PDA

View Full Version : Canon 20D Auto focus modes


Gidders
14-06-06, 00:17
I've always used the "one shot" AF mode but when I was up at the Appleby horse fair last week trying out a my new 70-300 Is lens I got a dissapointingly high number of slightly soft shots. Testing in the garden on Sunday on a tripod shows that there is no problem with the lens. As I was shooting the majority of the time at 1/800 with IS on I don't think it can be camera shake. Therefore I've come to the conclusion that, with the lens mostly at 300mm and f7.1, with the shallow depth of field, in the time between achieving focus & IS lock, and actually releasing the shutter, the subject has moved out of the plane of focus.

As they always say, if all else fails read the instructions. In Al Servo AF the camera keeps adjusting the focus until the final moment when the shutter is released. So that might have been a better focusing mode. Then there is Al Focus AF which is "one shot" AF which switches to Al Servo when movement is detected.

What I don't understand is this - what are the advantages of the 3 modes? Is the one shot mode mearly to conserve battery power or is there some other reason for using it over the servo mode. Al Focus seems to offer the benefits of both the other modes so why would anyone use either of the other 2? :eek:

hollis_f
14-06-06, 07:29
Hi Clive,

With One-Shot the focus locks and then doesn't change as long as you've got the shutter half-depressed. That allows you to focus and recompose. It's also useful if you've got interfering objects that may pass between you and the subject (wildlife in trees or long grass is a good example). In the other modes the camera may well decide to try to focus on the closer object as it moves near your subject. In One-Shot the camera will only take a picture when it's locked the focus on something.

AI-Servo is great for keeping focus on a moving subject - as long as it doesn't move behind something else. But you have to bear in mind that the shot will be taken whether or not the camera is focussed.

AI-Focus sounds great. But it's my experience that it seems to combine the disadvantages of the other two modes, rather than their advantages. With a static subject it'll switch to Servo mode just when you don't want it to. And it's very often too slow to cope with a moving object.

So I tend to use One-Shot or AI-Servo, depending on how I think my subject will be moving.

Roy C
14-06-06, 08:38
I use A1 Servo for bird flight shots and one-shot for static shots (inc perching birds). Having said that, recently I have been trying A1 Servo for almost all my bird shots and as it seems to work ok. I think I am right in saying that A1 Servo only uses the centre focus point for the intial 'lock' but after that it will use all availble focus points to keep track of the subject.

Gidders
19-06-06, 22:43
Thanks for the input guys. Tried A1 servo mode at a local horse show at the weekend and got a greater number of sharp shots :D

Wheeler
20-06-06, 00:07
Correct me if I'm wrong, but isn't the IS on the 70-300 just for static shots as opposed to the more sophisticated two mode version of IS on the 100-400/70-200 etc? If this is the case, isn't it better to switch it off when tracking moving objects?

bpw
20-06-06, 00:38
...recently I have been trying A1 Servo for almost all my bird shots and as it seems to work ok. I think I am right in saying that A1 Servo only uses the centre focus point for the intial 'lock' but after that it will use all availble focus points to keep track of the subject.

What you are saying about how AI Servo uses the AF points is correct, certainly with my old faithful 10D. Because of the way it works, I would imagine that using AI Servo all the time with birds could cause you problems at times?

Most agree that getting the bird’s eye in focus is critical, and when using a telephoto lens with large aperture, the depth of field is very shallow. As far as I understand it, if with AI Servo selected you focus on the eye, and part of the bird is in front of the eye, another AF point will take over and critical focus will move away from the eye. Also, if after focusing on the eye you move the camera to recompose the shot, focus may shift from the eye, especially if there are other objects (leaves, branches, etc) in front of the bird’s eye.

Have you experienced these problems? I’d be interested to know how you get on with AI Servo and birds.

Torpedo
20-06-06, 08:55
Correct me if I'm wrong, but isn't the IS on the 70-300 just for static shots as opposed to the more sophisticated two mode version of IS on the 100-400/70-200 etc? If this is the case, isn't it better to switch it off when tracking moving objects?

The IS on the 70-300mm is 2 mode.
I believe you are thinking of the older 75-300mm IS (which it replaced), which is 1 mode.

Wheeler
20-06-06, 16:26
Thanks, Torpedo, that's the one I've seen.

miketoll
22-06-06, 18:44
Correct me if I'm wrong, but isn't the IS on the 70-300 just for static shots as opposed to the more sophisticated two mode version of IS on the 100-400/70-200 etc? If this is the case, isn't it better to switch it off when tracking moving objects?
The IS on the latest 70-300 IS and the DO version are an upgraded version of the IS on the 100-400 lens you mention. They are supposed to give "up to 3 stops" rather than 2 stops and detect when the lens is on a tripod so you don't have to turn the IS off in that situation although still advised to. I wonder if there is any difference in practice?