This is the AF On button on the Canon 5D sr. This button can be made to trigger the autofocus (also remember to turn shutter autofocus off )
Most photographers rely on the shutter to activate autofocus. Press the shutter half way, and the camera focuses, then press it all the way to fire the shutter.
Ever have a situation in low light where the autofocus struggles? Same story in low contrast? Or something enters the frame and when you depress the shutter your true target is no longer in focus? Or maybe you miss a shot because an animal, child, or bride moves? Or you have to go into the focus menu and choose a new focus point so you can compose your picture and have the right thing in focus?
There are two functions available on higher end cameras that can fix all that. Back button focus can be turned on, and the focusing on the shutter can be turned off, and you’ll have autofocus and shutter release as separate functions on separate buttons. Look on the back of your camera. There may well be a button labeled AF On. This button can become the button you depress with your thumb to trigger the focus on your camera.
Next to that button is an asterisk button. That button can be triggered easily with the same right thumb just by adjusting your position. It does not require special dexterity to control when to push that * along with the AF On button. That button can be programmed to activate your servo focus mode for one frame (the one you about to shoot). If the subject is going to move, you can capture the subject accurately and you don’t have to go searching in menus to change focus modes (you’ll always miss the shot with all that adjustment time). In servo mode, the focus automatically tracks the moving subject. Setting these functions on a camera gets you ready to capture a bride walking down the aisle, a bird erupting in flight, or a child playing with a toy. Not all our subjects are still life landscapes. If you are looking to fire bursts of shots in a sports photography situation, this is invaluable.
Even if you only shoot landscapes, the low light advantage and recompose advantage make back button worthwhile. If you are set up on a tripod, the landscape won’t be moving about. Focus once, then you can recompose all you want. That original focal distance is maintained, but you can shoot all you want and no autofocus will occur unless you press the back button again. YOU get control of when the camera focuses. You don’t have to struggle in low light to figure out which switch on the side of the lens shifts to manual focus to kill the annoying failed efforts to autofocus. Being in control is always better than working pure auto.
This asterisk key may be programmed to provide one frame of servo focusing which will track a moving object. It is easy to control both AF and * with your thumb.
Don’t you like to set your ISO? (BTW, that’s pronounced “eye-so” not “eye–s–o”…it’s a word not an acronym.) And don’t you like to shoot in a mode other than “auto”? This is just another way you can have more mastery over the machine. You have better control over the focusing.
The downside? Well, that’s temporary. You have years of repetitions expecting the shutter to do the focus and give you that little beep. You’ll have to program the brain to get used to the new action with the back button for that to happen. It is just as easy to hit that button. Just as fast. It just is not as practiced at first so you have to remember to hit that back button. It becomes quite automatic.
OK, going to give this a try? Every camera is a little different, but Google and YouTube have examples for almost all of them. Just type in back button focus and look at the instructions for your camera. You’ll need to disable focusing on the shutter. Enable focusing on the back button (AF On button or program another button if you don’t have that option). Then enable one shot servo on the asterisk button. Sounds complicated, but videos included I did this on my 5D sr in under 15 minutes. It is really simple and it really does give you better control over the camera.
That’s the kind of thing that makes a pro a pro. But anyone with a good camera can do this one.