Thursday, September 29, 2011

Back Button Focusing, What? Why? And How?

Excuse the quality of the picture, this is the best I could get out of my wife's P&S

So you've heard about BBF or Back Button Focusing and was wondering what BBF is. Maybe you also heard that once you get used to it you will never go back. In this post I will tell you what is BBF and why it is one of the best things that makes using a DSLR more practical and enjoyable.


BBF is short for Back Button Focusing, which means that you will move the focusing function from the shutter button to one of the buttons at the back of the camera (usually the "*" button on a Canon), check the picture of my Canon 60D at the top, you will find three buttons on the top right part at the back of the camera (Rebels or xxxD series only have two).

Usually half pressing the shutter button initiates auto-focusing and auto-metering, and completing the press snaps the picture, the whole idea of BBF is to remove this focusing function from the shutter button and move it to a button on the back, but why would anyone want to do that?


For one very simple reason, in order not to let the camera focus everytime you press the shutter button to take a picture. You are going to tell me "but I can easily switch the lens to manual focus and achieve the same result", true, you can, but can you do the the same as in the scenarios that I will mention below?


But before that, I must tell you how I have assigned the buttons on my camera first, so that you can imagine what I am talking about when I am describing the scenarios.
  • A half press of the shutter starts metering (exposure calculation).
  • A full press on the shutter will snap the picture whether it is in focus or not.
  • A press on the "*" button at the back will engage auto-focusing (will focus and lock the focus once if "Single Shot" is chosen, will keep focusing and tracking if "AI Servo" is chosen).
  • AEL button remains as it is, locks exposure when pressed.
The above configuration is not the only available configuration, you can shuffle the different functions between the buttons as you wish, I have 10 different possible combinations on my 60D.

One advantage of the xxD and xD series Canons over the xxxD (Rebel) series is the extra "AEL" button, if you have an xxxD camera, you'll have to sacrifice the exposure lock function in order to use the "*" button at the back for focusing instead.

You can change the button assignment on your Canon via the custom functions "C.Fn." menu under the settings, they are listed on this Canon's Learning Center page for different cameras.
There's an easier way to do it on the 60D through the quick menu, press the "Q" button, go to button assignments and you will find something that looks like this.

Custom controls quick menu on a Canon 60D

For Nikon users I found this link that describes how to set it up, if you can't do it on your Nikon, please tell us in the comments and Mic will be glad to help.

[Mic: For Nikon, the process is similar, although it varies by camera model.  In some higher-end cameras such as the Nikon D300 there is an independent AF-ON button.  The camera will focus when that button is pressed.  However, by default the camera will also focus when the shutter is half-pressed.  To use BBF, you disable the focus of the half-pressed shutter.  This is an option listed under the Autofocus options.  In the D300, it's option a5 AF Activation.

On older cameras such as the Nikon D70, you can use BBF but you have to use the AE-L button as the BBF button, thus giving up AE-L.  To enable BBF, find the option for selecting the function of the AE-L button and change it to AF-ON.  Enabling that option automatically disables the focus on half-pressing the shutter.

On still other cameras, such as the Nikon D80, there is no dedicated AF-ON button but there is a user-selectable Fn button that can be used for BBF.  Alternatively, you can assign the AE-L function to the Fn button and use the AE-L button as the BBF button.  The process is similar to that of the Nikon D70.]


I have read almost all of the posts and the forum topics that talk about BBF, and some of them list some possible usage scenarios that are straight out of science fiction books. I will tell you about the real life situatioins where BBF has helped me.

- Focus & Recompose: Imagine you're shooting someone using the focus and recompose method, you've already focused the picture, moved the camera a little to a composure of your liking and took the picture. Now imagine that the person in front of you made an interesting face or you said some instruction to them and want to take another picture, if you press the shutter again the camera will try to focus again compromising the focus you've already set, and you'll have to focus and recompose from the start again,  while if you're using BBF you can happily shoot away while the focus remains set from the first time.

