Crop Factor versus Lens Compression / Perspective

While a 50mm full frame lens used on a Canon DSLR with a 1.6x crop

factor will yield a similar image area as 80mm full frame DSLR -

will the resulting perspective / lens compression of a 50mm full

frame lens used on a 1.6x crop factor DSLR still be similar (except

for image area ) to a 50mm lens used on a full frame DSLR ? To

clarify my question: If I want to choose a lens for my 1.6x crop

factor DSLR to take head and shoulder portraits with enough

compression to flatten facial features (i.e. protruding noses ) then

do I still need a 85mm to 100mm full frame tradional portrait lens

(or equivelent in APS-C format) to accomplish the necessary image

compression / perspective ? I want to be sure I am not confusing the

resulting 1.6x crop factor image area of a lens with other image

parameters (such as lens compression / perspective).

With a 50mm lens on a 1.5 DSLR, you will be standing in the same place as you would with a 75mm lens on a FF DSLR. So both pictures would have the same perspective. I suspect that the same aperture would (very roughly) produce the same DOF.

DOF is a product of f-stop and magnification

Try to think of it this way...if all else is equal with only the sensor's dimensions being different, the ONLY thing that changes is how much of the photo is cropped off. The image remaining from the smaller sensor will be identical to that part of the same image from the larger sensor. Nothing else changes.

CI P is wrong about DOF. See my answer in the thread immediately below this one:

 

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00GI22&tag=

 

and look at the links I provided. Perspective is a function solely of your point of view relative to the subject (ignoring any lens distortion such as fisheye, barrel and pincushion). If you shoot from the same position with a 50mm lens and an 80mm lens at the same aperture, and then crop the 50mm photograph and enlarge it more to give the same print as the full frame from the 80mm lens, you will have the same image.

Mark U,

 

I'm not sure that is entirely correct. If you think of an extreme example, then using a

300mm lens and a heavily cropped image from a 50mm lens; there will be substantially

more distance compression with the 300mm than the 50mm. This surely will be the same

principal with the 50 v 85 although to a much much less effect.

 

The perspective remains the same, but the compression alters.

 

But actually, I think any 50 to 85mm lens on a smaller sensor would make a good portrait

lens without regard to the compression issue.

 

David

Chris, the first thing you need to understand is that perspective has NOTHING to do with focal length. Perspective is totally related to the distance between the viewer (or camera) and the subject.

 

Like you said, the images taken with a Canon 20D (for example) using a 50mm lens will be virtually identical to images taken with a 35mm film body using an 80mm lens. If you use both cameras/lenses at the same location relative to the subject, then virtually everything about the two images will be the same (assuming you use the same exposure settings and equivalent ISO selection/settings). The perspective will be identical because you are taking the two shots at the same distance from the subject.

 

The problem with the "crop factor" thing is all the mis-information that gets spread around. The 20D (and all other digital cameras with APS-C sized sensors) is just a camera with another format to the "film". It uses a 31mm focal length for its classic "normal" lens. A 35mm film camera (using the standard 24x36mm film frame) uses a 50mm focal length for its "normal" lens. A 6x6cm medium-format camera uses an 80mm focal length for its "normal" lens. The "crop factor" is purely for calculating the difference in field of view for a lens being used on 35mm cameras vs the APS-C digital cameras.

 

The only reason the "crop factor" was ever invented is because, though the APS-C cameras are a different format, the manufacturers decided to make the cameras externally similar to 35mm cameras and to use the same family of lenses that were already available for the 35mm cameras.

 

One needs to realize the difference in the format for what it is. Also realize that lenses do not change anything when you put them on cameras with different formats. The focal length requirements of different formats are different, however.

David M (UK) - You are wrong about the "compression" issue. That is nothing more than perspective at work. Perspective is totally and absolutely a function of distance between the viewer (or camera) and the subject. If you shot two photos - one with a 50mm and one with a 300mm lens - of the same subject from the same position and then cropped the shot taken with the 50mm lens to match the field of view of the shot taken with the 300mm lens, both shots would look identical (ignoring resolution and sharpness issues).

"CI P is wrong about DOF"

 

Yes. It's true.

Yes, the lens characteristic do NOT change. Thus, if you are talking about the "compression" of teles that won't change either. Just like the field of view of a 15mm won't change. You will ONLY see a *cropped*, center portion of the image, just as if you would have cropped it in PS or darkroom. Nothing else changes.

 

However, becasue you would have to *stand further away from a subject* to get the *same framing* now, there is MORE DOF, not less. That's because the further a subject is from the lens the less the DOF. Or, greater distance=greater DOF.

 

The facial features will not look different if you use the lens from the *same distance*. But, because your cropped camera will "force" you to stand back to achieve the same framing as you would on a FF camera, the "distortion" associated with object close to the 50mm front element will be less.

 

For example: to have a person's head fill the frame using a 50mm lens with a FF camera you'd stand a XX meters from the subject. With a cropped camera it will will XX+the crop makup distance. So, you would be FURTHER away from the subject.

>>hat's because the further a subject is from the lens the less the DOF<<

 

Sorry, a typo> It should say "the MORE the DOF"

I did make one mistake in my post above - which was to say that the same aperture should be used in both shots. That will, of course, produce different depths of field comparing the cropped 50mm shot with the full frame 80mm shot - the aperture has to be adjusted to take account of the change in format to deliver the same depth of field. In fact, even when you do this there will be a residual very slight difference between the images because you can't simultaneously exactly match the point of sharpest focus and the near and far points of depth of field between the two cases - but you can get very close.

"Chris, the first thing you need to understand is that perspective has NOTHING to do with focal length. Perspective is totally related to the distance between the viewer (or camera) and the subject."

 

A basic rule which should be at the top of the camera instructions :-)

"If you think of an extreme example, then using a 300mm lens and a heavily cropped image from a 50mm lens; there will be substantially more distance compression with the 300mm than the 50mm"

 

Not if you take pictures of the same subjects from the same place. If you have a digital camera and a zoom lens you can verify this in a few seconds. There can be other effects causing differences in appearance but the size relationships between foreground and background objects will be exactly the same.