The focus unit used on the Rebels is not particularly accurate (the XT
unit is better, but not all that much), the 20D and up all have
significantly more accurate units, especially if you have a lens fast
enough to work in High-precision mode (f2.8 for the 20D, 30D and 5D, f4
for the EOS 1 series). The speed advantage is from how the AF unit
commands the motor, with in-lens units the AF unit tells the motor to
turn a certain amount then checks focus again. It usually gets it right
the first time as the motors are indexed so the AF unit knows how far
the lens has moved. Screwdriver AF drives use feedback, with the AF unit
watching the motor drive and stopping it when it's in focus which
usually results in a mild overshoot, which gives you the back and forth
hunting. Stronger motors brake quicker as well, so you get less hunting.
The Pentax AF units, for all their faults, are quite accurate. I never
had an issue with it not focusing properly or hunting in sufficient
light (Unlike the EOS 3 which replaced my *istD).
-Adam
Brian Dunn wrote:
Greetings all.
My wife and I photograph weddings. We use a bunch of Pentax and Minolta film
cameras, plus one original digi-Rebel, and we've just now added a *ist DS,
hoping to consolidate into one system.
First idea for Pentax: Allow us to select which four things are adjusted with
the function button. I'd like to replace the flash mode selection with
metering mode (spot / center / matrix) instead. Someone else might have
other ideas. Give us a choice.
Second idea is as follows:
While testing auto focus, I find that the Canon focuses and snaps a photo very
quickly, while the Pentax focuses just as quickly, but then it hesitates a
third of a second or so, then makes a few small adjustments to the focus, and
then finally releases the shutter.
At first I was very disappointed in the Pentax, but then I looked at the
images up close. The Canon focused quickly, but only 1/3 of the images were
in sharp focus. The Pentax was in sharper focus than the Canon's best image,
but it was also in focus every single time.
So on the Canon 300D, they give up a little final focusing accuracy in order
to gain a lot of speed. On the Pentax, they take their time and do it right.
Idea for Pentax: Let us choose between auto focus speed v.s. accuracy. Some
people will be happy with 98% focus if they can get 1/3 the auto focusing
time. Others will want sharp images even if it does take 3x as long to auto
focus.
Brian
---
Brian Dunn Photographic
http://www.bdphotographic.com