- Tripod Shooting: Imagine you have the camera on a tripod shooting some product, you have the focus set and you're taking several pictures, unless you change the focus button to manual, everytime you press the shutter button (and it is way more annoying when you're using a remote release) the camera will try to auto-focus and ruin your set focus.

- Difficult Focusing Situations: This happened to me a lot with the Canon G11, I was shooting some dark women dresses for an ad, the dress' position was fixed and I was standing in the same position, when I tried to focus the camera, it kept hunting and giving me the red rectangle which means it can't focus (dress is dark and has no contrast to help the camera focus), so I increased the room lights, the camera focused correctly, I turned the lights down and took my first picture, you can imagine what happened next, when I tried to take another picture the camera started hunting for focus (again!) and threw my correct focus away, ARGH!
If I had BBF then, I would set the focus once and shoot as many pictures as I like.

I assume you got the idea by now, the MAIN advantage is that you can set your focus, and shoot as many pictures as you like without the camera trying to focus again. However there are other useful scenarios for BBF:

- Canon's USM technology enables you to use the focusing ring at anytime even if the focusing button is set to "auto", that way using BBF you can focus manually at will and start shooting pictures without the camera trying to auto-focus, that way you can forget about the lens focus setting (manual or auto). A counter argument in this case would say "but I can set the lens to manual focusing and do the same thing", but there's a difference, sometimes (especially when shooting macro, and the camera's AF struggles) I quickly press the "*" button at the back to get the focus in the vicinity of correct focus, then resume focusing manually myself, after I''m satisfied with the focus, I know that when I press the shutter button the camera won't try to auto-focus and ruin my efforts.

- Finally, one great benefit for us Canon users (not sure about Nikon, Mic?*) is that we can use one focusing setting to either capture still or moving subjects. Let me explain, on Canon cameras we have three focusing settings (not entirely sure about the 7D and the 1D), Single Shot, AI Focus and AI Servo.
Single shot focuses once and stops when focus is achieved, AI Servo keeps focusing and tracking whatever it is that you're focusing on, and AI Focus is supposed to be an intelligent mode that detects whether it should use single shot mode or track subjects if they're moving, but I never had reliable success with it.
With BBF, I leave my camera on AI Servo all the time, that way I can use it as a single shot or track something without changing any settings, pressing the "*" button would focus until I release the button (when the subject is still), or I can keep the button pressed to track a moving subject. 

[*Mic: For Nikon, we have similar options, although it varies by camera.  At a minimum, there is a manual focus mode (M), a single focus mode (S), a continuous focus mode (C).  The function of the continuous focus mode can be changed from always attempting focus (similar to Canon's AI Servo), a mode called AF-A where it's similar to single but will switch to continuous when the subject moves (similar to Canon's AI Focus).  On some cameras there is a separate button for AF-A, whereas on other cameras AF-A is selected in the menus.  Some older cameras such as the D70 don't have AF-A at all.]


I will tell you a little story, when I saw all the rage about BBF, and all the "I tried it and I never went back" stories I was excited to try it myself, and I first tried it on my 550D I used for one or two times and decided that I hated it and it slowed me down and went back to the half-press shutter focusing.

After a few weeks I decided to try it again and force myself to get used to it, and I never went back. For Real! Now I feel very frustrated whenever I use some camera where every time I want to take a picture it has to focus first, ARGH!

The only problem with BBF is when I give my camera to someone not used to it. The solution is simple, I have a custom mode (amateur proof mode, if you will) where the camera is setup to aperture priority, auto ISO, all focusing points active, single shot focusing, single shot drive, and auto-focusing set to the shutter button. All I have to do is to set the aperture to f/8 and give my camera to the poor guy to take pictures of me.

UPDATE: I usually post technical posts that to the forums for one of two reasons, either to benefit other readers or to get expert opinions on the post material, this one of those posts, and it has created some valuable comments and insights, I'd suggest you check them here.