this out, very useful DOF2
K100D,
The link is about DOF, not perspective, and it is absolutely correct.

Notice that the camera has been moved, but the relative size of the background objects change. That is because of a change in perspective brought on by a change in distance from the camera to the subject.

Now if you reverse the experiment and change the focal lengths without changing the position of camera and subject you will get a series of pictures where the subject is smaller or larger depending on focal length. Now if you crop/resize each image so that the composition is the same for each one, you will note that the perspective is also the same.

That is how I made the two images in the earlier post.
  • Distance is kept constant
  • Focal length is changed
  • Crop the resulting images so that the composition is the same
  • Render the two images to the same dimensions and compare the results

To test that the perspective is the same in the images I supplied, measure the length of the back edge of the cutting board. It is the same in both images.

Steve
09-25-2008, 10:18 PM   #13
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you're right steve. i got a bit off topic, but still something to consider
09-25-2008, 10:23 PM   #14
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QuoteOriginally posted by k100d Quote
you're right steve. i got a bit off topic, but still something to consider
A dangerous thing. The last time we had a thread on DOF vs. focal length the battle raged for weeks!

Steve
09-25-2008, 11:48 PM   #15
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Well, I can see that perspective issue was analysed very deep, however noone gave direct and simple answer to Chaoron.

So, basically he asks if 35mm lens on a APS-C DSLR is a "normal" lens as 50mm on 35mm film camera.

Yes, it is.

As many already stated here, distance from photographer to object is the only factor which influences perspective. 35mm lens on APS-C DSLR gives approximately the same FOV as 50mm lens on f35mm film SLR and that means, that photographer will be at the same distance while taking identically composed picture with 35mm lens + DSLR and 50mm lens + film SLR. The same distance means the same perspective.

APS-C DSLR with 35mm lens will give a little bit deeper DOF though. That's because of shorter focal length of the lens.
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