@msmith1 wrote:
If I were going to use an EF lens on a camera that takes EF or EF-S lenses (Rebel T7i) you multiply the focal length by 1.6 to get the focal length the sensor sees. "Does the EF-S lens that is made for that camera follow the same rule? Mostly just curious."
From what I understahd, and you will find conflicting informstion on this. Some say the 1.6 factor should be aplied to both EF and EF-S lenses when used on a cropper. Canon has two deffinitions of the "S" in EF-S. Depending on what Canon rep you talk to, it refers to "Small focus circle" or "Short back focus". Either way the image circle is smaller to get more image on, as wadizzle pointed out, the cropper sensor. The image from the EF-S will project a small image circle on the APS-C sensor. I ran across a crop factor for the EF-S lens, but dont recall where. For example, an EF-S 18mm on a crop camera (1.6) will equall a 22mm FOV that you would get on a FF (35mm) sensor. So even though the EF-S was designed for the croppers, it still has a crop factor and different FOV as compared to a FF camera. It's not the 1.6 calc, but there is a factor non the less, I just don't recal (something like 1.3).
To do this, Canon designed the EF-S so that the rear element protrudes farther into the camera body (short back focus), even though the flange is still 42mm, which is why they will not fit a FF body as the mirror will hit the back element of the EF-S. They also designed them to have fewer elements which makes them lighter and they can produce them at a cheaper cost (they are generally plastic). Also, emphasis in design is more to the center of the image circle so with an EF-S lense, you will see some corner darkening. With 3rd party "S" lenses, it is really noticable.
Personally, I don't even consider any of this. We have both types of cameras, and we just don't think about it. We know what the lenses will do and that is what is important. I think too much emphasis is put on this "crop factor" thing. We have an EF 70-300mm f/4-5.6 IS USMnano II that has an LCD readout that shows FF equvilant FOV when on a cropper. My wife uses it on an R6 but bought it for her T7i. She is disabled and need a light rig, both the T7i and R6 with the aforememtioned lens are perfect for her because they are light.
Newton
