lens perspective and 1.6x focal multiplier

you will get an 85mm perspective or a 135mm perspective?


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toasty

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#1
Thinking about the hand-shaking and the focal length of a lens discussion from a previous post, I came to think of another question involving the focal multiplier. I thought I'd post this as a poll instead of a normal post to get a consensus, as I also think this is a rather subtle issue. The question is about the perspective rendered by a lens. We all know that the perspective of a given subject changes as we change the focal length of a lens. If you fill the frame with a 15mm lens, and with a 200mm lens, your subject will look different in the two frames. With the 15mm frame, the features closer to the lens (such as the nose) will be comparatively bigger with respect to features further away (like the ears). The shape of the head may even be different.

The question is: if you mount an 85mm lens on a DSLR (with 1.6x) and a 135mm lens on a film SLR, frame the same subject in the same way, will you get the perspective of an 85mm lens on the DSLR, or the perspective of a 135mm lens?

Reasons?
 

fleek

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#2
toasty said:
Thinking about the hand-shaking and the focal length of a lens discussion from a previous post, I came to think of another question involving the focal multiplier. I thought I'd post this as a poll instead of a normal post to get a consensus, as I also think this is a rather subtle issue. The question is about the perspective rendered by a lens. We all know that the perspective of a given subject changes as we change the focal length of a lens. If you fill the frame with a 15mm lens, and with a 200mm lens, your subject will look different in the two frames. With the 15mm frame, the features closer to the lens (such as the nose) will be comparatively bigger with respect to features further away (like the ears). The shape of the head may even be different.

The question is: if you mount an 85mm lens on a DSLR (with 1.6x) and a 135mm lens on a film SLR, frame the same subject in the same way, will you get the perspective of an 85mm lens on the DSLR, or the perspective of a 135mm lens?

Reasons?
If I am not wrong, perspective is not related to the focal length, but rather the distance between you and the subject. The compression factor is the one related to focal length
 

gooseberry

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#5
Don't think of it as a focal multiplier, but as a crop factor.

Take for example, this scenario. You have an 85mm lens. You have two cameras - a 1.5/1.6x crop factor DSLR and a full frame 35mm film SLR. You take a picture with each camera at exactly the same position using the same lens.

The picture on the DSLR will have the same perspective as the one on the film SLR, the difference being that the subject will appear larger relative to the size of the picture because of the crop factor. The picture on the DSLR will be the same as if you had taken the picture on the film SLR, scanned it in and cropped the picture in Photoshop.
 

Jun 20, 2004
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#6
If i'm not mistaken, perspective distortion has nothing to do with focal length. it is the subject to lens distance that affects perspective. we tend to get it more in wide angle lenses than in telephoto lenses because u would tend to move in closer with a WA to fill up the frame. :)
 

2100

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#7
gooseberry said:
Don't think of it as a focal multiplier, but as a crop factor.
Gooseberry, that means 135 is correct right? Since you gotta move further away.
 

gooseberry

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#8
2100 said:
Gooseberry, that means 135 is correct right? Since you gotta move further away.
To have the same subject size in the frame, an 85mm lens on a DSLR will have a different perspective from an 85mm lens on a film SLR, since you are at different distances to the subject. But is it the same perspective as a 135mm lens taken on a film SLR ?

Let's think of it this way.... we'll make the number a little easier, by having a 1.5x crop factor. Assume then that you have a 100mm lens and a 150mm lens. You take a full frame film SLR, take a photo of the subject at exactly the same position for both lenses using just the film SLR. You then take the 100mm lens picture, scan it in and crop it to the 1.5x crop factor. Will the cropped 100mm picture, taken at exactly the same position, be the same as the 150mm uncropped picture ?
 

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Yes it will be the same. See the link to the other thread I have posted with samples. :)
 

gooseberry

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#10
BurgaFlippinMan said:
Yes it will be the same. See the link to the other thread I have posted with samples. :)
Yep, that's right... it will be the same perspective because the camera to subject distance is the same. But I left the question open for the sake of the poll.

