Richard Lovison wrote:
>
> I re-read my original post (I've been going through some of the old OM
> list posts I've saved) and realized that I asked a rather dumb question
> or at least I did a poor job of explaining my observation. What I
> originally observed was that if I focused on an object (tree line in
> this example) that was approximately 3/4 of a mile away, turning the
> focus ring on the 14-54mm all the way to the left at 54mm produced an
> out of focus image... I had to turn the manual focus ring to the right a
> bit to compensate. I thought this unusual as I figured a distance of
> 3/4 of a mile would qualify as "infinity" on the lens distance scale.
> Oh well, I need to be a bit more precise with my explanations.
>
A true zoom holds focus from wide to long, but usually gives finest
focus is at the long end 'cause either eye or focus sensor can see to
focus more precisely with the enlarged image. The change you see may
simply be from less than perfect focus at the short end. It is not
uncommon for people concerned with MF focus to focus at the long end,
then zoom back to frame the subject.
Many complex zooms focus past infinity at normal temperatures so that
they can focus all the way there at extreme(s). Mirror lenses too.
It is easier to design and cheaper to make a varifocal lens, one that
changes focal length, but doesn't maintain perfect focus as it does so.
A few of these were made in the MF era. It is possible that makers of AF
zooms cheat a little, as the AF doesn't care. I'm not accusing Oly of
this, only speculating about all makers. The temptation to make this
compromise to get smaller, lighter, cheaper lenses with equal optical
performance must be considerable in a competitive market where almost
no-one manually focuses AF zooms.
Moose, who did some MF with his AF 90/2.8 macro today.
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