Better 400D autofocus question???

I keep reading about the improved autofocus accuracy on the 400D for lenses f/2.8 and faster. Does this mean that the XTi will only focus more accurately than the XT when a fast lens is set to 2.8 or below, or will the XTi focus the lens more accurately through the entire aperture range?

So, in other words will my XT will focus as accurately as an XTi except for the few times I am using a very fast lens and have the aperture wide open?

Furthermore, will a 2x tele-converter or extender not allow for autofocus on my 70-300 f/4-5.6 IS USM?

Bryklo said:

I keep reading about the improved autofocus accuracy on the 400D
for lenses f/2.8 and faster. Does this mean that the XTi will only
focus more accurately than the XT when a fast lens is set to 2.8 or
below, or will the XTi focus the lens more accurately through the
entire aperture range?

Click to expand...

The improved accuracy of the XTi is twofold.
1. It is better along the whole aperture range , for all AF points.
2. The center AF point is MUCH more accurate when using a f/2.8 or wider lens.

Bryklo said:

So, in other words will my XT will focus as accurately as an XTi
except for the few times I am using a very fast lens and have the
aperture wide open?

Click to expand...

No. It will be better with slow lenses as well. But the difference will be much more noticable with fast lenses.

Bryklo said:

Furthermore, will a 2x tele-converter or extender not allow for
autofocus on my 70-300 f/4-5.6 IS USM?

Click to expand...

Indeed, you won't be able to focus. Typically, the sensors don't allow for aperture slower than f/5.6. However, in practice you might be able trick it, and squeeze a little more out of it up to f/6 - f/8..

However, 2xTC on f/5.6 gives f/11. No way that the camera will focus on that.

Doesn't matter what stop you have the lens set on, the AF is done with the lens wide open and the iris is only closed when the picture is taken.

Bryklo said:

I keep reading about the improved autofocus accuracy on the 400D
for lenses f/2.8 and faster. Does this mean that the XTi will only
focus more accurately than the XT when a fast lens is set to 2.8 or
below, or will the XTi focus the lens more accurately through the
entire aperture range?

Click to expand...

The last month or so I've been doing a lot of reading about AF on Digital cameras. I wish I would have bookmarked all the threads and websites but I only bookmarked a few :-[

This is what I gather.... When the XTi body recognizes a f2.8 lens is attached, the high sensitivity portion of the center AF point is activated. It suggests other apertures will also be improved.

You can download the Canon White papers on the XTi here and see what you think..... http://www.robgalbraith.com/public_files/Canon_Rebel_XTi_White_Paper.pdf#search=%22canon%20xti%20white%20papers%22

The Canon XTi manual is available for download on their website. You'll have to click a few links to get there. Go here to select the XTi...... http://www.usa.canon.com/consumer/controller?act=DownloadDetailAct&fcategoryid=314&modelid=14256

Bryklo said:

So, in other words will my XT will focus as accurately as an XTi
except for the few times I am using a very fast lens and have the
aperture wide open?

Click to expand...

I don't think so. It seems the 20D, 30D and 400D (XTi) have a different 9 point AF system than the 300 and 350D's 7 point AF system. The white papers and manual explain it further.

I wish this information was more easily found. Either from Canon or review sites. I wouldn't have wasted so much time with the 350D. There was nothing much about AF points in the 350D manual.

But as you start reading the 30D and 5D manuals theres more information, which actually helped me understand the 300 and 350D's AF system.

Here are a few other links I found here in on these forums that might shed a bit of light on this AF thing.

This link is mostly about the 1 series, but there is good discussion about AF and lenses. http://www.juzaphoto.com/eng/articles/the_autofocus.htm

The 350D and 300 have a 7 point AF system. Here is a link to some images that show how the other Canon AF systems look. http://www.pbase.com/nelsonc/af_points .

I'm getting a good handle on this AF thing but I have much more to learn. What I don't understand is if there is such trouble with low light AF with the 300 and 350D AF system, why wasn't a larger view finder provided so AF could be verified and used for manual focus if there was a problem. If you start looking, there are many users having OOF issues which are not user error.

As is it was, I had no idea my images were OOF until I got home. Now I know I should have previewed for OOF. But I do a lot of low light stuff and there is no way I'm going to review every shot in the field wondering if it's OOF or not. I don't have that much time. If I shot low light stuff on a rare occasion, I might be able to live with it. I tried three different 350D's and returned them all. The third was actually much better, (but that's a different story.) That's when I started to research this AF stuff.

Good luck. Hope some of this helps you.

Cheers,
Jeff