I bought the Sigma 70-300 APO AFD around 1997. It gave good solid service on my N90s. I bought the F100 when it came out. Before I'd had a chance to put the two together, I happened to read postings in several venues from 'Bora' about problems he had with this lens on his F5. In short, I confirmed the error with my Sigma lens and F100. It goes like this:

At focal lengths beyond 200mm, the Sigma focuses 'short', while the camera thinks the focus is OK. For my test case, I Auto-Focused on a radio tower about a half mile away (or 'infinity') at the 300mm focal length setting. While the F100 says 'focus OK', the viewfinder is visibly unsharp. Looking at the lens barrel, the indicated focus distance reads 50 feet. This short focus behavior occurs at all focus distances. It occurs at all focal lengths between 200mm and 300mm for that lens / camera combo. At focal lengths below 200mm, it was fine.

'Bora' said he had contacted Sigma in France, and they acknowledged the problem. They said they fixed it in their latest design. They recommended he sell his old Sigma and buy a new one. I sold my old Sigma and bought the Nikon. All of the above is fact (the part that happened to me) or attributed to the guy who informed me about it. Following is my theory of 'why': Sigma doesn't pay Nikon to license the mount (I've heard that Tamron alone does this). Sigma 'reverse engineers' the electronic interface of the lens mount based on observing how the signals behave on sample bodies / lenses. At the time Sigma designed the lens, the Dynamic AF systems of the F5 / F100 didn't exist, but Nikon had already built the capability into lenses in anticipation of it coming soon. Sigma couldn't pick up on the feature, so when it came along their existing interface designs behaved like there were 'wires crossed'. Again, assertions in this paragraph are just my theory.

When I bought the Sigma, Nikon didn't have anything similar. Now they do, and at virtually the same price. Based on my experience here, when the price is even close, I would definitely opt for the Nikon lens. Would I ever buy another Sigma? Well, the 14mm is awfully tempting; Nikon doesn't even offer one, and their 15mm is over twice the price!

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