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.
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.