Is This True? 20D AF Question

FM Forum Rules
Canon SLRs, primes, and zooms lenses reviews

FM Forums \| Canon-mount SLRs \| Join Upload & Sell	Search Used
end

Archive 2007 · Is This True? 20D AF Question

Doug C.
Offline
Upload & Sell: Off

p.1 #1 · Is This True? 20D AF Question

I just heard today that with lenses that are 2.8 or faster, the AF on the 20D is more accurate because a cross sensor kicks in. Is this true, and if so how much more accurate is it?

Thanks,
Doug

Feb 22, 2007 at 05:29 AM

moondigger
Offline
Dedicated FM
Upload & Sell: Off

p.1 #2 · Is This True? 20D AF Question

True, only for the center focus point. It increases accuracy from "within depth of field" to "within 1/3 depth of field."

Feb 22, 2007 at 05:32 AM

mh2000
Offline
Dedicated FM
Upload & Sell: Off

p.1 #3 · Is This True? 20D AF Question

and it can still back or front focus within this range and be within spec.

Feb 22, 2007 at 06:28 AM

Alan321
Offline
Dedicated FM
Upload & Sell: On

p.1 #4 · Is This True? 20D AF Question

sort of true.

In the 1-series cameras with the 45 point AF system, the terms "cross-type sensor" and "high precision sensor" were synonymous. One and the same.

With the introduction of the 20D Canon clouded the issue by having "normal-precision cross-type sensors" and "high-precision cross-type sensors" as well as the "normal precision linear sensors". The central normal precision cross-type sensor has the ability to activate a usually-hidden high-precision cross-type twin sensor when the maximum aperture is f/2.8 or brighter. Three times more accurate than the normal precision equivalent.

At an aperture of f/5.6 the 20D can still use its normal-precision cross-type sensor whereas the 1-series have reverted to liner sensor operation at that aperture. Any cross type will be better than a linear type if the subject lines do not cross the linear sensor. So that means the 20D really has "normal precision linear sensors", a "possibly better than normal precision cross-type sensor", and a "high precision cross type sensor".

Now an observation: The high precision AF sensor may be faster, and definitely more precise, but the overall AF speed still depends mostly on the ability of the lens to move the focus group of lens elements. A slow macro lens will still be a slow macro lens. However, combine a fast-focusing large-aperture lens with the high-precision AF sensor and you'll get speedy AF and accurate AF.

Now another observation: The increase in AF accuracy is most noticeable at large shooting apertures where the DOF is small and is similar to the DOF at the focusing aperture. If you shoot at f/11 you will have a much greater DOF and that extra DOF will swamp any differences in the focusing accuracy .

So the warning is that you will not always notice the additional AF accuracy and you will not always notice the addition AF speed. Cruel isn't it.

Feb 22, 2007 at 02:59 PM

Doug Pardee
Offline
Upload & Sell: Off

p.1 #5 · Is This True? 20D AF Question

I've got to make a minor correction to what Alan said.

The center AF point on a 20D has a normal-precision cross-type sensor and a high-precision linear sensor. The high-precision linear sensor is only sensitive to vertical edges, and only with lenses of f/2.8 or faster.

So while the center AF sensor on the 20D is always a cross-type, it is only high-precision for vertical edges (and of course only with lenses of f/2.8 or faster).

Feb 22, 2007 at 07:29 PM

Alan321
Offline
Dedicated FM
Upload & Sell: On

p.1 #6 · Is This True? 20D AF Question

Well, that point had escaped my memory. Thanks Doug.

A quick check of the manual at page 68 indeed confirms that the vertical line detection is twice as sensitive as the horizontal line detection for the high precision cross-type sensor. That's interesting because other documents suggest that the sensor overall is three times more precise than a normal sensor.

Feb 23, 2007 at 01:07 PM

end
FM Forums \| Canon-mount SLRs \| Join Upload & Sell end