April 8 2016
Autofocus in DSLR cameras is a valuable tool, but photographers need to choose the right AF mode and configuration to ensure the area of interest is in focus.
The sophisticated optics and large digital sensors in DSLR cameras offer greater potential for sharp images than phone cameras, requiring accurate autofocus (AF) systems. This is balanced by the need when photographing moving subjects for fast AF.
This is a combination of the two main two AF modes. The camera starts off in Normal mode and if the subject moves fast enough it switches automatically to the Continuous focus mode.
This is the general purpose AF mode covering most situations where the subject is stationary or slow moving.
While taking photos using the optical viewfinder DSLRs use a phase detect AF method. This is significantly faster than the contrast detect method used in live view mode and phone cameras. The fast AF means the delay between pressing the shutter button and taking the photo is almost imperceptible.
When the shutter-release button is pressed halfway, the AF system is activated and chooses from a number of focus points, focuses the lens, and then locks the focus until the shutter button is released. Keeping the shutter button halfway depressed locks the focus and allows photographers to recompose the shot.
In normal AF mode, the camera will not shoot until focus lock is achieved. This is to help make sure every photo is focused.
Where the subject is not obvious, or the photographer wants a specific part of the image in sharpest focus, the photographer has the option of selecting which focus point to use, rather than using the camera’s choice. AF systems have a number of sensors because the main point of interest is not in the same position in every scene.
This is the most complex and varied implementation of Autofocus, with a large difference in capabilities between cameras and manufacturers.
In high-speed action shots, even the almost imperceptible DSLR shutter lag in normal focusing often results in subjects that are out of focus, or missing the key moment. The autofocus system starts to work when the shutter-release button is pressed halfway down, and while the button is kept half pressed continues adjusting the focus as the subject moves.
Advanced DSLRs often have a separate AF button conveniently placed on the rear so photographers operate it with their thumb. This button activates the autofocus system independently of the shutter button. This is much easier than trying to keep the shutter release button half pressed while tracking a moving subject.
Photographers have the option in Continuous AF mode to allow the shutter to fire even if the image is not in perfect focus.
If the subject briefly moves away from the chosen focus point the camera will use information from surrounding points to try and keep the subject in focus.
Photojournalist cameras such as the new Canon EOS-1D X Mark II have a range of settings in their Continuous AF modes to suit the type of motion expected in the scene. For example, there is one labelled for photographing tennis players that continues to track subjects while ignoring possible obstacles that may briefly come into view. Another choice is for figure skaters, or other subjects, where they change direction suddenly.
The main driver for the continual improvement in AF tracking ability is the development of faster and more powerful microprocessor chips for digital cameras. These enable the camera to process the increased data from more AF sensor points while keeping the lens focused on a fast moving subject.
In this mode, the autofocus system acts as an electronic rangefinder. The focus confirmation indicator in the viewfinder still operates when the lens has been manually focused on a point.
Despite all the computing power and multiple focus point in some difficult situations where AF systems do not work, although these are getting fewer.
When the shutter release button is half depressed the position of the active AF sensor, or sensors, is indicated in the viewfinder.
The AF system’s first choice for the focus point is in the centre of the view. This is not always the same position as the main item of interest. This difference can be an issue depending on the depth of field. This means the areas just in front and behind the point of focus appear reasonably sharp, the further you move away from the focus point the apparent sharpness decreases.
There is more than one way around these situations. One is the option of selecting another AF sensor as the main focus point.
Another is to move the camera so its choice of focus point is aimed at the main item of interest and keep the shutter release button half depressed. This locks the autofocus at that distance, then recompose the photograph and fully depress the shutter release button to take the photo.
When you buy a new DSLR you really do need to read the AF section of the manual.
Manufacturers keep updating the AF system of every new model DSLR. Couple this with the wide variations in AF capabilities between DSLR models means you need to understand the fine detail to get the best out of the AF system. Because of these variations, this article is only an overview of general terms. The aim is to help you understand the basic concepts of AF systems to help you interpret the camera’s user manual.
Don’t just leave it on automatic AF and hope, take control of your camera.