1.6X and Lense Perspective

I just purchased a 20D 17-85mm kit as my first DSLR. The camera and lens combo is great. I also want to get a prime portrait lens and am having a problem wrapping my brain around the 1.6x aspect of the camera. I understand that the “multiplier factor� is really more of a crop factor. Where I am having difficulty is with perspective-compression. It was always my understanding that perspective is a factor of lens to subject distance, not focal length, i.e. if you maintain the image size of an object in the foreground, the background will appear closer/larger the longer focal length you use due to the fact that you must be further from the foreground object in order to maintain its size. Conversely, if you use a wide-angle lens, the background will appear more distant/smaller because you must necessarily be closer to the foreground object to maintain its image size. What I take from this is that a 50mm lens on my 20D should give me the same perspective as an 85mm on my Elan if I keep my subject the same relative size because I should be about the same distance from my subject with each. Do I have this right? Also, one of the reasons 85-105mm lenses are so popular for portraits is because the compressed perspective resulting from the longer lenses tends to flatten features (like noses), while wider lenses tend to accentuate them. Will my 50mm lens on the 20D give be the same type of compressed perspective as a 85mm on my Elan. Based upon my foregoing assumptions I would think the answer is yes, but somehow I keep thinking that a 50 is a 50 is a 50. I have searched various forums but cannot seem to find a clear answer. Thanks for the help.

It's my understanding that the angle coverage of a lens is what will be affected by the crop factor. For example, a 17mm lens on a full frame camera will give you a true 17mm angle of view, but on a 1.6 aps-c sensor, it will give you the equivalent of what a 28mm lens on a FULL FRAME sensor will see. That means that you are losing part of what that lens is capable of seeing when/if you use it on a 1.6 crop factor camera. Perspective specific to a certain lens will not change. That is a lens characteristic, and not a sensor characteristic.

Yes. Every characteristic of the EF lens is preserved, but the camera records only the middle of the image, according to the "cropping factor" (1.6). Depth of field etc are unaffected.

Here is an accurate and definitive answer:

A 50mm lens on a Canon 20D will give you the same image as you would get from a full frame camera using an 80mm lens. i.e. same angle of view captured. The only difference is in the depth of field (the same as if you had used a slightly different aperture setting).

You are absolutely correct when you say that perspective is controlled by subject to camera distance and that with a lens that covers a smaller angle of view you will stand further back, altering perspective.

It is not that hard to understand really, the other thought you had that a 50 is a 50 is a 50 is wrong. A 50mm lens on a 35mm camera is different to a 50mm lens on a large format camera is different to a 50mm lens on a compact digital camera with a tiny sensor. Did my Nikon CP950 with its 7-21mm lens give me the same perspective as a 7-21 lens would on a 35mm film camera? (hint: NO!) On a large format camera a 50mm lens would be a wide angle lens suitable for landscapes, not a 'normal' angle of view lens like a 50mm on 35mm cameras.

The only reason we say the 50mm on a 1.6x crop D-SLR is the equivalent to an 80mm on full frame is that many photographers understand what shooting with an 80mm lens is like and it make sense to them. If all lenses were always refered to by their angle of coverage then we would understand what a 30° lens is and what a 46° lens is.

On your Elan an 85mm lens is a 28° 30' lens, on the 20D a 50mm is a 30° lens, hence a very similar image captured.

To relate how a lens works on a 20D to what you are used to on the Elan you should multiply the focal length by 1.6, this will give you a number that you can relate to, without having to relearn lenses based on angle of view.

To add to Mark's excellent description, here is an article that discusses how depth of field is affected. To quote the most relevant point from the article:

I think JBrown is asking about perspective not DOF.

Thanks Mark. I thought my original analysis was correct, but I kept on coming across posts such as the one below which lead me to believe that things were more complicated and I was missing something. Sometimes the simple answer really is the right answer.

“I personally would not use a 50mm lens for portrait work. I would use at least a lens in the 70mm range for the compression factor that you get in the perspective of the subject. A 50mm lens, especially when used for a head shot, still tends to exaggerate facial features...such as the nose.�

BTW, to be clear, that quote from my last post, while correct for a full frame 35mm, was from a 10D/20D forum hence my confusion. But, as it’s late and the mind tends to wander, I have a question regarding the new ultra wide angles lens, such as the EF-S 10-22mm. With a 17mm equivalent at the wide end, I wonder whether the distortion at 10mm will be the same or greater than the distortion of say the 17-40mm at 17mm? I’m not talking about perspective now, but the inherent optical characteristics of extremely short focal length lens.