Focus modes on the 40D

I shot a cyclocross race here in Boulder today - the first one with my 40D. I had the focus mode set to AI

Focus, which according to the manual is basically One Shot mode, that switches to AI Servo mode when it

detects movement. Reading the manual, one might summarize there's no reason to ever set the camera to

anything but AI Focus. I ended up with more out-of-focus shots than I expected. After I got home, I was

playing with the different modes and noticed that AI Focus doesn't seem to work the focus motor as much

as AI Servo when I hold the shutter and move the camera around, simulating subject movement. What

mode do you guys use for moving subjects, and do you think there is a difference in camera behavior

between AI Focus and AI Servo when the subject is moving.

 

I'm thinking the out-of-focus shots might be because I left the AF Point selection in auto mode, and I

should have just fixed it to the center point.

 

One othe question that the manual isn't clear on. It seems to me that regardless of which autofocus mode

is selected (One Shot, AI Servo or AI Focus), the camera won't fire the shutter until it has achieved focus (or

at least thinks it has). My previous camera (a Pentax) would release the shutter in servo mode when the

shutter release was depressed completely, regardless of whether focus confirmation was achieved or not.

On my 20D I have the best success with moving subjects when the <abbr title="autofocus">AF</abbr> is set to AF Servo and automatic AF sensor selection. I agree that AI Focus is pretty much useless, maybe it works for very slowly moving subjects. A fast lens with <abbr title="Ultrasonic Motor">USM</abbr> helps tremendously, my definitive speed-fiend is the EF 200mm f/2.8 L <abbr title="Ultrasonic Motor">USM</abbr> prime.

I consider AI Focus to be useless because of the additional lag it introduces--the camera has

to *notice* the subject is moving and *then* switch to Servo and try to maintain focus. With a

moving subject, I want it attempting to focus as soon as I hit the button, which is why I leave

mine in AI Servo with back-button focus almost all the time and just use a short burst of AI

Focus on stationary objects (well, except for macro and some landscapes, in which case I go

to either one-shot or manual focus.

Kevin - May I ask, what is the advantage of using the focus button on the back of the camera,

instead of activating focus by depressing the shutter button halfway down?

I agree with others, AI Focus is worthless.

 

In AI Servo, the lens does *not* have to find focus before it fires. It becomes shutter priority. I believe that is similar to what your Pentax did.

 

The AF On button can be used in several ways. Custom Function IV-1 gives you 5 choices. I sounds like Kevin has his set to #2 or #3 to do his AF with it, instead of with the shutter button. That's a pretty common choice. And by turning the camera to Servo you can have auto focus on command, but not have it keep moving around on you when you want it to hold it's focus.

"Kevin - May I ask, what is the advantage of using the focus button on the back of the camera, instead of activating focus by depressing the shutter button halfway down?"

 

The use of custom function four to move focus to the the * button is pretty powerful if you shoot sports. This thread covers it pretty well (it is for the XT but the principle is the same): http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00Coaw

peter - my experience with AI Focus is that it is not good for things that are moving quickly. I usually do one shot for nonmoving subjects, and AI Servo for moving. AI Focus is not very helpful for me.

Hi Peter,

 

I agree with the others, AI Focus introduces a bit more time lag that I find unacceptable with anything moving at a decent clip. You'd be better off either pre-focusing in One Shot or tracking with AI Servo. I've tried AI Focus a few times with my cameras, but never use it any more.

 

I also use CFn 4 to set up the * button on my 30Ds and 10D, but your 40D has an "AF On" button, too, that essentially does the same thing. (If you are using a vertical/battery grip, that lacks the AF On button, so you might want to program the * button with the CFn anyway).

 

If focusing is left linked to the shutter release button, too often focus goes off hunting when you least expect it, and you end up with missed focus. What happens is moving subjects are hard to track, and AF point "sees" something else and tries to focus on it, and your subject ends up a blur.

 

Personally, I have a lot fewer keepers too, when I let AF points select automatically. Too often the camera simply gets fooled into focusing on some point other than what I would have chosen. An exception to this is when using lenses pretty stopped down, so there's lots of depth of field to compensate if the camera is fooled a little, in AF point selection. (Say if focus locks onto the front wheel of a cycle, instead of the rider.)

