>I'm suffering from the same problem. I bought my F100 last January here >in Switzerland. When using a Sigma 70-300mm and focussing at 70mm, >the subject is not sharp when zooming to 300mm. If I focus at 300mm >(the subject is now sharp) and I zoom back to 70 and focus again, the >camera re-focuses and the subject become unsharp again. >This is exactly conform the article in the latest issue of Chasseur >d'Images. >By the way, the problem doesn't occur when using my Nikon 35-105mm. >It looks like a bug to me and I reported it to the Nikon Service. >Regards, >Toine The problem you're describing does not seem to be a problem with your F100. I think it is more likely to be with the lens design. This type of lenses are called Varifocal lens. It mean that it does not maintain focus through out its zoom range. As you've described, it means that, something in focus at the 300mm lens will not be in focus when you zoomed to the 70mm lens. You can test this out by focusing at something at the 300mm end, you should see that the object is in sharp focus. Now zoom out to the 70mm end WITHOUT CHANGING THE FOCUS. The object should be be out of focus, But now zoom back to the 300mm end, and the object should come back into focus. Nowadays, more lenses are Varifocal Lenses because the slight disadvantage have it have been more then compensated for by AutoFocus. This phenomena does now occus with your 35mm-105mm Nikon lens is because the design of this lens is NOT VARIFOCAL ! :-) that's all, it's not a bug in your F100. A few people responded to this mentioning about incompatability with 3rd party lenses as those lenses would hunt forever without finding focus, that may be true, this is not so for your case. Actually, a lens being varifocal does not mean it is inferior, the highly regarded Canon lens EF 28-105 (correct me if I'm wrong) is actually a varifocal. Canon has many lenses in this range which are varifocal. BUT, these lenses can maintain focus throught out the focal range with this thing Canon calls SIC (Super Internal Cam), I think I'm wrong with the name here, but anyway it sounds something like that. It is a system that re-adjust the focus when zooming is done to compensate for it begin Varifocal . Actually it is quite simple, I guess when you turn the zoom ring, there are also some gears that are connected to it to re-adjust the focus as well. Cheers RueyLoon 'I'm a poor photographer staying in a mountainous jungle'
