Today we’re going to be talking about auto focus. Specifically how it works and what modes are available.
Auto Focus Modes
There are typically three basic auto focus modes available on most cameras, Single (AF-S Nikon & One Shot AF Canon), Continuous (AF-C Nikon & AI Servo Canon), and Auto (AF-A Nikon & AI Focus AF Canon). These modes can generally be selected by pressing down your auto focus button and spinning one of the selection dials, but check your manual for details.
Single mode is best for subjects that are stationary. It will find its focus point then lock into place. If you or your subject move after the focus is locked then your subject will no longer be in focus.
Continuous mode is for moving subjects. The camera will focus on a point in the image and the try to stay focused on that point even when you or the subject move.
Auto mode is a hybrid of both single and continuous where the camera tries to figure out which mode is appropriate for your scene.
Typically I keep my camera set to single mode, since I want to know where the picture is focused and not have it change on me. I only switch to continuous mode when I know I will be shooting a subject that is going to be moving around a lot.
Auto Focus Area Modes
Along with choosing the auto focus mode, you can also choose the auto focus area mode which determines which auto focus points are used when focusing. These modes generally need to be changed in one of the internal menus, check your manual for specific instructions.
Single point mode allows you select a single auto focus point which will be the only active point. This is best used for stationary subjects.
Dynamic area is a hybrid version of single point in that you can select a single auto focus point. If you are in continuous (AF-C) or auto mode (AF-A), information from the surrounding points is used if the subject leaves the area. In single mode (AF-S), only information from the selected point is used even if the subject leaves the area. This mode is best for a subject that might move suddenly like children or pets.
Auto-Area is where the camera tries to detect your subject and automatically selects the auto focus points that should be used to focus that scene.
3-D Tracking is another type of hybrid selection mode that is similar to single point, in that the user selects a single point. When you focus on a subject then recompose the scene, the camera will track the subject and switch auto focus points to properly track your subject.
I often leave this set to Auto-Area, but will switch to single point mode when I want to specifically select my focus point.
In this technical section, I want to talk a little bit about how auto focus functions. Knowing how it works can help you figure out why your camera may be having trouble auto focusing and let you figure out what to do about it.
Contrast assessment works by evaluating the contrast in the image, moving the focus closer or further away, and then reevaluating the contrast. If the contrast increases, the camera continues to move the focus in that direction until contrast starts decreasing. At this point, it backs up to the maximum point of contrast.
This is typically the type of auto focus that is used in live view mode on SLRs and in mirrorless or point and shoot cameras. In live view, there is generally a marked rectangle or square, this is the only part of the image where contrast is checked. This is to let you choose the auto focus area and to limit the amount of processing the camera needs to do.
One of the major drawbacks to this type of detection is that it tends to be slow. It also tends to need decent lighting and may not work if the spot you’re looking at doesn’t have good contrast. Some of the big advantages of contrast detection is that it’s a simple form of detection and requires no additional sensors so is generally less costly. Since it requires no additional sensors with their own light paths it also tends to be a more accurate form of detection.
Phase detection uses the predefined auto focus points in your camera, it works because when a point is in focus, the light rays coming from it will equally illuminate opposites sides of the lens (in phase). If the point is not in focus, then the light will be in different positions (out of phase).
Detecting if a point is in or out of phase is accomplished by using a system of mirrors, lenses, or prisms to split the rays coming from opposite edges of the lens into two separate rays. If the image is in focus then both beams will strike the image a certain distance apart. If this doesn’t make much sense take a look at the diagram below.
Phase detection is the default of all SLR cameras. This type of detection is happening at each of the auto focus points. One interesting thing about this type of auto focus mode is that it tends to be more accurate when you have a wider aperture lens on the camera.
One of the drawbacks of phase detection is that it requires a large amount of physical and software calibration to work well. If the calibration is not done perfectly then the focus will not be correct. The big advantage of this type of detection is that it’s very fast. The camera can also use the array of focus points to detect if the subject is moving and adjust the focus appropriately.
End Technical Section
For those of you that are new readers, I don’t often have a technical section that is quite this dense, but how auto focus works is a very technical subject. If you would like to read some of my other topics check out my learning photography page.