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How do I use AF point selection

Donald A , Feb 24, 2008; 08:12 p.m.

I have an XTi that I'm just starting out using. When using lenses with a wide aperture in which focus becomes important I'm having some problems making sure my focus is where I want it. If I have a static subject, that's easy; I can just adjust the AF focus point. But if there is a lot of motion, for example a party, how do I make sure my focus is on for every picture? I could just focus manually, but that adds another thing to do (I'm still working on getting my exposures right). I could change the AF focus points in real time, but I don't think I could do it fast enough since the composition of the frame changes drastically from shot to shot. Any suggestions?

Responses


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Colin Southern , Feb 24, 2008; 08:39 p.m.

Select only the center AF point - focus by holding the shutter release 1/2 way down - whist still holding the release 1/2 way down recompose and take the shot.

Amol Kumar , Feb 24, 2008; 09:22 p.m.

Colin's suggestion is right on. I almost always use the center-point, especially in situations that are unpredictable (parties, social situations)

Jim Larson , Feb 24, 2008; 09:27 p.m.

the only time I have ever used a NON center AF point was for portraits with F1.8 lenses. I don't bother with F4 lenses.

And if focus could be dodgy. . .stop down a little bit to F8

Donald A , Feb 24, 2008; 09:30 p.m.

OK, that makes sense, especially since the center AF point is high precision compared to the other AF points, right?

Michael Ging , Feb 24, 2008; 09:54 p.m.

Each point is as accurate as the next, you just want to make sure that the point is over what your want to be in focus. I am constantly changing my AF points as I shoot,especially with a wide angle.

Keith Lubow , Feb 25, 2008; 12:04 a.m.

The problem seems to be with your expectation that auto focus actually focuses automatically. In fact, you have to tell your camera what to focus on. It is motorized, not automatic. So, for what you are talking about, I would pick a point, any point, and stick with it. Put it over the subject, hold down the * button and then frame the shot and fire. Some people call this "focus-compose" method. In practice, it is no different than using the split circle or micro prism on a manual focus lens. You still have to tell the lens what to focus on, but the focusing is motorized. If anyone actually expects AF to pick the intended subject, they have probably more than a few millenniums to wait for mind reading cameras to come out.

Keith

Amol Kumar , Feb 25, 2008; 12:59 a.m.

"especially since the center AF point is high precision compared to the other AF points, right"

Well, yes, this is true. But the main reason is what Keith wrote. If have a subject that YOU want in focus, you need to select one-focus point for the camera to use (then recompose). Otherwise, the camera could pick some other focus point that you didn't want.

Abdul Smith , Feb 25, 2008; 09:28 a.m.

is there any reason canon hasn't migrated the 'E' eye-controlled focus of the Elan lines to their DSLR line. I had a 7n, and i know i've read mixed reviews of the 7NE..is it just that it didnt work accurately? took too much calibrating for the average user? In theory it seems like a great tool "if" it works

Alan Myers , Feb 25, 2008; 07:02 p.m.

Hi,

Using the described procedure is fine, except that you'll end up with your subject (or at least your primary point of focus) exactly centered in every single shot.

There are at least a couple solutions:

1. Crop the image later in post-processing, to de-center the subject. Be sure to frame to allow for a little of this, or you'll end up doing a lot of cloning and pasting in Photoshop to make it work!

2. Use Custom Function 4 to reassign AF function to the * button on your camera. You can then use the center AF point to focus separate from the shutter release. Focus, recompose however you want the image framed, then fire the shutter. With a little practice, this becomes second nature.

The next question you'll be asking is when to use One Shot and when to use AI Servo. You might even ask when to use AI Focus. Let's start with this last one:

1. AI Focus: I personally don't use. This is intended to choose between the other two methods, when the camera decides whether the subject is moving or static. I gave it a try some years ago and felt there was a slight delay, and that the camera didn't always choose right.

2. One Shot is intended for use with static subjects. It's usually best to use with at least relatively slow moving subjects, that give you time to focus and finish making the exposure before moving out of focus again.

There is at least one way to use it with some moving subjects and that is to pre-focus on a point where you know the subject will be coming and you want to take the photo. Pre-focus, then wait for the subject to arrive, timing the shot at just the right point. An example: A horse and rider going over a jump. You know they will be going over it and just need to wait.

The * button technique works well in either case. Just press it, achieve focus (which is confirmed by the beep and by the green LED in the viewfinder), then release the * button and finish taking the shot.

3. AI Servo is intended for use with moving subjects. Using it with the * button is slightly different than with One Shot, however. The AF system starts when you press the * button and continues to track and try to lock onto the moving target so long as you keep that button pressed with your thumb and keep the AF point on your target.

This is a bit trickier, keeping the AF point on the subject and the button pressed, and still forces centering the subject or primary point of focus. But, it works.

In this case, there are more likely times when you would change the AF point. But, you really have to plan and set it up in advance. There's usually no time to make changes while the action is happening.

There are usually some missed focus and lost focus shots with this method. Practice helps, but still don't expect every shot to be a "keeper".

You can use all focus points with this method, in some cases. Then the camera tries to track the subject from one AF point to the next. Still, sometimes the camera will choose wrong! Expect more missed focus shots, fewer "keepers".

Note: You can use AI Servo with static subjects, too. The only downsides are that it A., may not be quite as accurate as One Shot and B., there is no focus confirmation beep or green LED.

A lot of people only use AI Servo, all the time. I imagine they are people who typically stop down their lenses a little and that covers up minor focus errors. I shoot too often at large, wide open apertures with shallow depth of field, so I use One Shot quite a bit. I'll switch to AI Servo when things are moving too quickly and unpredictably for One Shot. I wish all Canon had a high level and very accessible button to make this switch easy, without removing one's eye from the viewfinder. Depending upon model there might be some ways to set this up in Custom Functions, at least in part.

With either method, but especially any pre-focus technique, be careful not to manually change focus by accidentally turning the focus ring on the lens, overriding what AF has set up.

Regarding Eye Controlled Focus (or ECF) as seen on A2, some Elan 7s and all EOS-3s, there are three reasons I can think of that we won't be seeing it on any of the D-SLRs any time soon. One is that it adds a lot of expense to the cost of the camera. Another is that it uses processor power to function. The last reason is that it didn't work all that well... At least it didn't for me. I tried it for a couple months on my EOS-3s... Gave up, turned it off and have never used it since. I have to add that I wear eyeglasses, which may have been part of the problem.


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