A Quick Guide to Focusing
OK, I’m a guy who doesn’t always need to know how things work as long as they work. This means that when I take a photo, I don’t really care as much about the technical details of taking a photo as long as the end result looks good to me. So in keeping with this, today’s post will introduce you to a quick and dirty guide to auto focusing.
Knowing how to use your camera’s auto focus system is essential to taking great photos. So let’s get started.
Auto Focus control
First off, let’s take a quick look at a typical layout of the viewfinder of most Digital SLR cameras:
See all those little squares? Those are auto focus points that your camera is able to focus on automatically. What can be confusing with all those spots though is that it can lead people to believe any object under those spots can be in focus at the same time. Unfortunately, this goes against physics
SLR lenses can only focus within a limited range of distances from the camera at any one time. Objects within that range will be sharp (ie. in focus), and objects outside that range will be less sharp. The further the object is from that range of distances to the camera, the more out of focus it will be.
So when you point your camera and place your active AF spot over an object (usually the square that is lit up when you press the shutter), the camera will try to automatically select the focal distance for the lens so that whatever object is under the active spot is in focus. If multiple objects are present, the camera will usually try to focus on a spot in between in an attempt and get as much in focus as possible. This doesn’t always work though.
Aperture settings and focal distances
The problem though is that simply looking through the viewfinder will not let you see what is truly in focus and what is not when the shutter actually clicks. This is because the range of distances is controlled by the aperture setting of the lens. All DSLRs set the aperture of the lens to the widest possible opening until the shutter is clicked. This is to allow as much light into the viewfinder as possible and give you a bright view of the scene. When you click the shutter, the camera sets the aperture to whatever setting you have it set at (eg. F4 or F2.8 etc.) – and this aperture value in turn effects what is and isn’t in focus!
By placing the spot over a subject to focus on it and using a small aperture value, one can create photos like the one below, where the subject is clearly in focus and the background is blurred out.
Confusing I know but all you really need to know is 2 things:
- When you focus on an object, there is a range of distances that will be in focus. With a wide aperture (smaller f-stop, eg. f/2.8), the range of distance is small. At a smaller aperture (large f-stop, eg. f/11), this range is greater.
What exactly is this range? It depends on the lenses and with experience you should learn to gauge these distances just by eyeballing a scene.
What this means, is when you’re shooting at small f-stop values, remember to keep the objects you want in focus within the range of distances valid for that aperture.
- This range of distances starts and ends perpendicular to the lens. When you’re lining up your subject to be in focus, ensure that they’re perpendicularly equidistant to the camera so that they are all in focus. People forget this all the time when the subject is at an angle, so remember to keep it mind when you’re moving your subjects around for portraits.
So that’s all you need to know to get some well focused shots. It can be confusing but through some practice, I think you’ll find it easier and easier to focus more quickly and more predictably and shoot some great pictures.