Originally posted by KxBlaze I noticed that on my Tamron 18-200mm lens that when I focus on an object at 18mm, it will be in focus then when I zoom in on that object the object is out of focus, even though the focus distance has remained the same. For example, when I focus in on a car that is 25 feet away at 18mm the focus ring will show 75 feet, when I zoom in on that same car to 200mm the focus ring will show the correct 25 feet. So I guess my question is, is it normal for the focus distance to be different at 18mm than at 200mm?
The lens is a varifocal lens, to some of us old-timers it's not really a zoom lens. Back in the days of the dinosaur, they were two distinct breeds. With manual focus, it was really a lot better to have a zoom (focus does not change while zooming) to a varifocal, which gives the effect you have noticed. A "true" zoom lens is much more difficult and complicated to build. It shows in the prices, and we are not talking small change difference.
Another poster mentions zooming to the longest focal length and focusing, then zoom back. This works more or less on varifocal lenses because of the increased depth of field at shorter focal lengths.
Most modern "zoom" lenses are varifocal lenses. With autofocus, it really does not make much difference, but with a manual focus lens it can be a royal pain.