Does changing the focal length change focus?

Question

When using manual focus or back focus on D60, I sometimes zoom in the lens, set the focus, and then zoom out. Does changing the focal length (zooming out), also change the focus?

In case the answer depends on the lens, I use Nikkor 55-200 and 18-55.

Answer 1

It's not supposed to, but usually does.

The basic difference between a "varifocal" lens and a "zoom" is that a zoom stays in focus as the focal length changes. That's typically done by moving different lens elements simultaneously. The problem is that it's (at least normally) done mechanically, so manufacturing tolerances and wear in the lens prevents it from working quite perfectly in most cases.

There have been a few lenses that were poorly designed (at least in this respect) so the changes in focus were consistent, but in most cases it's more about mechanical tolerances, so it's mostly specific to a particular lens, not a design in general.

Answer 2

The technique of zooming in to check focus and then zooming back out to the desired focal length is common in the video industry, where lenses do tend to maintain the same focus distance across the zoom range. In the still photography world, however, lenses may shift the focus distance as you zoom in and out.

This makes sense from the standpoint that in still photography you will generally have time to refocus between shots, but when recording video the subject must be kept in focus the entire time.

See http://notesonvideo.blogspot.com/2010/06/video-terms-parfocal-lens.html for further reading.

Answer 3

In most cases, yes - zooming does change focus. There are some exceptions, google up on "parfocal" lenses.