Back Button Focus
Article by Mark Rayner, Trekabout Photography
There is one feature of your camera that you may have just brushed over in the past or ignored altogether that holds the key to improving your 'in-focus' hit rate enormously.
Back button focus.
This one tiny button will change the way you shoot forever.
Traditionally, you hold down your shutter button half way and then, when the moment is right, depress the shutter button the rest of the way to take your shot. But what if you want to focus, hold focus and re-compose, and then take the shot. No problem. Just make sure you are on 'One Shot' or 'Single Shot' focus, hold the shutter half way, re-compose and shoot. Fine, except now the exposure is not right because the area that the camera metered from is different to where you original focused (mainly a problem with spot metering for portraits or wildlife). So you redo the shot and hold down the AE-L button on a Nikon or the * button on a Canon to lock the exposure or use exposure compensation..... hmm, it's starting to get complicated.
Let's simplify everything. How about one button to focus and another to lock exposure and activate the shutter. Now you could press a button to focus and if you needed to recompose, by simply half depressing the shutter button, and then recomposing and shooting, your exposure would be correct based on the reading taken from your subject before you recomposed. Easy.
But it gets even better. How about never having to change your focus mode from One Shot or Single Shot to AI-Servo or Continuous A/F or whatever. Just use one setting that works for everything.
Here is how you do it.
In your camera's menu find the section where you can allocate different functions to different buttons.
Set you Back Focus Button to 'enabled' or 'on'.
Set your shutter button to activate exposure lock. Make sure your shutter button does NOT activate focus. It would take another couple of pages to go through the menu settings for each camera so you'll just have to work through it.
In the end you should have a back button that activates focus and a shutter button that activates and locks metering and takes the shot. Pretty simple really (but it can take a bit of digging around in menus to get there. Consult your manual if you run into trouble). Just a quick note here to advise that some of the entry level Canon cameras don't have an AF button on the back. All is not lost as you allocate the * button to carry out this function.
Having set your buttons up change your focus mode to Continuous or AI-Servo and leave it there. Forever.
So here is what happens now.
Back button focuses when you are depressing it. If you take your thumb off the button focus stops, effectively locking focus.
Shutter button activates metering and locks it when half depressed and then activates the shutter when fully depressed.
Now let's look at two common scenarios. In one situation we have a moving subject and another where we have a stationary subject.
Focussing on moving subjects
As long as you hold your new focusing back button pressed your camera will continue to focus, changing constantly as the distance to your subject changes. So, provided you have your focus point lined up on your moving subject, as you track along with your subject you will continually maintain correct and accurate focus. Remember your camera is now set to Continuous Focus or AI-Servo, exactly the same as you would have used in the past for a moving subject.
Now though, you don't have to shoot off a heap of unwanted shots by keeping your shutter button depressed to maintain focus. Just keep you thumb pressed on your Back Button and fire off the shutter whenever you want. Perfect. If you want quick burst then fire of a quick burst. Want one shot, then simply press the shutter once. The focus remains independent and you subject will remain sharp and in focus as long as you keep your back focus button pressed.
So for this shot of the Impala in the Okavago delta, I just kept my thumb pressed on the back focus button, tracking along as the Impala ran along and fired off a few shots, all of which were in focus. Easy.
Nikon D800 Nikkor 400 f/2.8 VR with 1.4x teleconverter. 1/1000s f/4 ISO 125. 560mm focal length.
Focussing on non-moving subjects
Standard shot first, no recomposing necessary: Leave your camera on Continuous A/F or AI Servo. Line up your focus point, press your back focus button and, when you are ready to take the shot, press the shutter button. Too easy.
Now, the recompose method: Again, leave your camera on Continuous A/F or AI Servo. Line up your focus point, press your back focus button and, when focus is achieved, take your thumb off the back focus button. Your focus now won't change. Unless you press the back focus button again your focus is effectively locked. Now, half press the shutter button and hold it half-depressed. Recompose and press the shutter button the rest of the way. The result is a correctly focused, correctly exposed shot.
For this shot of a Spiny-cheeked Honeyeater near Alice Springs, I focused on the bird’s eye, released my thumb from the back focus button, half pressed the shutter to activate the metering (spot metering in this case), recomposed (half holding down the shutter button) and took the shot.
Nikon D800 Nikkor 70-200 f/2.8 VR with 2x teleconverter. 1/640s @ f/5.6 ISO 450. 400mm focal length.
Like anything, you will need to practice this technique but, once mastered, you will never go back.
Enjoy your tack sharp photography.