New answers tagged lens-design
What you've missed is the lack of space behind the lens mount for placing your proposed diverging lens. Most SLRs need almost the entire registration distance to allow for the mirror to move up and down.
Lifting the relevant section from another answer of mine: Basically, the larger the aperture is, the larger the angle of light rays on the outside of the lens has to change: Look at the image and imagine that D increases while f stays the same - it should be clear that the light rays then need to "bend" more. And making optics that refract light rays at ...
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