In article <4vo4qs$2ge@netnews.upenn.edu>, gasser@linc.cis.upenn.edu says...
[most deleted]
>Don't camera lenses maintain focus while you zoom? [....]

Some do, but many don't - which makes the advice (found in
many of their instruction booklets) to focus at the long end,
and zoom to what you want (for more accurate focus) absurd.
Hope This Helps