05-28-2019 04:13 PM
Size of the image circle has to do with other things besides "classic" lens design.
For example, you can use an EF-S lens on a Canon R, but it restricts the image to the size of an APS-Sensor. You probably have "image" out there, but Canon cannot guarantee the quality. I imagine the field curvature goes out of whack, among other things.
05-28-2019 04:15 PM
And the answer is no, a 70 mm lens is a 70mm lens. It will provide the exact same magnification. It only makes a difference when you are talking about the size of the format.
05-28-2019 05:41 PM
"If this is true, then there is nothing being thrown away,..."
"It was about EF-S vs EF lenses with the same focal length dialed in and on the same APS-C camera, identically placed, seeing the exact same subject. Are the resulting images different?"
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.
05-28-2019 06:08 PM - edited 05-28-2019 06:09 PM
"... the part of the FF image circle that does not fit ..."
OK, if your Rebel throws away anything so does your FF. Lenses produce a circular image which is projected on a rectangular sensor. Some light is ignored because one is round ant the other ain't.