I was recently given a Sigma AF-k lens and told it was Nikon compatible. I am trying to get a better handle of what the AF-k stands for. I have both a Nikon F4s and a D5100 that I use and not certain of what after market lenses are compatible with these.

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    See: photo.stackexchange.com/questions/496/… if that doesn't define it already someone who knows the answer should update it.
    – dpollitt
    Mar 17 '13 at 20:38
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    AF would no doubt refer to autofocus. I can't find any reference on Sigma site (sigmaphoto.com) referring to the "K" designation. There are eBay references to AF-K and Zoom-K, but couldn't find any definitiive explanation. Doesn't appear to refer to Pentax K-mount though.
    – MikeW
    Mar 17 '13 at 21:11
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    Here is a picture of a Sigma lens with the "K", although the font of the K makes me wonder if it is even a "K" or not: pentaxforums.com/userreviews/…
    – dpollitt
    Mar 17 '13 at 21:14
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    The "K" designation predates autofocus in Sigma's lineup. I can't find a reference, but I seem to recall that it was their designation for parfocal zooms (lenses that retain focus as you zoom, as opposed to their older varifocal zooms). It has nothing to do with the Pentax K mount, nor does it indicate the focus drive type (AF-K lenses were also available in the Canon EF mount). If it's an AF lens in the Nikon mount, it'll work on the F4s. Whether or not it works (with AF) on the D5100 depends on whether it has its own motor or relies on a body drive.
    – user2719
    Mar 17 '13 at 21:18

Thanks to dpolitt and Stan Rogers I think I've found the lens. The "K" seems to be a Kappa.

The name Sigma being a Greek letter, it seems Sigma have used many greek letters in their lenses, alpha, beta, gamma and so on. I can't see any rhyme or reason to the use of these symbols however. You'll see many examples if you scroll through Sigma History

On the same page are many 70-210 Zoom Kappa lenses, including the one dpolitt linked to ( 70-210 Kappa ), although that is a manual focus lens. There are other 70-210 zooms, but none that I can find that are listed as AF lenses with the kappa designation. The last 70-210 kappa zoom listed is from 1987, which would mean it could have been an AF lens.

Because the D5100 has no AF motor built-in, the lens would have to be focused manually anyway.

  • So the kappa "k" stands for nothing?
    – dpollitt
    Mar 18 '13 at 14:05
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    No doesn't seem to. The 70-210's had the Kappa designation, 21-35mm Gamma, 35-105 and 35-135 Alpha etc. My guess would they just picked a letter for each zoom range
    – MikeW
    Mar 18 '13 at 17:54

Today I received a Sigma Zoom-K III 75-210 and I tested it with my Nikon D5100.

Automatic modes don't work but it works in manual mode without a problem.

First test shots just great, details super - well you must know how to handle not only the manual mode but also the lens setting! I am curious how the macro mode will behave.

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