Despite modern advancements in photography making it easy for every man, woman, and child to get their Annie Leibovitz on, taking breathtaking photos isn’t always quite as simple as just hitting the shutter button. To make the most of your camera, there are a wide range of prerequisites, skills, and advanced functions you should understand in order to get terrific-looking photographs. Whether it’s the successful staging of your desired picture, choosing the right lens, or simply figuring out how to set white balance, proper photography requires more than just an artful eye.
Perhaps one of the most important steps to taking beautiful photography lies with understanding how your camera’s focusing system works. All cameras — whether you own a high-end or an entry-level interchangeable lens model — feature a collection of autofocus (AF) sensors and processors that work tirelessly to focus on a subject or object. To help you get a better understanding of autofocus systems, we’ve put together this simple rundown on how these features work. We can’t guarantee reading our guide will allow you to achieve National Geographic acclaim, but it’s a great reference for any amateur or novice photographer, and will definitely help you capture better photos.
How do autofocus sensors work?
First and foremost, it’s critical to learn exactly how a camera’s autofocus sensors work. The sensors serve as the driving force behind clear, focused pictures. Each camera sensor measures a subject or object’s relative focus by determining changes in contrast across areas of the desired image for the highest possible sharpness and contrast.
During the autofocus process, after you half-press down on the shutter button, a camera’s AF processor makes minor alterations in its focus distance, which relay focus improvements to the sensor. After assessing the focus improvements, the processor then sets the camera’s lens to a different distance, continuing to assess the improvements and make changes until it discovers optimal focus. The process may sound exhausting, but thankfully, cameras can carry it out in the blink of an eye.
In some occasions – say, when you attempt to capture challenging landscapes — the camera enters into a bout of what’s called “focus hunting.” Focus hunting occurs when the autofocus processor is unable to achieve a desirable focus setting, and resorts to focusing on different regions of the frame over and over again. If this happens, use a different aperture, or simply move the camera slightly reframe your desired shot.
Phase detection and contrast detect systems
Interchangeable lens cameras – DSLRs and mirrorless Compact System Cameras (CSC) – use either one type of focusing system or a combination of the two. Without getting too complicated, they are called phase detection (PD) and contrast detect (CD). PD is used in all DSLRs and features a combination of mirrors and sensors to sharply focus the subject. CD, found in mirrorless cameras, measures the change in contrast in the frame to achieve focus. PD is considered superior by many photographers for speed and sharpness. That’s why many mirrorless manufacturers use a hybrid system that features both.
Understanding autofocus points
While every camera features an autofocus sensor and processor, the range of specific autofocus points varies from model to model. A camera’s autofocus accuracy – and pure capability – greatly depends on the location, type, and outright number of autofocus points relative to its model. For example, top-tier DSLR cameras tend to have roughly 50 or more different autofocus points, whereas entry-level models may feature just one, central autofocus point.
High-end DSLRs typically feature cross-type sensors that cover most of the desired image, allowing for the highest accuracy. Surrounding these cross-type sensors are a series of lower accuracy, vertical line sensors that provide one-dimensional detection. Though these are inferior to the cross-type sensors, they complement the framed image to help achieve the best possible focus.
Lower-end cameras also feature cross-type sensors, though the flexibility – and sheer amount – drastically differs compared to the above mentioned. Most entry-level models tend to feature just one cross-type sensor for f/2.8 apertures, and lose the functionality of these sensors with smaller apertures. One way to assist with the focus process on these cameras is to initially use the maximum aperture afforded, even if you plan on snapping on a smaller aperture. Though not a definitive solution, this does tend to help the smaller aperture achieve a desired focus even though it lacks several autofocus points. Moreover, using the central autofocus point — with either a high-end or entry-level SLR — on objects outside of the center of your desired image helps achieve a better focus lock for the entire photograph. Simply point the center of your camera at an off-center object, allow the autofocus sensors to find a desirable focus level, then reframe your camera back to the original position.
Unless you prefer shooting moving objects most the time, chances are you’ll likely use one-shot focusing while taking pictures. Because its forte lies with still objects, using one-shot focusing tends to stumble during attempts at snapping any moving object. Nearly all cameras require an achieved focus lock before taking any photographs using one-shot focusing.
Most cameras also offer a type of continuous focus mode which continuously tracks moving objects, while simultaneously adjusting its focus distances accordingly. Depending on which brand of camera you own, this function may go by several different names. For instance, Nikon calls this feature its Continuous focus, while Canon refers to it as AI Servo focusing.
Continuous focus functions by anticipating the movement of objects in the frame, based on the movement speed and location of the object at prior distances. To account for its own lag in shutter speed, the camera uses its prediction of the object’s location to focus on a spot in the frame where it intends the object to be. Using continuous focus greatly improves your chances of shooting clear pictures of moving objects.
What’s an autofocus assist beam?
Though it slows down the autofocus process a bit, an autofocus assist beam is an incredibly useful feature found on many cameras. This feature uses a visible or infrared beam to help a camera’s autofocus sensors recognize and identify objects in the frame. Autofocus assist beams greatly improve the focus function for pictures of objects or landscapes taken in poorly-lit locations, or when the contrast hinders the camera’s basic autofocus process. It’s also recommended to only use the autofocus assist beam on objects that stand completely still, as the assist beam typically struggles with locking focus on moving landscapes.