When making the change from a smartphone or a compact camera to a DSLR, having a comprehensive understanding of how your camera’s autofocus (AF) system works and how you can use it are the first steps to controlling the quality of your image.
A camera's AF system intelligently adjusts the camera lens to obtain focus on the subject. An AF system consists of focus points which can be configured to adapt your shooting style to your subjects. This allows a greater freedom of photography styles and techniques when different autofocus modes are used, while ensuring reliable performance in every situation.
Focus points refer to the points within the camera’s frame that the camera uses to lock onto the subject being captured. When the shutter-release is pressed halfway, the DSLR usually emits a “beep” and some of the focus points will light up in the viewfinder or on the LCD display while using Live View mode. This means that the DSLR’s AF sensor has automatically identified the areas which will be focused on.
You can choose to use a single focus point to hone in on an exact spot on your subject, or use multiple or all focus points to capture moving subjects. The central focus point is the most sensitive and accurate of all and exists in the centre of the viewfinder, great for use in varying light conditions. It is also most suitable for when the object of interest is in the centre of the frame.
Entry-level cameras typically feature less number of focus points in the AF system. On the other hand, DSLRs catered to professional photographers have many focus points that are strategically positioned, including the far left- and right-hand sides of the image frame. This difference in the positioning of focus points can be seen through the viewfinder.
A DSLR equipped with a higher number of focus points provides wider coverage, minimising the odds of your subject being out of focus, and tracking subjects even as they move toward the edges of the frame.
The difference is best observed when shooting with telephoto lenses, where a higher percentage of sharp images are achieved during longer bursts.
AF systems feature three types of focus points – vertical, horizontal and cross-type:
• Vertical and horizontal sensors are one dimensional and only detect a horizontal or vertical line
• Cross-type sensors are two dimensional, taking into account both vertical and horizontal lines, making them much more accurate
Cross-type sensors are especially useful when shooting fast-moving subjects typical of sports photography, but other situations also benefit from utilizing these sensors. Portrait, family, and wedding photographers often use the focus-and-recompose method to nail focus with a cross-type sensor, followed by shifting their camera’s field of view to get precisely the composition they want.
Dynamic Area AF, one out of multiple AF Area Modes available in Nikon DSLRs, allows you to select a main AF point with the surrounding focus points as backup, a significant advantage when shooting moving subjects.
Nikon’s AF system provides several options – 9, 11, 21, 25, 39, 51, 72 or all points up to 153 (varies depending on DSLR model) for a wide variety of shooting situations. The resulting images are crisp, sharp and highly-detailed every time.
Generally, it is advised to choose a higher number of focus points for low-contrast subjects if the camera has difficulty focusing, or for active subjects that are hard to keep framed in a single point. Photographers can increase the number of focus points if the subject is moving unpredictably or occupies a large area of the frame, or when the subject is at the edge of the frame.
For example, the 9-point option is recommended if you want to focus on a specific subject in the frame, such as a single athlete in a group of players, or the eyes of your model in a portrait.
Alternatively, a 25-point dynamic-area AF tracks a wider area in your frame; for instance, a figure skater executing a move.
A 51- or 72-point dynamic-area AF may be preferred if a background contains objects with regular, detailed patterns, like a billboard or the stands.
Nikon’s 153-point AF system covers a wide area across the frame, with minimal spacing between each point, including 99 cross-type sensors for improved subject recognition. This is particularly effective even when tracking subjects with erratic movements that veer toward the corners of the frame.
You can always refer to the top control panel and viewfinder of your Nikon DSLR to ensure you are using the right AF-area mode.