Archived posting to the Leica Users Group, 2001/03/20
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>With gels the amount of focus shift is negligible. In practice other
>errors will be greater and mask any actual focus shift.
That's good to know -- but what about glass filters? I have a 4x5 lens
with a rear filter thread, and I do take care to focus with the filter
in place.
>No, it's the same for any f-stop. It's purely a geometric function.
I think I see what you are saying (but it took me a long time to think
about it): the error in focus due to any film buckling (for example)
will be the same for any f-stop. i.e., if you are in focus from 3ft to
infinity with a wide angle lens at f8, or from 100ft to infinity with
a tele at f8, a buckle which harms the focus with the wide angle lens
will harm it for the tele just as well.
However, in practice I often stop down more with longer lenses or on
larger formats, so the "same for any f-stop" business doesn't tell
the whole story.
For example, the Alpa website has an interesting quote:
12. Why the need for extreme precision and robustness?
The laws of physics make wide angle lenses with their large depth
of field and their tiny depth of focus much more demanding in
their tolerances than lenses with a narrower field of view.
Byron.