f/2.8 autofocus, 350D and 400D

I've owned the 350D for 1.5 years now and the 400D for 2 weeks. I don't have hard evidence other than the photo's I've taken, but I believe I am indeed seeing more reliable and accurate autofocus on the 400D with my 17-55IS at f/2.8 in low light indoor, shallow DOF shots (1 - 3 inches total DOF). I've read this was one of the advantages of the 400D over the 350D in that it uses the "same" autofocus system as the 30D. I believe I'm experiencing this advantage in my 17-55 low light f/2.8 autofocus shots on the 400D.

Good question. I do not have an XTi, but do have an XT and a 30D. I "believe" the 30D (and I guess the XTi) focuses a little better. I have done no exhaustive tests, but I have two 2.8 lenses and the Sig 1.4 and I "feel" the XT does not focus as well as often. I look forward to other more scientific people's responses here.

Gordon

No need for scientific responses... the 350D does not have any ablity to do what Canon calls 'high precsion' focus... that is none of it's focus points are capable of this... The 400D like the 20D, 30D, and 5D all have a center point focus that Canon callse 'hybrid standard and high precsion'...(sometimes also called dual precsion) that is when the camera detects a f2.8 or faster lens, it uses an addtional sensor that gives three times better precsion then the 'standard' focusing system does...

Note this hybrid standard/high precsion focus point is ONLY on the center focus point.

This type of focus system first showed up in the Canon 20D and is described on page 10 of the following document...

http://www.robgalbraith.com/public_files/Canon_EOS_20D_White_Paper.pdf

The 300D and 350D apparently have a differnt system that does NOT have this Hybrid center focus point.

Gordon Leitch said:

Good question. I do not have an XTi, but do have an XT and a 30D. I
"believe" the 30D (and I guess the XTi) focuses a little better. I
have done no exhaustive tests, but I have two 2.8 lenses and the
Sig 1.4 and I "feel" the XT does not focus as well as often. I look
forward to other more scientific people's responses here.

Gordon

Click to expand...

Yes the faster lenses work better with the 400D and the 20D as well as the 30D. Canon has a white paper on this. I had unrelaiable focusing with my f2.8 lens on my 10D, but on the 400D the focus is great and spot on every time.
Check this out as it applies to the new Rebel as well.

http://forums.dpreview.com/forums/read.asp?forum=1031&message=20137838&q=10d&qf=m

My f/2.8 AF precision with center point focus is better on the 30D than on the XT. I credit part of this to the extra precision with fast lenses, and part to the AF points being smaller (Lee Jay brought this up and it makes sense).

I was playing with my 400d in a room that was only lit by the tele and it was focusing with the 18-55mm in some case witohut assist in no time at all. I was very impressed with that. The Tamrom 70-300mm however couldn't do a thing at close distance but you can go manual and it was exceedingly dark.

I personnaly still have my 350D and don't plan to upgrade for 400D. (don't want to spend too much for photo since I don't take that much photos in fact)

But for sure I must say that canon made a good choice when adding the 20D autofocus system on the 400D. This kind of feature is very important but more difficult to market on an entry level DSLR than 10 MP I guess.

The 350D was a great enough camera to justify using expensive lens but autofocus was not always reliable at F/2.8 and below...

In fact this AF improvement is the most interesting update on the 400D in my opinion. Well I would probably enjoy the 10MP , the dust protection and the large LCD too ;-) but having a reliable AF at large aperture seems much more important to me!

mmiikkee