Perspective Distortion of Telephoto Lenses across Formats?

christian.rudman

christian.rudman

digital to analog convert
Local time
11:49 PM
Joined
May 16, 2011
Messages
99
Location
ATX
View My Gallery
I was having a lengthy discussion here at the store the other day with a customer who was interested in shooting portraits with a fixed lens MF camera like the Fuji GW690. While we were discussing the various ins and outs of MF and film cameras, he brought up a very interesting point. Now I have googled a considerable amount of terms trying to come up with an answer and that is why I have turned to you guys. Hopefully someone here knows the answer and has the ability to provide a reference for their answer.

Down to the nitty gritty:

He was wondering if the pleasing perspective distortion (flattening effect) he achieved with his FF DSLR 85mm would be the same with the converted 170mm (rough conversion and unrealistic focal length, but you get the idea) or would the perspective distortion change with the focal length increase?

My assumption was that because the angle of view was technically the same across the formats that the perspective distortion would be the same. But I wasn't sure. I'm no optics scientist, not even a geek, but it has sparked my interest in the answer. I want to be able to give a confident answer, so I turned to my trusty group of know-it-alls! 🙂

Any answer on where to possibly look would be much appreciated as well. Thanks!
 
If the lenses are rectilinear it's simply the FOV that dictates the look ... this is a 12mm overlaid with a 50mm both on 135.

If the sitter is around 3m from the camera they look good.

 
Every time I shoot from the hip on this stuff, I am corrected by folks who know more about optics than I do (search for: painfully long threads about differences in DOF among different formats). With that caveat: my gut says you are right. However, I don't think you've chosen the right example. The FF shot with an 85 will be just fine in terms of perspective distortion (that is why 85 is a popular portrait length lens). The test should be with a 35, which I find too short for a FF headshot close up (too horsey), but which would cover a 70mm "field of view" on m 4/3.
 
On 135 a 50mm for half body, a 90mm for head and shoulders and a 105mm for just the face puts the subject at the most flattering distance, well unless they look like a horse ... then 200 and stand well back
 
Not trying to be rude, so I apologize if it sounds that way, but I think all of you have missed the point of the post. I'm not trying to answer "what is the most flattering position and focal length for portraits." I do appreciate the answers, but I'm looking for one that correlates to the question.

What my question is simply this: "Is the perspective distortion the same across equivalent focal lengths in varying formats, or does it change with the physical focal length?"
 
What changes perspective is not the lens at all but the distance from the subject. The further you are away the more compressed things look.

So equivalent focal lengths (in 35mm terms) will look the same (as far as perspective goes ignoring lens "character"). So a 50mm on a 35mm would look similar in regards to perspective to an 80mm on Medium Format, because to get the same framing you would stand in the same spot. So yes your assumption is correct.
 
Sparrow said:
Sorry, not making myself clear ... the thing that matters is the Field of View regardless of what focal length is required to achieve that Field of View
Click to expand...

ChrisP said:
What changes perspective is not the lens at all but the distance from the subject. The further you are away the more compressed things look.
Click to expand...

Ahhh, you did say it and I'm just not paying attention! Thanks for the replies.

Makes sense when it's put that way ChrisP, thanks.
 
Sorry to drag this up again, but I was going to post a smilar question and found this existing thread.

I want to make sure I understand correctly.

If I stand 20 meters away from my subject, mount an 18mm lens on a 135 body and snap a shot, then mount a 180mm lens and take the same photograph from the same distance, I end up with images that show the same subject but with vastly different perspectives, angles of view, and spatial relationships. Now, if crop the picture from the 18mm lens to only show the part of the image captured by the 180mm, and blow the print up to match the size of the print from the telephoto, I will then end up with images that show the same type of spatial distortion, perspective and FOV.

Is that correct?
 
froyd said:
Sorry to drag this up again, but I was going to post a smilar question and found this existing thread.

I want to make sure I understand correctly.

If I stand 20 meters away from my subject, mount an 18mm lens on a 135 body and snap a shot, then mount a 180mm lens and take the same photograph from the same distance, I end up with images that show the same subject but with vastly different perspectives, angles of view, and spatial relationships. Now, if crop the picture from the 18mm lens to only show the part of the image captured by the 180mm, and blow the print up to match the size of the print from the telephoto, I will then end up with images that show the same type of spatial distortion, perspective and FOV.

Is that correct?
Click to expand...

No. The 18 will show the exact same spatial relationships before it is cropped as after it is cropped. Both of these will be exactly the same as the 180 (although the 18 will have more things in the image before it is cropped).

But yes assuming you had an infinite resolution they would look identical once you cropped it (assuming the 18 and 180 both had no distortion and you didn't move between shots).
 
I may be wrong but I don't think there is any such thing as perspective distortion. All subjects have natural convergence points which is what perspective is. These are exagerated if the film plane is tilted from vertical but it is not distortion. Distortion is what lenses do when they can't render straight lines as straight. Perspective doesn't bend straight lines.

Angles from the position of the camera don't change with a change of focal length. They remain the same.
The "Field of View" (FOV) changes with a focal length change. But that just means you get a wider view with a shorter lens. Thats not distortion or perspective. Its just a wider view.
Only by moving camera position does the "Point of View" (POV) change.

Essentially ChrisP is correct except an 18mm lens is very likely to have some lens distortion where it can't render straight lines as straight. This usually increases the further out from the lens axis you go. So if your crop was from the corner of an image taken with both an 18mm lens and a 180 lens the crop from the 18mm lens is likely to have more lens distortion than a 180mm lens. But if the crop was dead on the lens axis then and only from the central portion of both images then essentially they would be the same.

The flattening effect may be exagerated in the 18mm lens because the background is most likely sharp at 20m distance regardless of aperture. If the depth of field on the 180 lens makes the background out of focus with the 180 lens then the flattening effect may be different. But if its sharp then they'll both look the same.
 
Thanks 135format!

That last bit about where the crop is taken makes it clear. I really found it hard to believe the neat straight lines of my 50mm would be matched by the slightly distorted lines of an ultra-wide, but that's because I was looking at the edges of the frame.
 
froyd said:
Thanks 135format!

That last bit about where the crop is taken makes it clear. I really found it hard to believe the neat straight lines of my 50mm would be matched by the slightly distorted lines of an ultra-wide, but that's because I was looking at the edges of the frame.
Click to expand...

One other thing. Perspective only changes with a change of "Point of View" (POV) which as I already said is when the camera moves postion. That includes tilting the camera.
 
You must log in or register to reply here.
Back
Top Bottom