If you spend any time surfing photography forums and Facebook groups, you will undoubtedly see a constant flow of questions asking for the best way to nail focus. Maybe you are one of those people that find themselves struggling. The trick is that most cameras have a setting that will help you focus like a pro. That trick is called back-button focus, and once you use it, you’ll never want to go back.
When you get a camera and turn it on for the first time, it’s automatically set up with some default settings. One of the main settings that have become standard across the board is how your camera focuses. That standard practice is to halfway press the shutter button, the camera focuses, then you press the shutter down the rest of the way to take your image. What back-button focus does is remove the focus actuation from the shutter button and instead assign this function to a button on the back of the camera. Now when you shoot, you’ll press your back-button to lock focus and then press the shutter to take an image. If you don't press the focus button, your camera acts as if you have the camera set to manual focus. At the beginning, this may seem like you're over complicating things. I get it, why make it so that you have to use two buttons to accomplish what was done with one? But there are some very real benefits that make this change more than worth your while.
No More Having to Constantly Refocus
The first advantage comes when dealing with those certain compositions where your subject sits in the part of the frame where there is no focus point. In the past, you would have to focus, recompose, shoot. The problem here is that after that first frame if you release the shutter button all the way you will have to focus again the next time you press the shutter. So now you have to focus, recompose, shoot, repeat over-and-over again each time you press the shutter button. Another problem with this is that you are constantly moving the camera to lock focus which distracts you from fine-tuning your composition. With back-button focus, all you have to do is focus one time, recompose, and shoot until you're done. Unless your subject moves forward or back, you don't have to press the focus button again. Now you can shoot and make small adjustments to your composition while you shoot as needed without having to worry about focusing each time.
No More Having to Choose Between Single and Continuous Focus
One of the biggest struggles with autofocus comes from the use of single and continuous autofocus. Before, you had to choose what setting to use. If you needed to focus and recompose, then you had to choose single. This way you could half-press the shutter to lock focus, recompose, then take your image. But then if something with movement started to happen you would miss the shot because you didn't have the right focus mode.
If you had continuous autofocus selected, you would be fine for anything with motion as well as anything that was still. The problem here is that your subject had to be located in part of the frame where there was a focus point. If you needed to move the frame so that the focus point was no longer on the subject, the continuous autofocus would make the camera automatically refocus on wherever the focus point was placed.
When you have back-button focus set up you actually get the benefits of both modes without any of the drawbacks. The way this works is that you have the camera always set to continuous autofocus. This way when you are tracking the movement of a subject you just hold down the back-button as you simultaneously press the shutter to capture images. When you run into a situation where you need to focus and recompose, all you have to do is press the back-button to focus, release the back-button, then recompose and shoot. Because the camera only focuses when you press the back-button, you’ll never have to refocus until you or your subject moves.
While this process may vary from camera to camera as well as manufacturer to manufacturer, below is a short list walking through how to set this feature up. If you any of the below doesn't work for you, I suggest referring to your camera's owner's manual or doing a quick YouTube search for your specific camera.
For this, I’m referencing the Nikon D750 but it should be similar for most Nikons.
Menu - Custom Settings Menu (pensile) - f (Controls) - Assign AE-L/AF-L button - Press = AF-On and Press + Command dial = OFF
From here you select the AF-C focus mode from the AF selection button.
Menu - Custom Controls (C.Fn).
In this menu, you’ll need to do two things. First, you’ll remove the AF-ON from the shutter button by selecting the shutter button and set it to AE-Lock (*). Next, you’ll select the AF ON button and set it to AF. After this, you set your camera's AF mode to AF-C.
The first step here is to remove the AF function from the shutter button. To do this we go Menu - Custom Settings (gear icon) - AF w/Shutter = OFF
Next, we need to assign the AF function to the back button. For this we go Menu - Custom Settings (gear icon) - Custom key settings - AE-L Button = AF ON. Now when the AF/MF / AEL lever is in the down (AEL) position, the button acts as the back-button focus.
We can stop here if we want, but because Sony offers some nice features such as focus peaking when in the manual focus mode, we can also set a very quick and easy way to access this feature by going Menu - Custom Settings (gear icon) - Custom key settings - AF/MF Button = AF/MF ctrl Toggle. Now to quickly toggle between AF and MF, you can switch the AF/MF / AEL lever up to AF/MF and press the button once to switch to manual focus and then press again to switch back to autofocus.
The first step is to set the focus selection dial to M (manual). This will make it so the camera won't focus when you press the shutter.
Next, we need to make sure what focus mode is used when manual focus is selected. To do this we go Menu - AF/MF - Instant AF Setting - AF-C
Lastly, we need to decide what button we want to use to trigger the focus. Some cameras have the ability to map a custom button as AF on. For my X-PRO2, I have to use the AF-L button. For me though, The AE-L button is a better choice. Thankfully, you can swap these two buttons around so that the AE-L button acts as the AF-L button and vice versa. To do this, we go Menu - Set Up (wrench) - Button/Dial Setting - AE-L/AF-L Button Setting - AF-L/AE-L
While this is a bit different because you have to set the camera to manual focus in order to use back-button focus, I do like that I can easily switch the camera from manual focus to single or continuous and the focus will be back on the shutter button. This is great for when you hand your camera to someone else to take a picture since that's what most everyone will be used to using.