Focusing – The Basics – Technical Fundamentals

Focusing and Autofocus

Focusing an image is pretty important if you don’t want to look like a complete idiot – a bit of blur is acceptable now and again, but if your work is continually soft around the edges, people will start to talk! Thankfully, nowadays, focusing is almost always done automatically, and it is usually very accurate, quiet and fast. Many autofocus lenses also offer the ability to focus manually as well, should you feel the need, but you’ll find autofocus is OK for 99% of the time – I certainly do.

Autofocus (AF) has 2 main modes, continuous and single shot. Their precise names will vary from camera to camera, but the functionality will be the same. Single shot means that the camera focuses on the subject, will only re-focus if you press the shutter (or focus activation button) again, and will generally only allow you to take a shot if the image is in focus. Continuous mode will keep focusing and re-focusing as long as you have your finger on the shutter (or focus activation button). Single shot is ideal for static subjects, as you’ve got a better chance of what you want being sharp, whereas continuous is perfect for things that move around a lot, as it allows you to keep moving subjects in focus.

Continuous focus will often allow you to do “focus tracking”, whereby you keep your finger on the shutter or AF-activation button, and the AF motor will try and keep a specific subject in focus as it moves around the frame.

Focusing with the “Back Button”

Why do I keep mentioning this “focus activation button”? Well, many cameras have a 2nd button that can operate the focus besides the shutter, and many photographers (including myself) use that rather than the shutter. There can be many advantages to separating out the actions of taking a picture and focusing, although at first it may seem like an inconvenience. For the most part it allows you to choose when to take the picture, rather than waiting for the autofocus to allow you, and to compensate for any delay that may occur between seeing a shot, pressing the shutter, the lens focusing, and the camera actually taking the picture.

One example, which I’ve had to shoot many times, is catching a golfer swinging, and trying to shoot very close to the time they make impact with the ball. Even at 10 frames per second, the speed of a golf swing means that I often won’t catch that precise moment, so it’s a case of simply timing it right. My response needs to very quick indeed – we’re talking fractions of a second here! If I focus and shoot with the same button, then at the precise moment I decide I want to take the picture, the camera will start to focus. No matter how sophisticated the AF system is, that will add a small delay to the procedure as the camera and the lens try to ensure the image is sharp. If I have focusing and shooting as 2 separate actions, then I can prefocus on the golfer before they start their swing (they’re not about to start running around at this point, so I can be confident it won’t go out of focus) and then fire the shutter at the precise moment. The result is much less delay, and a better chance of capturing the right moment.

Focusing - the importance of back button focus
A good situation for back button focus.

Focus Points

If you’re using a modern camera, you’ll instantly spot lots of little squares or rectangles in the viewfinder. These are your focus points. In the original days of autofocus, the camera and lens could only focus on one point – usually the dead centre of the viewfinder – and the AF would only work on whatever happened to be on this spot. If what you wanted to shoot was off-centre, you either had to use manual focus, or move the frame until the subject was under the centre point, then recompose and shoot. Some of us (me) still do this quite a bit, as we’re very slow to learn!

Nowadays, you’ll see most cameras have loads of focus points. This not only saves you from having to recompose your shot if the subject is off centre, but means that the AF can track any moving subjects better. Broadly speaking, the more focus points you’ve got in your camera, the more capable the AF will be. Many cameras now have 50+ points, although you’ll find you can select varying numbers of them to be active at any one time, and fine tune how many will activate as subjects move – all that sort of clever stuff which you should read your specific camera manual for information on! I’d suggest you use them in the way they’re designed, and if you’re shooting something off centre, select the most appropriate focus point, rather than reframing to use the centre one. Moving the frame each time introduces a risk that you’ll actually be out of focus when you shoot, particularly if you’re already shooting with a shallow depth of field.

The precise number will vary from camera to camera, but generally speaking, the centre point, or points, tend to be the most accurate and sensitive, so if you’re tracking something fast moving, you may find you get better results if you try and keep it towards the middle of the frame.

Focus on what you want to focus on.

Make sure the thing you want to focus on is what you want to focus on. With complex subjects, and lots of AF points, the camera can easily start to focus on something in the frame that’s not what you intended, so pay attention. You may need to override AF sometimes, either by shifting focus points, or by switching to manual. Many modern AF lenses on DSLR’s will allow you to simply grab the focus ring and yank it round, and assuming you’re using back button focus as I described earlier, you know the lens will stay focused on what you choose, not what the camera does.

As an example of when you might need to use manual focus – you could have your camera on a tripod to shoot a landscape shot, but the precise area you want to focus on lies outside the range of the focus points in your camera. You can’t re-frame to use one of the points, as your camera is locked in place, but each time you press the shutter, you end up focusing on the wrong bit – here is a perfect time to use manual focus and over-ride the camera’s AF.

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