If we will use crop lenses only on crop bodies, why do we call crop lenses for example "10mm" when they are effectively "16mm" (with a crop factor of 1.6 in this case)?
I am familiar with the history and the difference between a full-frame and a crop body. My question was about why we call these lenses different in focal lengths since they both yield the same field of view in degrees (the crop lens on a crop body, and the FF lens on a FF body). I know the lens doesn't actually turn into a longer focal length optically on a crop body, but since the result is the same, why not just label it that way?
Answer
Because a 10 mm lens is a 10 mm lens.
Crop factor has nothing to do with the real mm of a lens.
Crop factor is the same as if you take your Photoshop and crop the center of a photo.
Take a look at this answer: Do I use the crop factor in calculating aperture size and area?
The crop factor equivalent is to give you "an idea" if you have being using a 35 mm film camera on how your framing will look compared to that size.
A "crop" lens is diferent vs a full frame lens, becouse if you use it on a full frame camera you will have a vignetting. "Simply" because it is cheaper to project a smaller image. Smaller glass, less weight, etc.
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