> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Jim Davis
> Sent: Sunday, February 25, 2001 7:28 AM
> To: [EMAIL PROTECTED]
> Subject: Re: EOS EF 20-35 with extension tubes
>
> Wide angles have a small depth of focus? HUH?
> Wide angle lenses have inherently large DOF last I checked.
> If you held
> the lens out by hand I guess it was wide open, maybe that gave
> you a pretty
> small DOF...
> I think I need a really small extension tube. Hopefully 12mm
> will do the
> job.
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 Skinner
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