The key to digital bliss – Understanding Autofocus
Controlling autofocus is one of the most essential functions of any digital camera. Few things are as frustrating as going for a nice portrait or a potentially spectacular wildlife shot only to realise a second later that it’s all blurred and you’ve missed your chance.
Before we continue, we need to clear up a frequent source of confusion: there are two kinds of blur in photography.
Motion blur is something that happens in time and is a creative side-effect of your shutter speed. It has nothing to do with your lens and your focus.
Blur caused by lack of focus happens in space. If it’s a desired, creative effect we tend to call it depth-of-field. It it’s unwanted it’s just a blurry image. This has everything to do with your lens and your focus.
Autofocus can only help with the latter.
WHY DO WE NEED AUTOFOCUS?
For one simple reason: a lens can only be focused at one distance at a time. This is how your eye works (and if you’re wearing glasses like I do, you’re reminded of how your eyes can fail to focus every time you’re looking for them) and the lens on your camera is no different.
Lenses made for manual focusing have a very precise focus ring with markings most of the time both in metres and feet. Older, pre-digital cameras have viewfinders that help you achieve precise focus. Many of them have a split prism in the middle: if you look at a vertical line through your viewfinder the line will be split in half if it’s out of focus and will appear joined when it’s sharp.
Digital cameras and the lenses made for them are simply not made for manual focusing. On consumer, smaller sensor DSLRs the viewfinder doesn’t magnify your image to much. It appears quite small and there is no prism to help you focus. And while modern lenses provide us with stunning image quality, their manual focusing rings do not offer precise control and often even lack focus markings.
DSLRs are made for autofocus.
The three basic styles of focus rings. A Leica stills camera lens with precise focus markings and depth-of-field indicators. A Cooke lens used by cinematographers: see how huge and incredibly detailed the focus marks are. Made in heaven for the utmost in precision and fine control over focusing. And finally, one of the basic kit zoom lenses for current DSLRs. A plastic ring with no distance marks whatsoever. Okay for autofocus but utterly unfit for manual control.
HOW TO USE AUTOFOCUS?
There are two separate aspects of autofocus that you need to control. First, you need to select the autofocus area that you want to use. DSLRs always confirm the AF point or bracket selected by flashing a red dot or a small rectangle at the chosen AF area. Second, you need to choose the AF mode you want your camera to use.
1. PICKING YOUR AF AREA – SETTING IT TO SINGLE POINT
All brands do it differently.
On Canon DSLRs the AF point selection button will be under your right thumb, it’s the top right button at the back of the camera, the one with the icon of the frame with five small dots in it. Push it and pick your point either with your dial or with your arrows.
On lower-end Nikons you need to set the AF area to single point using your back display (push the ‘Info’ button at the bottom left or the one with the green dot) and then you can move the point around with your four-way dial.
On higher-end Nikons there will be either a dedicated button or the AF/M switch may also work as a button. On Nikons you need to hold down these buttons and then dial to change the AF area mode to the single point setting. Move the point around with your arrows.
2. SELECTING YOUR AF MODE
Again, different brands call it different names, but it works the same way.
Canon calls it ONE SHOT and AI SERVO. Nikon and most other brands call it Single AF (AF-S) and Continuous AF (AF-C).
The difference is straightforward. AF-S means that when you push your shutter release halfway down, your camera will focus on whatever is inside your AF point. And as long as your finger stays on the shutter release, your focus will be locked. Which means that you can reframe your shot by panning your camera left or right or tilting it up or down. As long as you do not change the distance between you subject and yourself, your image will be sharp. [illustration: perhaps at tiny video or even an animated gif of a photographer doing this would be cool]
This is perhaps the most efficient and widespread way of focusing.
AF-C means that as long as your finger is on the shutter release, the camera will try to focus on whatever is in your AF point continuously. It’s been developed for photographing moving subjects. Sports, wildlife and sometimes even street photographers use it. However, it has its obvious pitfalls. It may fail to provide good results with an unpredictably moving subject or if you fail to keep your subject in your chose AF point. Play and experiment with it if you want to, just don’t expect miracles.
Autofocus will always need your help.
Don’t forget: the camera cannot read your mind. Only you can make the decision of what you want to focus on! Your camera can only help you by using its internal computer to ‘look for’ sharpness (i.e. clear edges) inside the AF point that you have chosen.