Auto focus on Digital SLRs

Posted on November 29, 2011


Recently, I was asked by someone who was assisting at a shoot how I focus the camera because they saw a lot of extra movements by me and they noticed in previous shoots that I have a 99.9% in focus rate when I shoot. I use a Canon digital camera for most shoots, but I originated on older film based cameras. This entry will only concentrate on how I focus, and not where you should focus. This isn’t the only way to use an auto focus system, but it is the way I use it.

Let us start by explaining in the older days, we didn’t have an auto-focus. Instead, we had ground glass screens with prisms and micro prisms that you used to focus (similar to the ones today that are available as options on some camera models). One of the most popular types of screens was the split diamond or split circle. This screen worked by displaying two images and you had to align the two images to make one image. Once aligned, you knew the focus was set. The split diamond or split circle was always in the center of the viewfinder area. Don’t think that this was an error free process. Typically, in low light, only one of the images would show and the other would be black. In bright light, just the opposite occurred: one image would be all white, and the other either dark or barely viewable. You always made the best of what you got.

Today, with digital cameras, there is an auto focus. Just point the camera, it focuses, and there you have it. The camera has done all the hard work for you… but why is the image not in focus? The reason is that each camera is programed to focus in a certain way. For instance in point and shoot cameras, they frequently have face recognition and will make sure that all the faces in the image are in focus.  But, what if we didn’t want that face in focus? There is a reason why this happened: you relied on a program that guesses to do the focusing.  Why is this?

Today’s digital cameras have “focus points” in the viewfinder or view screen. The more expensive the camera and the more modern the camera, the more focus points there are (camera phones typically have one to high end digital SLRs with 45 or more). In some cameras, you can also choose a single focus, continuous, or an intelligent tracking focus. Single focus only focuses once, and stays there with a continuous half depression of the shutter release button. Continous is constantly focusing and never stops until the shutter is fully depressed. The intelligent tracking focus varies from manufacturer to manufacturer, but works on a program that predicts where the focus is going and attempts to keep the object in focus. When you focus, the program uses the information from these focus points, the focus mode you choose, and chooses what it thinks is best for focus from those focus points. That may or may not be where you want focused. Somewhere, at least one of the dots, were most likely in focus. This brings up a point: which dot is being focused on?

Let’s go back to the old method: there was one place where the focus could always be determined. I highly recommend that you choose one point, and set that one point to be the focus point. I know many people say to use the focusing, you need to use all the dots. My $0.02: keep it simple. To keep it simple, you only have one and only one point to adjust focus.  Once you have the one point defined, you simply focus at that point, keep the shutter partially depressed, recompose for the image you want taken, then depress the shutter fully. It works. It is easy. It is simple. For sports, you probably would be better off using the intelligent focus option for your camera and using all the focus dots. This allows the camera to predict where the action is going, and adjust focus appropriately.

This should help you understand why your camera focuses the way it does, and what you can do to control that auto focus system.