> Depth of focus is not the same thing as depth of field. Depth of
focus is
> basically a measure of how critical the back-focus distance of the
lens is;
> that is, how much the back-focus distance can be "off spec" while
still
> providing the ability to focus to infinity. Telephoto lenses have
large
> depth of focus; wide-angles have very shallow depth of focus.
>
> I doubt that even a 12mm extension tube will allow much of a usable
focus
> range with a 21mm lens.
>
> Chuck
I felt you misused the term "Depth of Focus" in your original post, or
at least placed it out of context.
I almost replied then but couldn't be bothered ... but since you bring
it up again.
1. DOF is indeed not the same as DOF ;o)
2. Wide aperture on ANY lens reduces DOField but decreases DOFocus.
3. Just as with DOField, DOFocus is a function of magnigication (is
it not?) rather than focal length per se.
(Please correct if you think I'm wrong, but it could go way off topic.
If nothing else people might actually look the term up for themselves
;o)
Infinity focus has little relevance to extension tube users in the
main. A 12mm tube on a 20mm might be a way to identify those little
bugs that get inside lenses from time to time.
DOFocus as a concept is relavent to all focussed distances not just
infinity.
Right OFF Topic ... I spoke to Bob Turner at the "Focus On Imaging"
show at Brum. today, by phone that is. Unfortunately he was in a
seminar most of the day so we didn't actually get to meet ... ho hum.
Bob
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