Back from Photoshop World Vegas and I wanted to take a quick post to describe one of my favorite shooting tips/techniques. I demonstrated this post to about 20 folks at the convention and thought I should share with everyone.
Most SLRs are equipped with an "AF-On" button on the back of the camera. For Canon users this button first appeared on the 40D. If you have an earlier model Canon you can easily program the AE lock button (the star button on the upper right of the camera's back) by swiching Custom Function 04 (Shutter/AE Lock) to option 1 (AE lock/AF.) This is Custom Function 10 on the new Rebel series.
Per default settings the AF On button will initiate auto focus (as long as the lens' autofocus is enabled) and metering. The ergonomic and control benefits of using the AF On button for focus should be immediately apparent when you press the button with your thumb. It feels very natural, comfortable, and simple to press with the thumb when holding the camera body in your hand. This makes sense as the button actuation is a normal extension of the gripping action. Beautiful to have an opposable thumb, don't you think? You should also notice there is no intermediate step for pressing the AF On button; it is either depressed or not, unlike the shutter release.
Adjusting to use of the AF On button for focus/metering takes a little time but the benefit is felt almost immediately. The simplest and fastest method for switching your technique is to divorce the shutter release button from anything but releasing the shutter. Focus/Meter = AF On and Shoot = shutter release.
The only gotcha in this process is if you release the AF On button once focus is achieved, metering is complete, and exposure is set (in Manual by your input of shutter speed and aperture / in Av by selecting an aperture and then alllowing the camera to set the correct shutter speed with AF On depression) and then continue to shoot the image by depressing the shutter release. Does not seem like this should cause a problem but it does! By default settings the shutter release controls auto focus, metering, and releasing the shutter. So when you press the shutter release the camera will also focus, meter, if in Av change shutter speed, and shoot. This is a major issue for the photographer as our desired focus and, potentially, exposure as set by the AF On button are changed at the last millisecond.
The fix is to not only think in terms of divorcing the shutter release from anything other than releasing the shutter but also to reprogram the camera via custom functions to disable focus from the shutter release button. As a Canon shooter I will describe the method to set the appropriate custom function. Would love to have input from some of our Nikon shooters as to the correct method to do this on the Nikons.
In order to reprogram the shutter release do the following on 40D,50D, and 5D Mk II. Go to the Custom Fucntions tab (Orange) on the menu display and select c. Fn IV: Operations/Others. Use the secondary control dial on the back of the camera to find the first function or 1 called Shutter button/AF-ON button. Hit the set button and then scroll down to select option 2:Metering start/Meter+AF start. The options describe what function will be set to each button shutter first and then AF-On button. So with option 2, the shutter release only initiates metering and shutter release not focus and the AF-On button does focus and metering. As you have probably noticed this isn't exactly what we want but it is as close as we can get. I will describe the potential gotcha in a minute.
For 7 D users, follow the same procedure until you get to the first function of c. Fn IV:Operations/Others. Your first function will show a list of programmable buttons on the camera. Select the upper left icon for the shutter release and program it for "metering."
20D, 30D, and Rebel users do not need to perform this additional reprogramming.
With reprogramming complete, you now have no focus actuation with the shutter elease. For Manual shooters, the shooting process is simple. Depress the AF-On button for focus and metering. Adjust exposure as required and ensure focus locked at the appropriate point. Release the AF-On button and shoot.
For Av shooters, a slight modification is needed. After locking focus and exposure by pressing the AF-On button continue to hold the button down. Fortunately, this is easy to do since our natural grip allows/encourages keeping our thumb down. This prevents the shutter button from remetering and modifying exposure if we recompose. As always, remember friends don't let friends shoot aperture priority. Of course, there are always exceptions :).
After a few shots using this technique you will never want to go back. Whether locking focus and then recomposing or continually tracking a target in motion, the back button focus WILL improve your photography.