 

I usually limit it to just the center AF point, use the * button to activate and achieve focus, then re-frame if need be. With really faster moving objects, it might be necessary to re-crop the final image, skipping even the re-framing step.

 

If you want to avoid centering the subject and have a pretty good idea how you are going to frame the image in advance, you could pre-select one of the other AF points, if you wish. This is even more possible with the 40D than earlier models, since all the camera's AF points are now the more sensitive cross type. With 30D and earlier, you might have also been limited to only the center AF point if light and/or contrast are low.

 

As you probably know, there simply isn't time to be fiddling with AF point selection while the action is passing you by.

 

You might also want to try One Shot with pre-focusing.

 

This is where you are taking shots at a pre-determined point, when the subjects pass you at regular intervals. Works well with most sorts of sports run on a track, where you can predict where the subject will be and when you will want to capture them with a high degree of accuracy. Simply use the * button to pre-focus at the pre-determined point, and be careful not to change focus manually. Then pickup your subject as they approach, track them to the point where you pre-focused and release the shutter once they get there.

 

This is pretty similar to what we had to do with manual focus lenses back in the "bad old days". Few could continually focus on moving subjects with any degree of accuracy and consistency.

 

Of course, this doesn't work if using 3 or 5 or 6.5 frame per second drive. Use it only with single shots (I use fill flash a lot, indoors and out, so am limited to single shot by the flash recycling anyway.)

 

If the subject is more unpredictable, or you are using a multi-shot frame rate (and no flash), switch to AI Servo. Set your AF point on the subject early and track them with the * button held down all the time. You just have to be careful to keep the AF point on them, not let it drift off and focus elsewhere. Release the shutter whenever the subject is close enough and framed the way you want it to be.

 

You can increase your keeper rate by stopping the lens down a little, to increase depth of field. Trade-offs here are keeping the shutter speed up where you want it, and at the same time ISO as low as possible for best possible image quality.

 

Another reason to stop down a little is that pretty much all DSLRs (except full frame) tend to exaggerate DOF in the viewfinder. Working with a lens wide open you are actually going to get a lot shallower DOF in the final image than it appears in the viewfinder. Partly this is just because the image in the VF is so small, making it hard to judge DOF with any accuracy. Slower lenses are less of a concern than faster ones, where DOF can be pretty shallow when used at max aperture.

 

With practice, these techniques can work pretty well.

 

To give you some idea, I used all of them at an outdoor horse show a couple weeks ago - on two 30Ds with 24-70/2.8, 70-200/2.8 and 300/2.8 lenses - and my keeper rate from nearly 800 photos was 60% (and many of the "throw aways" were due to poor framing, closed eyes, unattractive poses, or even just too-similar duplicates... not focus issues). Despite pretty fast moving subjects, over jumps and at pretty fast gates at times, I managed about 80% or better in good focus, I'd guess.

 

At more recent event in a covered arena I shot a lot more in part because I knew I'd have more missed-focus issues, in part because of the more challenging lighting, but also because I couldn't use flash at all. My keeper rate was a lot lower, even though the action was slower. I expected and allowed for it by taking more than twice as many shots.

 

I ended up with 479 keepers out of 775 shots, at the first event.

At the second, I had almost the same final tally - 465 keepers - out of 1787 shots total.

 

 

I use a signle AF focus point exclusively (typically the center point), since the camera will never do a consistent job of figuring out what exactly is the main subject in your photo. I would label both multi-point AF and AI Focus as "Random Focus".

 

I use either single shot or servo AF. I am getting used to the AF On button which has several advantages. With it, you can use the AF Servo mode just like single shot (using a momentary press).

 

For some really fast action like football, where the subject can jump from person to person very quickly as you follow the ball, I still feel slightly more comfortable with the half-shutter press. (I suspect I will eventually develop the coordination and muscle memory to use the AF On as efficiently and comfortably in that situation as well).

 

I have had mixed results with servo mode on moving subjects. With a slow lens (i.e. either aperture or focus motor) it just doesn't do a good job. In low light, even a fast lens (aperture and USM) like the 70-200 f/2.8 can be hit or miss in servo mode (although that lens does the best on AF Servo of any lens I have used).