Putting a Neutral Density filter on an f2.8 lens reduces the light
reaching the auto focus sensor, it does not necessarily degrade auto
focus performance to the level of an f5.6 lens.
Auto focus is affected by a variety of factors including depth of
field, light reaching the sensor, sharpness of the lens, contrast of
the subject, etc.
My experience has been that Depth of field of the lens is quite
important. With a small depth of field, slight changes to focus make
a big difference. With a deep depth of field, small changes in focus
make a much less noticeable difference.
Many people think that a fast lens is better for auto focus because
it allows more light to reach the auto focus sensor. I think the
real reason it's better for auto focus is that it has a shallow depth
of field.
Adding a Neutral Density filter or polarizer to a fast lens will not
affect the maximum f stop. An f2.8 lens will still be an f2.8 lens
as far as depth of field is concerned. As long as there is an
abundance of light, adding the filter should not have a significant
effect on auto focus.
Adding a teleconverter is a different matter. Putting a 2x
teleconvertor on an f2.8 lens presents an f5.6 lens system to the
camera. This f5.6 lens system has a deeper depth of field then an
f2.8 lens, and hence is harder on the auto focus system.
-Michael Fryd
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