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Old Dec 6, 2006, 2:13 PM   #1
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Sorry to bother with such trivia, but I'm putting together a program for a local photo club and can't find this anywhere on the Nikon site. Apparently I can't find the right search terms to get it here either. So please don't flame me if you've just talked about this a month ago.

What is a smallest lens maximum aperture that allows autofocus on Nikon DSLRs?

On Canon, for example, it's easy. All the consumer grade cameras (10D, 20D, 30D, Rebels, and the 5D) allow autofocus to operate on lenses of F5.6 or larger maximum aperture. On the pro-series cameras--1D, 1Ds--the AF extends to F8, but only with the center AF point.

Thanks.
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Old Dec 6, 2006, 4:23 PM   #2
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With the D50 and D70 it is implied that they are both f5.6 max. It is under the "specifications" section of the manuals under "compatible lenses". For non-CPU lenses it says the "electronic range finder can be used if maximum aperture is f/5.6 or faster". While this doesn't really say that CPU lenses won't autofocus if the lens is lower than f5.6, it would seem like a reasonable assumption (unless someone knows the real answer of course:-))

Keith.
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Old Dec 6, 2006, 8:38 PM   #3
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Thanks. That's what I needed. I'm discussing extenders, and their effect on autofocus. With Canon, you put a Canon 1.4x on a 400mm F5.6, and it becomes a 560mm F8 manual focus lens. Do the same with a Tamron standard 1.4x (not the Pro version, it works like the Canon), and while the effect is the same, the camera doesn't know it, as the Tamron lacks three extra electical connections to talk to the camera. In such a case, given sufficient light, the lens will still autofocus, because the camera "thinks" it's still a 5.6. Of course your EXIF data is incorrect--still showing 400mm F5.6--but you can always take notes.

There is a trick where you can cover the extra three pins on the Canon, Tamron Pro, or Sigma extenders, so that they don't communicate, and supposedly that works. However, I found with the Canon, that while the lens tries to focus, it mostly just hunts. Why that is different from the Tamron without the extra pins is a mystery.

Anyway, thanks for your help.

Bill
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