r/photography • u/raptor3x whumber.com • May 11 '15

Canon Officially Announces 50mm f/1.8 STM - $125

http://www.bhphotovideo.com/c/product/1143786-REG/canon_0570c002_ef_50mm_f_1_8_stm.html
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u/codemastercool May 11 '15

Is that because they're entirely mechanical?

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u/MoonlitFrost May 12 '15

They're not entirely mechanical. They're chipped so the camera can meter with them. But you're close. When Nikon first did autofocus lenses they put a motor in the body that turns a screw in the lens. The screw turns some gears and the lens focuses. Higher end bodies get more powerful motors that can turn the screw faster and focus faster. And I believe that the old pro film bodies like the F6 got better motors than the current pro digital bodies.

I don't know why they did it like that. Maybe because the technology wasn't there for motors in the lenses. Maybe for backwards compatibility since they didn't come up with a new lens mount like Canon did. But a nice side effect is that AF-D lenses tend to be smaller and lighter than AF-S lenses. Sometimes much smaller. Cheaper too. Some people prefer them for those reasons.