"F. Craig Callahan" wrote:
>
> Thomas Bantel wrote:
>
> > While, with an extension tube, the nominal max aperture communicated
> > to the lens will not change, the effective max aperture will. So this
> > means, an extension tube will not prevent the camera from *trying*
> > to AF, but it will lower the efficiency of AF just like using
> > a slower lens with a third party TC.
>
> This has in fact been my experience in the field. Even with a fast lens, use of an
> extension tube often leads to some measure of hunting (depending on light and
> subject), although the AF still works.
>
I guess there is a third factor involved. With an extension tube attached, the lens'
inherent focusing properties change - but the camera doesn't know. Again similarly to
using a third party TC, this should cause more hunting. With the tube, any movement
of the focusing ring causes much more defocus than without.
Thomas Bantel
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