>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.
The problem is not exaclty DOF, but can help. A 28mm f/2.8 has more dof
than a 200mm f/5.6. It also depends on the body you are using.
The high accuracy sensors of the EOS 3, for instance, need the f/2.8
aperture (f/4 in the central one) son can be used with the 28mm f/2.8 lens,
but they cannot be used with the 200mm f/5.6 lens.
Af operation works, in the end, the same way that typical rangefinders.
Instead of using two windows separated by a baseline distance, you compare
the image seen from several zones of the lens. In a linear horizontal AF
sensor, the image, as seen from the left side of the lens is compared with
the image seen from the right side of the lens. In a cross type sensor, the
image from the left is compared with the image from the right, and the
image from above is compared with the image from below.
A sensor optimized for f/2.8 has a greater baseline, so the AF accuracy is
bigger that with one sensor optimized from f/5.6. The problem is that a
sensor optimized for f/2.8 NEEDS a f/2.8 aperture to operate. Of course it
also wors with a f/1.4 lens.
As you cannot always use f/2.8 lenses, High-end EOS cameras use two kinds
of sensors, some for big apertures and the rest for low apertures.
The central cross sensor of the EOS 1n, for instance, is composed on two
linear sensors, one optimized for f/2.8 (high-res) and the other optimized
for f/5.6 (low-res).
If you use a f/2.8 lens you can use the high-res sensor, but if you add a
teleconverter you only can use the low-res one.
The EOS 30 body only has f/5.6 sensors, the AF gain of using a f/2.8 lens
instead of a f/2.8 lens is more light a less DOF. But if you have a EOS 1n,
you add a higher accurancy sensor to the list of advantages.
>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.
Your statements are true, but for a high-end body you must take into
account the two kinds of sensors the body uses too.
>- -Michael Fryd
Best regards
Vicente
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