How Focusing works
There is no such thing as a camera that will focus accurately every time you press the button. Even the most expensive cameras will miss focus. Understanding focus therefore will help you to take better photos.
There are many focusing systems, but at this stage you only need to know about two - contrast detection which is found in cameras without mirrors, and phase detection which is found in DSLRs. Very simply, a camera with contrast detection will look for differences in contrast on a surface by hunting backwards and forwards to find them. In poor light or on surfaces with little contrast this means a camera may sometimes hunt backwards and forwards continuously and not lock focus, which can be extremely frustrating if you're trying to capture action, wildlife, or that special moment. Cameras with phase detection use mirrors to split an image into two and analyse the contrast along the edges. Once the camera has determined the best focus, it then snaps straight there.
Both methods have their advantages and disadvanges. Phase detection cameras (DSLRs) are lightning quick but often miss focus, and they have problems either focusing just in front of or just behind where focus should be. Contrast detection cameras (mirrorless systems and compacts) may hunt around sometimes, but they are reliably accurate when they acquire a lock. Additionally, with advances in modern technology, contrast detection systems are now beginning to compete with phase detection systems for speed, and now they can even mix the two systems. Before much longer, mirrorless cameras will be just as quick as DSLRs.
Now that we know that focus is not a perfect technology, it is important to check important photos are in focus when we take them by checking them on the rear screen. This is known as chimping. Here I am chimping with renowned street photog Chris Porsz in Paris. Thanks for the photo Melanie! Oh, and don't listen to those who frown at chimping, if they want to go home with out of focus images, that's up to them. The only safe way to ensure you have an important photo in focus is to chimp. The technology is there so use it.
Cameras come with three focus modes, an auto single focus setting, an auto continuous tracking setting, and a manual focus setting. As most subjects don't move, the default camera auto single focus setting should be selected. This setting allows you to half press the shutter button to attain a focus lock before taking a picture. This method is without doubt the most widely used. You can even use this setting to lock focus on a subject and then recompose your shot before fully pressing the shutter button. The continuous focus or tracking focus setting is for moving subjects. Your camera will focus on a moving subject and track its movements to keep it in focus so you can continue to take photos. The last setting is manual focus, where you switch off auto and focus yourself. Most cameras have excellent technologies like focus peaking to help you. We will now look at these in more detail.
Auto Single Focus
This is typically what you will see through your viewfinder or on the rear display screen when using single focus. The white box is your focus area. The image is out of focus. When you half press the shutter button, the camera will then focus and the white box in the centre turns green. If for some reason the camera doesn't find focus, the white box on this camera turns red. Once the camera indicates that it has locked focus, press the shutter button fully and take your photo. If for some reason you take the pressure off the shutter button, the camera will lose its focus lock and you will have to do it again.
If you don't want your subject in the centre, there are two things you can do. First, centre on your subject, half press the shutter button to get a focus lock, and then move the camera to recompose your photo. As long as you don't release the pressure on the shutter button the focus will remain locked. Once you're happy, press the shutter button. Had I simply focused and pressed the shutter button in this photo, the background would have been in focus rather than the lovely Tina. With practice, focus and recompose becomes second nature.
The other technique is to move the focus box around. It's simple to do, but you may need to read the manual to find out how to do it with your camera. On mine I simply press a button on the back and then thumb the direction buttons until the focus box is where I want it. When your photo is composed, half press the shutter button again to acquire focus lock and then press the shutter button fully to take your picture. Once you're happy, remember to move the focus box back to the centre so you're ready for the next photo.
Auto Continous Focus
Often called tracking focus, this is a more difficult technique to master, particularly with wildlife. Auto tracking focus works differently from camera to camera but the principles are basically all the same. While your camera is continuously auto focusing it will attempt to track anything moving within the centre of the tracking sensors. The trick is to half press the shutter button on your moving subject to aquire focus and then while keeping the shutter button half pressed, track your subject in the viewfinder and the camera will keep it in focus. This isn't an easy skill to master and will only come with practice, so start with big slow moving things like cars until you are comfortable with it.
There will be times you will not want your camera to do any focusing for you, particularly when you're using a tripod. How you switch from auto focus to manual focus varies from camera to camera. If you're unsure how to do it, a quick glance at your manual will sort it out for you. Switch off image stabilisation when using a tripod.
When your camera is in manual focus, you will not be able to focus accurately simply by looking through the viewfinder and turning the focus ring on your lens because you will not be able to see any details. Therefore, cameras have manual focus aids built in. For example, you can zoom right in to 100% view to see details. Most cameras also offer focus peaking, which are highlights you see when your subject comes into sharp focus. Another way you can focus is to use auto focus and then switch to manual focus so the focus is locked. When would you use manual focus? Taking landscapes on a tripod requires manual focusing to ensure the focus is where you want it. Another time you might want to use manual focus is in situations where you would use what is called zone focusing, where you preset the focus beforehand and then wait for a moving subject to reach the point in your frame where your focus has been set. Sports and street photographers use this method frequently.
Credits - George with Chris Porsz, Melanie Barrett. All other photos copyright George Maciver, ScottishBrochs.com, all rights reserved.