I've posted the following in another forum, but received no feedback. Hopefully someone here can tell me if they've experienced this 'phenomenon' with the Nikon 17-35, or other wide angle lenses.
I recently tried my Sigma 18-35 zoom lens on the D100. In auto focus mode, the lens will focus on the infinity mark at its 35mm position, but does not go all the way to the infinity mark at the 18mm position. Intermediate distances also register differently between the two zoom extremes. Went to Sigma USA, and had them check the lens, and they said all was fine. When it still didn't focus in sync at both ends of the zoom, I asked for a better 'answer'. I was then told that because of the extreme depth of field at the 18mm position albeit 27mm on the D100), the auto focus sensor 'stops' its focus point when it enters the lens' depth of field for that distance/focal length. In 'manual focus' mode, the rangefinder does not show a 'crisp' focus point, but rather a range from the point where the green indicator first comes on and when it goes 'out'.
When I checked my other zoom lens (of course, in the tele range) the focus hits the same mark consistenly (but then again these have much shallower dof).
My question is do owners of the Nikon wide angle zooms experience this same focusing 'phenomenon' or is this just a problem with Sigma wide angle lenses and the Sigma explanation simply 'smoke'??
Sorry for the long post, and thanks for any feedback on this question.
Hans: Thanks for your response. "This" is a strange phenomenon. But it seems like its only occurring on non-Nikon lenses??????????? I have NO problem with my Nikon lenses. Additionally, when at Sigma offices, they tested my lens on an F5 body, and it focused in exactly the same manner, IE: same subject distance registered at different reference marks on the distance scale at the two extreme ends of the zoom. YET, Sigma said that my lens was NOT defective on their test bench (whatever that means). So maybe the 'fix' is to stay with Nikon lenses?
Just to clarify this question, is the lens out of focus at infinity, or are you getting crisp focus and worrying about where the infinity mark is aligning when the image is in focus?
If it's the latter, be aware that not all lenses offer perfect focus as you go through the zoom range (acting as a varifocal lens rather than a true zoom). If your Sigma is a little varifocal toward the wide end, that would explain the phenomenon. If so, it's nothing to get worked up about unless you use manual focus a lot. It can get annoying to manually have to touch-up focus as you zoom, but if AF is handling it and your image is in focus, what's the stress?
Remember that "infinity" on a lens will change with temperature. Much less with a wide angle than with a long tele, but the point is that infinity isn't always in line with your index mark.
Hi BJ: The lens does not align at the infinity mark on the lens at the 18mm length, but does align at the 35mm length. Additionally as I said, focusing on a subject which is 10 feet away does not hit the same distance mark on the lens at 35mm as it does at the 18 mm point. The images 'seem sharp' but with a wide depth of field, its hard to tell. Maybe I'll just forget about the 'manual adjustment' and hope for the best. My main query was really whether this happens with Nikon lens too or is it just with these 'other' brands.
When you are viewing through the lens depth of field does not play a role. In normal viewing the lens is wide open and sharp focus is simply sharp focus. To see the effect of depth of field you use the depth of field preview button. That is the prime beauth of SLR - what you see is what you get.