Mark Cassino wrote in message <01bceef2$7ed64840$cde531cf@armadel>...

>I know that the effect of a teleconverter on aperture for
>exposure adjustment is equivalent to the lens aperture multiplied
>by the teleconverter's power -- so a lens set to F4 with a 2x TC
>will function as if set to F8 for exposure purposes.
>
>But what is the effect of the teleconverter on aperture in terms
>of sharpness and depth of field? Does the same rule apply?
>
>In thinking it through, it seems like the "circle of confusion"
>for a lens would be no smaller than the aperture setting, and
>that the teleconverter would not do anything to make it smaller.
>As such, it seems that using a TC results in the disadvantage
>of the smaller aperture in terms of light loss, without the
>corresponding advantage of increased sharpeness.
>
>But then maybe I'm missing something.
>
>So -- if I put a 200mm lens set to F4 on a 2x teleconverter, do I
>have a setup that functions like a 400mm, F8 for both metering
>and sharpness / depth of field, or do I have a setup that
>functions like a 400 F8 for metering and a 400 F4 (or less?) for
>sharpness / depth of field?


Do not confuse sharpness with DOF - they are quite different
(even opposite - a less-sharp lens may appear to have greater
DOF). A 200 at f4 on a 2X would have the same (theoretical)
DOF as a 400mm set at f8, but definitely not necessarily the
same sharpness! While most good 1.4X converters work fairly
well with most good lenses (stopped down at least a couple of
stops...), the resultant image quality is rarely as good as it
would be without the converter. With 2X converters, the
situation is even worse - few 2X converters and lenses match
well enough to produce high image quality unless used at
small stops (there are exceptions, like the Nikkor 300mm f2.8
and 400mm f3.5 lenses used with the TC300/1 converter -
sharp wide-open, contrasty by f5.6). The one area where 2X
converters shine (other than in examples like the above) is in
macro work, where you want to be able to use effectively very
small stops without suffering the image quality loss due to
small-stop caused diffraction effects. Here good 2X converters
do seem to give one a "free lunch" - image quality appears
better with the macro lens stopped down to no smaller than
f16 and used on a 2X converter for an effective f32, than when
using the macro lens for the same magnification alone, but
set at f32.
Hope This Helps
David Ruether - http://www.fcinet.com/ruether