40D focusing question?

I have a XTi and am considering the XSi or the 40D as an upgrade. I'm leaning towards the 40D due to it's slightly superior high ISO noise performance and better focusing ability than the XSi. However, I don't have lenses faster than F/2.8 and don't plan on buying any.

My question is, will the 40D focus more accurately than my XTi or a XSi with lenses slower than F/2.8 or will it be the same?
Thanks!

Jim

The 40D has nine cross-type focus points, where previous models in the xxD and xxxD range have only one, the one in the center.

This implies that the chance that the 40D finds anything at all to focus on is higher.
The autofocus computation is also faster in the 40D than for example in the predecessor, the 30D.

apersson850 wrote in post #5889725
The 40D has nine cross-type focus points, where previous models in the xxD and xxxD range have only one, the one in the center.

This implies that the chance that the 40D finds anything at all to focus on is higher.
The autofocus computation is also faster in the 40D than for example in the predecessor, the 30D.

Thanks for the feedback. I'm not sure that the additional cross sensors will help me much since I rarely if ever use anything but the center focus point and combine it with center weighted metering. That kind of levels the playing field with my XTi since I'm shooting with slower lenses. Hence the reason for my question. I'm not sure I'm going to gain much if anything with the 40D's advanced focusing mechanism since the rest of my hardware doesn't take advantage of the high precision focusing it offers.

Any XTi/40D owners or upgraders that can give personal experience?

Thanks!

Jim

I just upgraded from my trusty XT to a 40D. The whole feel of the 40D is a world apart from a Rebel, IMO. I use f3.5-5.6 range lenses mostly. I find the AF on the 40D much easier and more successful but that could also be due to its better feel and larger viewfinder as well.

Well, that was actually exactly what I did. I have both the 400D and the 40D, and my experience says that the 40D has a better autofocus than the 400D.

That's true even if you use the center point only, due to the enhanched algorithms and computing power in the 40D.

But to take any advantage of the all cross-type sensors you need to use them, of course. Following subjects with irregular motion patterns, using Servo AF, is sometimes more efficient with all focus points enabled, depending upon the background.

To take advantage of the fact that the high-precison center point is a cross-type as well, you need at least an f/2.8 lens.

To take full advantage of the fast focusing, you also need a lens with good autofocusing performance, like for example the EF 70-200 mm f/4L IS USM or the EF-S 17-55 mm f/2.8 IS USM. The latter also enables high precision autofocus, due to being f/2.8.

As an occasional sports photographer, I notice the upgrade from the 400D.

There seems to be a misconception about the center AF and 2.8 or faster lenses. It's not an "advantage" as such unless you have faster lenses since they require more precise focussing... due to their shallower DOF.

In other words, this enhancement was done to address problems with focussing with 2.8 lenses or faster with previous bodies. They would be of no help with slower lenses which is why it is disabled with > 2.8 lenses.

bohdank wrote in post #5891113
There seems to be a misconception about the center AF and 2.8 or faster lenses. It's not an "advantage" as such unless you have faster lenses since they require more precise focussing... due to their shallower DOF.

In other words, this enhancement was done to address problems with focussing with 2.8 lenses or faster with previous bodies. They would be of no help with slower lenses which is why it is disabled with > 2.8 lenses.

This is very interesting. If I understand your post correctly, the only reason the high precision center is required is to aid F/2.8 and faster lenses focus due to the very shallow depth of field. The focusing of slower lenses would be just as accurate since they don't require the extra help. In other words, the high precision cross sensor is only there to help fast glass equal the focusing accuracy of slower lenses.

Did I understand that correctly?

Thank you!

Jim

Yes, that is correct.

The 2.8 and faster lenses require more precise focus since the DOF is shallower. The AF system tells the lens how far to move the glass and the lens acts but does not return any info back to the AF system. In other words, the AF sends a signal to the lens and that is the end of the interaction. It doesn't send any corrections. This is why problems like FF/BF exist.

