Bob Talbot wrote:
>
> > Therefore, while technically the aperture does not
> > change, for metering and AF purposes the "effective aperture" does
> > change, since less light reaches the sensors. Since with AF what
> > matters is the amount of light that reaches the sensors (no?),
>
> Mmmm...
>
> I'm not sure: I expect it is a function of the amount of light for
> sure. But I'm not sure if the effective "width" of the beam is also a
> factor. I.E. is AF efficiency at f2.8 with an ND 0.6 (2 stop) filter
> the same as no filter on an f 5.6 lens?
>
> need someone really clever to answer that ;o)
Not really ;-) AF efficiency is a function of both factors. Every AF Test
I read until now showed, that the time AF needed to focus on the same pattern
is different at different light levels (generally faster at higher levels of
course). Using an ND filter is equivalent to shooting at a lower light level.
OTOH, the effective aperture, IOW the "width" of the cone of light passing
through the lens is also important. 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.
Thomas Bantel
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