Author: Vern West
All images © Vern West
Back in the film days at the advent of auto focus lenses there were two options for initiating focus. The first one was initiated by the shutter button. This is the way most of us have our cameras set up today. Your camera probably came this way by default from the manufacturer. The other way was to initiate auto focus using a button on the back of the camera and the shutter button only controlled exposure and shutter release. This method usually had to be reprogrammed through the menu system on the camera. Most modern digital cameras still have the option for back button focus and in many ways it solves some focus and exposure issues that we all have to try and deal with.
Let’s look at some of these issues.
- First we decide what we want to be in focus (our subject) but then we recompose to get the desired composition. One way is to move the focus point around to put it on the subject. Of course this assumes that there is a focus point that falls on the subject and gives us the desired composition. If it doesn’t we have to depress the shutter button ½ way to lock the focus and then recompose the frame. This may cause the exposure to be over or under because the holding the shutter button ½ way down also locks the exposure and that is not always the correct exposure after we reframe for composition.
- If the subject is stationary such as a mountain then the focus is achieved and locked until after the shutter is released. Canon calls this One Shot. Nikon calls it Single AF. In this case the camera will give an audible beep when focus is achieved as well as visual indication in the view finder. If the subject were to move the focus would not change and that subject would be out of focus.
- In the case of moving subject set your camera to AI Servo (Canon) or AF-C (Nikon) and the camera lens will try to follow the subject. For a moving subject only a visual indication shows in the viewfinder.
- Exposure is set where ever the focus point is adjusted and is usually affected by the overall scene brightness. There are several exposure options that you can set but this is a discussion for another time. As an example, if your subject is darker colored, and you lock the exposure by depressing the shutter ½ way, but the overall scene is bright you camera meter can be fooled into over exposing the image.
Now let’s look at moving the focus to a button on the back of the camera. You may have to read your manual to determine how to achieve this but with
some practice it will become natural. When you do this there will be a learning process because you are probably used to just pressing the shutter release for everything.
- Set your focus mode to AI Servo (or whatever your camera calls the continuous mode). You won’t hear the beep anymore but you will get a visual
indication of focus.
- On my Canon camera I have the * button set to initiate AF. This is where you may have to use your manual to get your camera setup properly
- I usually leave the focus point set to the middle position, on most cameras this is the most accurate focus point anyway. I also usually have the exposure set to evaluative although I change it depending on the subject.
So now we can look at some real world scenarios.
- When you are photographing a stationary subject, put the focus point on the subject and momentarily depress the back focus button. Now you are free to recompose and take the photo, in fact as long as you or your subject do not move you do not have to refocus and the camera will set the exposure when the shutter is depressed. This can be particularly helpful when photographing birds with a telephoto lens and the lens keeps hunting for focus on a small subject in a tree.
- When you are photographing a moving subject such as a flying bird or panning such as with a bicycle and running dog all you need to do is keep your thumb on the back button and the focus is set to continuous it will track the subject.
- When you want the lens in manual focus while doing a macro shot of a cactus flower and Cactus Wren flies in and lands on the cactus next to you. You are immediately able to take a properly focused image of the bird by depressing the back button focus button instead of having to fumble with a switch on the lens or finding some menu item. A Cactus Wren may not hang around long enough for you to get set up. This ability to change from manual focus to AF quickly is one of the advantages that I really use and value.
- This past spring my wife, Barbra & I were scouting for a good place to photograph the full moonrise. We saw a pair of Gila Woodpeckers flying into a hole in a saguaro, feeding baby chicks. When photographing flying birds like this you must pre-focus and then shift the lens to manual because the bird is moving too quickly for the lens to achieve focus. I simply pointed the lens at the hole, pressed and released the back button to achieve focus and I was ready to capture the action.
- The proper way to do a stitch panorama is to set you lens to manual focus so each frame is not focused at a different point. Instead of switching the lens from AF to manual and back again, I simply press and release the back button focus as appropriate.
Is this a better way to autofocus your camera. It all depends on how you work and what your preferences are for switching AF back and forth from single shot, continuous and manual. Give it a try, it might work as well for you as it does for me.