On faster lenses, the AF tolerance has to be tighter, hence the precision AF focus point.

bohdank wrote in post #5897987
Yes, that is correct.

The 2.8 and faster lenses require more precise focus since the DOF is shallower. The AF system tells the lens how far to move the glass and the lens acts but does not return any info back to the AF system. In other words, the AF sends a signal to the lens and that is the end of the interaction. It doesn't send any corrections. This is why problems like FF/BF exist.

On faster lenses, the AF tolerance has to be tighter, hence the precision AF focus point.

Thank you for the excellent and very concise explanation. It makes a lot of sense now.

Best regards,

Jim

bohdank wrote in post #5891113
There seems to be a misconception about the center AF and 2.8 or faster lenses. It's not an "advantage" as such unless you have faster lenses since they require more precise focussing... due to their shallower DOF.

In other words, this enhancement was done to address problems with focussing with 2.8 lenses or faster with previous bodies. They would be of no help with slower lenses which is why it is disabled with > 2.8 lenses.

What's your source for this?

bohdank wrote in post #5891113
There seems to be a misconception about the center AF and 2.8 or faster lenses. It's not an "advantage" as such unless you have faster lenses since they require more precise focussing... due to their shallower DOF.

In other words, this enhancement was done to address problems with focussing with 2.8 lenses or faster with previous bodies. They would be of no help with slower lenses which is why it is disabled with > 2.8 lenses.

I thought that the reason it only worked on f/2.8 lenses or faster is because Canons focus with the aperture wide open and the added sensitivity simply needs more light for greater accuracy. You would still get the same AF accuracy on an f/2.8 lens stopped down to f/32 as the body would focus with the aperture wide open.
I may be wrong though as this is just what I heard.

HankScorpio wrote in post #5898968
I thought that the reason it only worked on f/2.8 lenses or faster is because Canons focus with the aperture wide open and the added sensitivity simply needs more light for greater accuracy. You would still get the same AF accuracy on an f/2.8 lens stopped down to f/32 as the body would focus with the aperture wide open.
I may be wrong though as this is just what I heard.

Yes, the added accuracy would be applied to whatever working aperture the fast lens is using. However, the advantage would be progressively less apparent as the aperture becomes smaller because it woild be concealed by the greater DOF.

It would be interesting to know where you got this information from.

Chuck Westfall, Canon USA, is usually well informed. According to him, all standard precision focusing points provide focusing with a precision that's equivalent to the depth of field for the maximum aperture of the lens used, regardless of which lens it is.
He continues to say that the high-precision points provide a focusing accuracy equivalent to 1/3 of the DOF of the lens when using the maximum aperture, regardless of what that aperture is (well, 2.8 or larger, of course).

So according to what he says, you are wrong.

Personally, I don't know, so I have no opinion.

gooble.... the post above this one confirms what I said although in a slightly indirect way.

This topic was also discussed in great depth on another board awhile ago with some very knowledgeable posters contributing.

Shot with the XTI for almost a year and just got a 40D last week. Really wish I had made the jump sooner. Better build quality, sturdier shutter, more control over my shots and faster AF. I will say however that the XTI is noticeably crisper shooting the same lens. Images off my 40D require a bit more sharpening but overall look better to me, proabably due to the DIGIC III. With the 40D rebates now, I would go with one over an XSI. The 40D is only about $135 more.

As far as fast lenses go, 2.8 and less (or more) are awesome. Take today for example. I pulled out my 50mm f/1.8 to shoot a macro shot of a flower before sunset. I was able to shoot at ISO 100 with a shutter speed of 250 at f/1.8 and got a great photo. Just for fun, I then put on my 28-135 IS. Most it would go to was f/4.5 and the pic looked horrible with the same settings of course. I had to go to a shutter speed of 40 and bump the ISO up to 200 to get it to come close to matching the shot from the 50mm.

Having at least one fast lens of f/2.8 or so is a must I think unless you are always shooting in very well lit situations.