How to Use Autofocus Effectively

*This post is going to be aimed at DSLR users, so all apologies to point and shoot owners (although parts of this may apply to you).

Autofocus Modes: Single vs. Continuous

Autofocus settings can be confusing, because they repeatedly used the same words in different combinations.  Let's start with autofocus mode, which controls how often the camera attempts to autofocus.

Single servo means that when you press the shutter button halfway down, the camera will focus once, and hold that focus as long as you hold the shutter button halfway down.  This is good for off-center compositions, since you can focus on your subject, and then reframe your shot without losing your original focus.

Continuous servo (AI servo for you Canon users) means that when you press the shutter button halfway down, the camera will focus, and keep tweaking the focus as long as you hold the shutter button halfway down.  This is useful for maintaining accurate focus on moving subjects.

On more expensive DSLR's the focus mode switch tends to be on the front of the camera next to the base of the lens.  For example, on my Nikon D200 the available choices are "S" (single servo), "C" (continuous servo) and "M" (manual focus).  On entry-level cameras, this may be a menu setting instead.

BTW if the word "servo" is driving you nuts, just think "lens motor" instead.

Autofocus Area Modes: Single vs. Multiple

Very similar name, very different function.  Autofocus area mode controls WHERE the autofocus tries to focus.  Although the names can vary widely from brand to brand, they ultimately boil down to two types: single area and multiple area.

Before we get to that, look through the viewfinder of your DSLR.  You should see a matrix of small brackets or cells, as few as 3 or as many as 50.  These are the available autofocus areas for your camera.

In single area mode, only one of these autofocus areas can be selected at a time.  By default this is the one in the dead center of the frame, but you can cursor over to other brackets using the directional button on the back of the camera.  When you try to autofocus in single area mode, the camera will only attempt to focus on the one focus area that you have selected (if you're not sure which focus area is currently selected, it generally flashes red when you attempt to autofocus as a reminder).  The advantage of single area mode is it's predictability, although it can be slower since you must either move the focus area or "focus and reframe" for off center compositions.

In multiple area mode, the camera looks at all the autofocus areas and tries to decide where you think the subject is.  Obviously this can be hit or miss, but it's fast and can evaluate the entire frame at one time.  This is good for erratically moving subjects, quick-draw shots where you don't have time to focus, or "from the hip" shots where you can't see the viewfinder.

Autofocus area mode may be controlled by a switch or menu, depending on your camera.  Read your manual! :)

Focus vs. Release Priority

Another important autofocus setting.  In focus priority, the camera will not allow the shutter to release until autofocus "successfully" focuses on something (even if it wasn't your intended subject).  In release priority, if you press the shutter button all the way down, the shutter releases, in focus or not.

I spend all my time in focus priority, because it reminds me not to shoot if I can't establish a proper focus.  Release priority is good for "once in a lifetime" shots, where you won't get a second chance and you need to get SOMETHING, even if the focus isn't perfect.  However, release priority shoots no matter what, so you may get surprised by an image that looked in focus through the viewfinder, but wasn't quite so sharp when you got back to your computer.


Other Fancy Autofocus Stuff


Some cameras feature other specialized autofocus modes.  Predictive or tracking focus tries to anticipate the progress of a subject moving towards or away from you (such as a car) and adjusts the focus ahead of the object so that it will be properly focused by the time the shutter releases.  Some cameras also feature face recognition so that the camera knows to focus on the people instead of the background.  Another common one is "closest object" mode, which pretty much describes itself.  There are also a handful of other focus modes that may be particular to a certain camera, but they should all be a variation on or combination of one of the basic types listed above.

Autofocus (AF) Explained

Autofocus is just what it sounds like: the camera focuses the lens for you.

Passive vs. Active

Autofocus systems are broadly categorized as "active" or "passive".  

Active autofocus systems do not use the light coming through the lens to determine correct focus, instead they have a separate mechanism for determining the distance to the subject.  Some active autofocus systems use infrared light from two separate sensors to triangulate the position of the subject, others have sonar to "echo-locate" the subject like a bat.  Pretty slick, huh?  Well, active auto-focus is not commonly used in modern cameras.

Instead, most cameras now use "passive" autofocus.  All this means is that the camera analyzes the image coming through the lens to determine correct focus.  There are two ways this can be done, "phase detection" and "contrast measurement".

Phase Detection vs. Contrast Measurement

In phase detection, the camera uses mirrors and prisms to split the light coming through the lens into two similar images.  Because of the way the prisms are positioned in the camera, the images are focused slightly differently from each other.  The camera then compares the images to calculate the correct distance to the subject and focuses the lens.  Phase detection tends to be faster than contrast measurement and is the most common autofocus method used in DSLR cameras.

In contrast measurement, the camera analyzes the boundaries between light and dark areas of the image to determine correct focus.  While the image is out of focus, the boundary between a light and dark area will appear as a gradient (i.e. gradual change in brightness from one pixel to another, low contrast).  When the image is in focus, the boundary will be sharp (i.e. quick change in brightness from one pixel to another, high contrast).  The camera adjusts the focus until the image has the highest possible contrast, which corresponds to the edges in the image being in focus.

This is the method used in digital point and shoot cameras, most video cameras, and DSLR's when using the "Live View" or "Movie" modes.  Contrast detection is traditionally much slower than phase detection, although this technology is quickly improving.

Some Light Please?

As I mentioned above, almost all digital cameras use some form of passive autofocus.  Since passive autofocus relies on the light coming through the lens, the autofocus system can really struggle in dark environments.  To overcome this, many cameras have an AF-assist lamp, a LED that turns on whenever autofocus is engaged in a low-light situation.  However, autofocus still does best when the subject is brightly lit.  Autofocus can also struggle if your subject is strongly backlit (the sun is directly behind them), as it will struggle with the comparably dim light on your intended subject.

Not So Good Up Close

Autofocus also has trouble with very nearby subjects.  This is more of a concern for "macro" or close-up lenses on DSLRs, and also for point and shoot cameras which are sometimes able to focus from only inches away.  I couldn't tell you exactly what causes this problem, but it is a common and predictable problem for macro photography.  This is a great time to use manual focus!

Better Than You or I

Despite its limitations, autofocus generally focuses more accurately and consistently than people do, especially when it is used properly.  Most modern cameras are not optimized for manual focusing, and unless you have a big, bright viewfinder and excellent eyesight, you're probably better off using autofocus like the rest of us.

Up next, how to use autofocus properly!