Another good visual example to look at is here...

http://www.dpreview.com/learn/?/key=perspective

specifically, pictures A, B and C.
 

toasty

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#11
Yeah I agree with the thoughts which have been mentioned here. The perspective depends on the subject to camera distance, and not on the lens used (directly). However the lens used tends to affect the subject to camera distance. Therefore I believe you will get the same perspective if you use an 85mm lens on a DSLR and a 135mm lens on a film SLR. However at this time, the poll seems to show that more people believe (or believed perhaps?) the other way.
 

Zerstorer

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#12
The poll is inherently a trick question and neither answer given in itself can be correct.

Whether you use a 135mm or an 85mm on the same camera or different cameras as long as you maintain the distance and crop down to get the same framing you should see hardly any difference in perspective.
 

toasty

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#13
Zerstorer said:
The poll is inherently a trick question and neither answer given in itself can be correct.

Whether you use a 135mm or an 85mm on the same camera or different cameras as long as you maintain the distance and crop down to get the same framing you should see hardly any difference in perspective.
So wouldn't you say that if you put an 85mm lens on a 1.6x DSLR, you'd get the perspective of a 135mm lens on a full frame no?
 

dkw

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#14
Zerstorer said:
The poll is inherently a trick question and neither answer given in itself can be correct.

Whether you use a 135mm or an 85mm on the same camera or different cameras as long as you maintain the distance and crop down to get the same framing you should see hardly any difference in perspective.
As Zerstorer says, the poll options in themselves, in relation to the question you have asked, are equally correct (or wrong).
 

dkw

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#15
toasty said:
So wouldn't you say that if you put an 85mm lens on a 1.6x DSLR, you'd get the perspective of a 135mm lens on a full frame no?
Ah! Now this question is a little different from your original one. If the distance to the subject is the same, the answer is "Yes".
 

toasty

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#16
dkw said:
Ah! Now this question is a little different from your original one. If the distance to the subject is the same, the answer is "Yes".
Actually, it's the same as my original question, as I stated that you should frame the subject the same way. ie: the subjects need to be the same size in the two frames. From that it follows the distance to the subject needs to be the same if you have an 85mm on a DSLR and a 135mm on a full frame. And then it does follow that the perspective in the two photos will be the same even though the lenses are different. Perspective doesn't depend on the lens you use, so much as your position relative to the subject. But in a sense, your position relative to the subject is influnced by the lens you use. For example, you would not use a 35mm lens to go birding :) But ultimately, it's the camera-to-subject position which determines perspective, or the position of elements in your frame relative to other elements.
 

2100

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#17
I also blur already.

I will just go home and try with a regular p&s at 105mm and my dSLR + 70mm. :bsmilie: My thought is that 70mm will have perspective at 105mm, that is if you frame properly. In my case, it should have the same picture, same distance to the subject, dSLR is 70mm and p&s is 105mm (35mm terms).
 

dkw

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#18
toasty said:
But in a sense, your position relative to the subject is influnced by the lens you use. For example, you would not use a 35mm lens to go birding :) But ultimately, it's the camera-to-subject position which determines perspective, or the position of elements in your frame relative to other elements.
Exactly, but how you compensate for different FOV is a different issue. Anyhow, I think we all understand what we are each trying to say, just that we are viewing this from different "perspectives" :) .

Cheers,
 

Jed

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#19
And again I ask, without referring specifically to the initial question, does it matter? If I have an 85mm lens in my hands, that I then attach to the front of my (ahem) 1.5x multiplier camera, and shoot with that, does it matter which perspective I'm getting, 85mm or 135mm, if ultimately at the end of the day I frame and compose the shot through the viewfinder. Does it matter that it's actually not the perspective of an 85mm lens?

That's assuming there *is* a difference in the first place. As explained above already, this isn't necessarily the case.
 

marcwang

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#20
Hmmmmmm......

The answer is yes. With the 85mm lens, whether you use a 2x crop factor cam, a 4x crop factor lens.... or whatever, you will get a 85mm perspective.
The compression (depth) of an image will still look the same.

And thats one reason I prefer a film.

On the wide end, when I want a 30mm FOV, I need a 20mm lens. And for that, I find the perspective of the 20mm lens way too small, or it exaggerates depth far too much and not too distant subjects become way too small for the sensor to sometimes resolve.

I like the way 50mm is on a film camera, its perspective. On the 1.5x crop camera, its a bit too tight for my liking. Sure I can use a 35mm lens for that extra space taht I like, but then... the perspective changes.
 

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