In the old days we used to focus our cameras manually. In the really old days that meant sticking your head under a black cloth and looking at the upside-down image on the ground glass and seeing if the image was in focus. Even before the advent of autofocus (AF) cameras, most rangefinder cameras has some kind of mechanism to assist when focusing. Single lens reflex (SLR) cameras allowed the photographer to see the image coming through the lens, and most were equipped with a split circle that allowed adjustment of the lens focus until the top half and the bottom half were aligned, confirming proper focus.
Nearly every camera sold these days is autofocus, which is an advantage, and a disadvantage. For me, I can’t see through the viewfinder on my DSLR very well, because I wear glasses and my eyes are tired. I don’t think I could focus manually if I had to… So AF is how I focus when using my camera, and for that I’m really grateful. But AF makes for lazy photography if you let it. Lazy because you point the focus spot–usually the center of the image screen–at what you want in focus and expect (hope) it is indeed in focus. And most of the time it is. There are times when it’s not, and that could be due to the subject moving, or because you accidentally lifted your finger off the shutter, and then depressed it again. Most cameras a set up so that when you depress the shutter button either half-way, or fully, the camera sets the exposure and releases the shutter once focus has been confirmed (I call this one-button AE/AF). Pressing the shutter halfway sets the AF on most cameras. You can use this to focus off-center, then holding the shutter down halfway, recomposed the image and make the photo by fully depressing the shutter. It’s simple and both proper exposure and proper focus are achieved in one move by one finger.
Recently I have starting using two fingers. Weird, I know! It’s called Back Button Focus (BBF) and it appeared first on a Canon camera in 1989. What Canon did, and many other cameras allow this now, is offer the option of separating the exposure function from the focusing function. The shutter button will still control the setting of the exposure when pressed, but the camera will not start autofocusing until another button (set by the user) is pressed. Most users assign this function to a button on the back of the camera, close to where it can be pressed and held by the thumb. This usually means one of two possible buttons. Using BBF, the photographer must set the exposure and focus in two explicit moves–which might seem to defeat the purpose of an AE/AF (modern) camera. But it seems to give me more control over where I focus, and when I choose to set the focus. The camera I use has two types of focus modes: one shot and Ai Servo. This sounds pretty fancy, but really it just means the camera either sets focus and sticks to it, or sets focus and then constantly adjusts it if the subject moves, which is great for sports. With “one-button AE/AF” the only way to refocus is to lift off the shutter button. With BBF, focus can be set, and reset, by pressing, holding or releasing the assigned button.
You can read more about the technique here at Canon’s digital learning center.