I was recently asked what's the best way to ensure focus. Apart from using your eye perhaps the best way to do this is an attempt to understand the focus systems within your camera and how they work together to create focus within your composition. Sure, you can use autofocus and get the shot but there are reasons that you can customise your focus selection to suit the scenario you're in...
Where to start... There are area selections that you will want to explore first. Single point which allows you to select the focus point manually so the camera focuses only on the area in the selected focus point. Dynamic area allows you to select the focus point manually as with single point but if the subject briefly leaves the selected focus point, the camera will focus based on information on the subject from surrounding focus points. Then there's also automatic which, as the name implies, is an automatic selection made by the camera.
AUTOMATIC FOCUS POINT SELECTION: When you are focusing automatically you need to have the active AF point over the subject in the viewfinder to get it sharp. Broadly speaking, there are two ways of selecting the AF point using this camera focus technique. The easiest is to let your camera decide for you and use the automatic AF selection point option. Provided there is enough light, the camera will do a decent job and this is a useful option when you don’t have much time to get the shot. However, your camera will usually try to focus on the closest object near the centre of the frame or the most well lit and it’s not usually very good at pin-pointing finer details. For this reason, I use the single AF option more often than not. You can move the slider over the image to see where the focus was concentrated in the image below using dynamic area and automatic focus point selection.
SINGLE SHOT AUTOFOCUS: Often abbreviated to single autofocus or single AF (forever alone?), this camera focus option sets your camera to focus when the shutter release is depressed half-way and to keep the lens focused on the selected subject until the shot is taken and the button is released. If you need to refocus you have to release the shutter button and then depress it again so it's more of a mechanism within the focus system. You can move the slider over the image to see where the focus is concentrated in the image below using single shot autofocus. Bree Street wasn't well-lit and the infra red beam on my strobe gun/flash managed to assist in focusing but you can see that the depth of field within this image is an issue which meant that only the subjects closest to the camera are in focus.
CONTINUOUS AUTOFOCUS: This focus option allows the camera to continue to focus the lens as long as the shutter release button is half-pressed. This makes it a very good option when photographing moving subjects because the camera will adjust the focus distance as the subject moves (provided there is enough light). Newer cameras have options that enable you to specify which AF points the camera will use to track the subject as it moves through the frame. When using continuous AF it’s usually best to set the starting AF point manually so the camera knows what the target is before it starts to track it. If you like shooting sport or fast action then this will most likely be the focus option that you make use of.
MANUAL AUTOFOCUS POINT SELECTION: This focus option allows you to manipulate the AF point you want while you look through the viewfinder. Setting the AF point yourself gives you ultimate control over where your camera focuses and it’s a good option for when you have time to compose the shot that you're chasing. Once you reach the AF point that is over your subject, you’re ready to focus and take the picture. You can move the slider over the image to see where the focus was concentrated in the image below using manual single point with AF point selection.
FOCUS AND RECOMPOSE: Although most digital cameras offer a collection of AF points so that you can select the one that sits over your subject, there may not always be one exactly where you need it. In these instances the camera ‘focus and recompose’ technique comes in very handy – and it can be quicker than selecting an AF point even if there is one over your subject. Imagine, for example, that the central AF point is selected, but your subject is off to one side of the frame. All you need to do is move the camera so that the AF point is over the subject and half-press the shutter button so that the lens focuses. Now, with the shutter button still half-pressed to keep the focus locked, recompose the image so that the subject is where you want it in the frame and press the shutter release home to take the shot. This also a useful focus technique to use in low light, as the outer AF points tend to be less sensitive than the central one. When using this camera focus technique, it is essential that the camera is set to single AF mode. If it is set to continuous AF, the camera will refocus the lens on whatever subject is under the active AF point when you recompose the image. You can move the slider over the image to see where the focus was concentrated in the image below.
BACK-BUTTON FOCUS: The standard way to focus a lens is to press the shutter release button half-way down. It's useful when using this back-button focus technique to photograph moving subjects so that you press the AF button without locking the exposure settings until the moment that you want to capture the image. This camera focusing technique allows you to see the subject sharp in the frame and only take the shot when the composition or lighting is as you want it for the frame. Using this technique means you won’t waste time while the camera is attempting to focus every time the shutter release is pressed and you can wait until the subject is in the right position to take the shot.
Focus options vary from camera to camera but for the most part you should be able to select the number of focus points in addition to the focus mechanic that you want to employ. For the most part, I tend to use 5-point / manual autofocus point selection / back-button focus because the majority of my subjects are in dimly-lit clubs so it helps me maintain more control over the focus within each frame.