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05-22-2013, 09:49 AM   #16
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QuoteOriginally posted by rawr Quote
But how? Is it standard in modern AF lenses, in a chip somewhere? Focal length, aperture etc I can understand being transferred to the body by the lens, but the MTF performance profile?
Exactly. The lens communicates a lot of information about itself.

Just think about newer Sigma lenses which can be updated via USB. Or some flashes which must be updated ("re-chipped") periodically. There's a lot going on inside these components.

05-22-2013, 10:10 AM   #17
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In lens disassembly tutorials you'll typically see a flexible PCB mounted on the inside of the barrel somewhere, with one or more chips on it. Basically you need something to decode a pattern that allows determining the focal length (think rotary encoder), and a couple hundred bytes or so worth of EEPROM. Nothing too fancy even by the standards of 20 years ago, it just has to work reliably.

Incidentally, I had the same issue of different AF performance on different camera lately. A Pentax 16-45 showed noticeable backfocus on my K-x, but was spot-on on a K-5. Go figure. All I can think of is that the AF sensor sees through a different part of the lens or somesuch, and a non-ideal lens alignment then causes these discrepancies. This sort of problem seems to be pretty much limited to complex zooms with multiple moving lens groups, probably because a lot can go wrong mechanically in those, in terms of tolerances and whatnot.
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