BACK BUTTON FOCUSING WITH THE T2i
I think this feature is GREAT and use it all the time. YMMV
First here is how to set it up on the T2i.
Turn the T2i on.
Press the MENU button to display the menu
Select the tool symbol just to the left of the STAR
Select Custom Functions
Select Custom Function 9
Highlight OPTION 1 AE lock / AF
Press Menu to exit menu
Now here is what you can do that helps me in a lot of different situations.
You shooting procedure is now slightly different - less automatic if you please.
I shoot mainly in modes OTHER THAN AUTOMATIC ANYTHING!
When you half press the shutter button now the camera reports the exposure only allowing you to make any adjustments you like however IT DOES NOT AUTOFOCUS!
To AUTOFOCUS you press the * button I have found I can do this with my thumb while still keeping my pointing finger posed over the shutter button.
Each time you press the * the camera will auto focus and each time you half press the shutter button the camera will calculate or report the exposure depending on which mode you are in.
1. I can press * and autofocus right away then half press the shutter and set my exposure as long as I keep the shutter button half pressed my exposure will be locked in regardless of where I then aim the camera. I move the camera if I want to another subject maintaining the previous exposure and press the * to autofocus the new subject and press the shutter all the way to take the picture.
2. Have you found that occassionally the camera just does not focus sharply on what you want it to. Well with this feature you can be in control of when the camera does its auto focus. If it isnt as sharp in focus as I think it should be I just press the * button again and it will re auto focus. I find this very handy since I use only the CENTER FOCUS POINT to focus on. I am in control of when and what the camera will focus on.
3. Normally when I don't have interring bright backgrounds its very quick. I aim press * to focus press shutter halfway to expose and shoot.
Once you get use to doing this you can shoot a lot of pics using continuous shooting and pressing the * to refocus as you deem necessary as you aim at different subjects without the fear of the camera slowing down because it cant seem to find what you want to focus on.
In other words it seems to me that doing it this way speeds up the auto focus process and my ability to shoot faster.
Hope this helps someone ... it works great for me!
I tried that in Live View Contrast Focus, and it works there as well. Very nice! Many people complained that on the 450D you had to use the star button to focus in this mode, but I liked it. Focus is very slow in this mode, but once you are focused, it's nice to be able to take several pictures without the camera going through a slow refocus each time as it does with the T2i. You can turn off autofocus once it's focused, but that's a pain to do so and remember to turn it back on for a new scene. This would be much better for my uses.
i also occasionally use "BBF" (back-button focusing).
the reason i like it is because it puts your camera into a "shutter priority" mode. not to be confused with "Tv Mode". shutter priority is not a setting, it is a behavior.
in "focus priority", the camera's main concern is autofocus. that is why when the camera cannot find focus, it will not take a picture. however there are occasions when a picture is already in focus but the camera gets confused. and even if a frame is in focus, the camera will still confirm that it is in focus by running its AF routine before firing a frame.
in "shutter priority", the camera will fire a frame regardless of focus. this will give you the quickest response in taking a photo. of course since the camera fires even when out of focus, you will get out of focus pictures.
i like this setting when i am taking a picture of a subject at a pre-determined distance from me. you can AF on the subject once, and then you are done. unless you or the subject moves and physically changes the distance between the two of you, you should never need to AF again. and if you don't need to adjust focus, why would you AF before every frame?
my favorite use for this is baseball. if you are sitting in the stands, each base is a constant distance from your camera. so let's say there's a runner on first base. you can use BBF to focus on first base. from now on, you don't need to AF if you are taking a picture of first.
so now you can wait for the pickoff play. you can concentrate on other things, as opposed to holding down the shutter at half-press. if you see a pick off play develop, you can drop the shutter and the camera will start firing instantly.
BBF also works well with AI-Servo and continuous shutter.