I've been engaged in a similar discussion under a different subject line. I want to disentangle myself from the question, asked under a different OP's post.
I'm asking this question with specific parameters:
It is a given that the image taken using the 35mm EF lens with APS-C sensor will have an EFFECTIVE focal length of 56mm when compared to the same lens on a full-frame camera with all other variables remaining constant.
Yes. A 34mm lens is a 35mm lens is a 35mm lens. No need to restate it. However, two different lenses, with different mounts, casting different sized image circles, at two different distances from the focal plane, may produce non-identical images. The unknown being solved for here is whether the image taken using the 35mm EF-S lens with APS-C sensor will have the same EFFECTIVE focal length of 56mm or have the stated 35mm focal length.
Why do I ask this?
Owners of APS-C cameras must make a mental calculation of effective focal length when purchasing EF lenses, because of the cropping and enlargement produced by this combination. I have never yet seen any discussion of whether such a calculation is necessary when purchasing an EF-S lens. If the results are the same between EF and EF-S, then what's the bloody point of making EF-S mounts and lenses in the first place. Just accept the cropped image and move on would seem logical.
Solved! Go to Solution.
I answered this on the thread but here it is again.
This topic which isn't difficult to understand. It has been made difficult by some folks in the beginning seeing a difference in their photos from what they saw with 35mm film. This whole crop factor myth is a joke and totally necessary. Medium format and large format guys don't use it although they have it.
Let's get out of the "weeds". A lens FL is what it is when it is manufactured. It never changes nor can it change. If the lens is a 70mm ef-s or a 70mm ef or a Hasselblad 70mm lens, it is a 70mm lens no matter what camera it bolts on to. The printing on the lens body is what the lens physically is. The AOV (angle of view) will be different but not the FL. Second, and probably the most difficult concept to get over is nothing is cropped. Nothing is thrown away. If you believe that, something must added when you go up in sensor size, if something is throw away as you go down. I am sure the larger sensor doesn't create it out of the air.
Another confusing part or at least what most people don't know is just what is a lens? They think a lens is that black tube with rings and glass and markings on it. Actually a lens is a piece of glass with curved sides for either concentrating or dispersing light rays. WHat is FL, for camera lenses the distance between the center of a lens and its focus point. It is not a measurement of the actual length of a lens. FL tells us the AOV (angle of view). The longer the FL, the narrower the AOV will be, higher magnification. And of course, the shorter the FL, the wider the AOV will be. This is where the "equivalent FL" or "crop factor" got its beginning. People were talked into using a crop factor, 1.6x, in favor of just learning the AOV a lot less confusing.
Hopefully this has clearned it up a bit.
"one lens is an EF, the other is an EF-S"
One thing that might be tricking you up is the two lenses may not have the same physical size. The "S" in ef-s stands for Short back focus. This tell us right off the two 35mm most certainly not look the same. That has nothing to do with their physical FL.
"Owners of APS-C cameras must make a mental calculation of effective focal length when purchasing EF lenses, because of the cropping and enlargement produced by this combination. I have never yet seen any discussion of whether such a calculation is necessary when purchasing an EF-S lens."
Oh, but they do, or they should. Because there is no difference beside the letters ef and ef-s.
BTW, the short back focus is why you should never attempt to use an ef-s lens on a FF body. The lens rear element sticks back far enough that it may contact the mirror in a FF body. In fact Canon makes the mount a little different so you can't even do it.
Easily at least!