focusing problems

Ist DS camera

My SIGMA 18-125 lens appears to be giving me a problem.
When I focus while the lens is in wide angle mode and then zoom in the image is out of focus. It's only in WA that I have a problem. If I zoom in and focus (auto) and then go to WA to shoot it's not a problem. Has anyone else had this problem? It's not the lens, the same happens with other lenses.

Tony
Australia

I'm not a lens design expert but I always understood that this is normal - most so called "zoom" lenses should properly be described as "vari-focus". Very few lenses are "true" zooms.

I stand to be corrected here.

I think you will find that this is normal behaviour for a lens construction where its overall length changes as you adjust the zoom. Older Pentax/Takumar zooms (mine is a Takumar-A 70-200mm Macro circa 1984) have a dual-function sliding/rotating barrel to adjust the zoom/focus and the overall length of the lens (front element to rear mount length) remained the same at any specific focus point. If focussed at the long focal length, the image remains in focus at all times when zoomed to a shorter focal length and back again. The difficulty is holding the zoom/focussing barrel at the correct focus whilst zooming as the one barrel performs both functions and it is all too easy to inadvertantly adjust the focus. That said, my old lens is mighty heavy when compared to a modern equivalent but the images are superb.

Hope this helps.

A zoom lens that retains the focus setting throughout the zoom range is termed para-focal

My Tokina AT-X 24-200 is parafocal. It works a dream on the Z1, but on the *ist-D it does not - apparently the camera focuses at the hyperfocal distance, which of course changes with focal length.

So, you may find the same with the *ist-DS. Even though the lens may be parafocal, it won't work that way because camera's focusing not quite where you think it is.

Hope that helps
Matt

I thought the zoom that retained its focus was a parfocal lens, and one that doesn't is a varifocal.

In practice, a two ring zoom is easier to keep in focus as you zoom - trombone types can be very difficult, especially as they wear and get sloppy.

My mistake - just looked on Google. It is "parfocal", not "parafocal". DOH!

But the point about the *ist-D (and probably the *ist-DS) is valid - my Tokina was parfocal on the Z1, but not on the *ist-D. It's not the lens, but where the camera focuses.

Matt

Thank you for your advice. I was beginning to lose confidence in the camera and lens. Not a good situation. I'm truly impressed by the depth of knowledge that some contributers bring to this forum.

Thanks, again
Tony

Australia