What is Back Button Focus and do I need to use it?
If you have a DSLR you may be aware of Back Button Focus and may already be using and benefitting from it.
If you haven't heard of it then you have come to the right place!
Back Button Focus (BBF) enables you to change the method of focussing from the default 'half press' of the shutter button to a button on the back of the camera.
'But why would I want to do that?' I hear you ask.
I'm glad you asked, let me explain :-)
The biggest use for it, at least in my world is for action photography.
Imagine you are at a race track in a great spot for cars coming fast round a corner and then into some straight on shots. With the default setting, the moment you you press the shutter button the camera will focus then take the shot. This is great if the car isn't going too fast and your DSLR has a fast shutter speed.
But if that car is going fast and you have a slow shutter release, by the time you have focused and taken the shot that car may be a few feet closer than the point you focussed on and therefore the image may not be as sharp as you want it.
Of course this can be overcome by using the AI Servo mode (Canon) on your camera but even if you are using this mode you have to keep the shutter button half pressed to keep the focus tracking the subject which can be tricky when there is a lot of action or its January and your fingers are freezing.
This is where Back Button Focussing can be very useful.
On your camera, depending on the model, there will be either a dedicated AF button or a AE lock button which is marked with an asterix *.
Normally if it has an AF button it will also have the * button.
This button can be programmed to take over the half button press of the shutter button.
(More details and instructions to set it for Canon cameras to follow)
When this is set it means that the main shutter button will no longer auto focus but will still meter the scene and the back button will also meter but will take control of auto focussing.
So lets go back to the previous scenario and explain how this will benefit:
Once again the car is coming around the corner and is heading towards you at speed.
You have AI Servo enabled so now all you have to do is keep your thumb firmly planted on the AF (*) button which will then keep focus on the moving target (Make sure you are focussing on the car in the first place!) then you can press the main shutter whenever you feel is the right tome knowing that your focus is always tracking while you hold the back button down without worrying about half presses and potentially missing that awesome shot.
Here is another couple of scenarios because its not all about action.
You are shooting a performer on stage but you were a bit late getting to the venue so you are not at the front and keep getting shots focused on the person with the big hair in front of you rather than the performer.
If you have Back Button Focus enabled you can lock focus on the performer with a quick press of the back button and be assured that the focus will stay there and not pick up other distractions that move into your line of site. But remember to re focus if the performer moves backwards or forwards. Once you have done it for a while this will become second nature.
Finally lets say you are in full tourist mode and you want to get a picture of your mate with the Eiffel Tower (Other landmarks are available!) but you don't want the cheesy shot of them filling the frame grinning inanely, instead you want them off to one side and the Eiffel Tower in the center of your frame but you want to make sure your friend is in focus.
You could have all focus points enabled and hope the camera knows you want them to be the point of focus which is a bit hit and miss or you could move the focus point (if your DSKR allows it) to be on the subject which is a pain.
Or you could focus on your mate by pointing the camera at them and tapping the back button then recompose the shot to include the Eiffel Tower knowing that the focus will remain on your mate.
You will find when you make the move to using Back Button Focus, you probably won't get on with it and you may even hate it. You may forget and get blurry shots which will be annoying but if you stick at it you will most likely wonder how you ever managed with out it.
One thing to keep in mind though is that Back Button Focussing only applies to the 'professional' modes. e.g. TV, AV and Manual. If you use the dreaded 'auto' modes (Why would you?!!) then the AF will be back on the main shutter button, although this is handy if you want to give the camera to Uncle Bob to get in the shot with your mate and the Eiffel Tower.
Here are some brief steps for how to set Back Button Focus on a Canon 5D MkIII
Bring up the menu on the back of the camera.
Go to C.Fn2:Disp/Operation
Go down to Custom Controls and press set
On the first button - Shutter butt, press set
Choose the middle option - Metering Start, press set
Next scroll down to the next button (AF-ON), press set (Or choose * if your camera doesnt have a dedicated AF button
Choose the first option - Metering and AF start.
Press set and you are done. This now means that the main shutter button will meter when half pressed and take the shot when fully pressed and the back button will also meter and take control of the focussing.