Autofocus Lenses: What They Are and How They Work

Autofocus Lenses: What They Are and How They Work - Rokinon

A sharp image shows the professionalism behind your photography. You can purposefully blur parts of an image for artistic effect, but you don’t want to blur your subject. Most modern cameras come with an autofocus (AF) lens, so whether you are using a DSLR or your smartphone, you’ve probably used autofocus many times. If you’ve been curious about what autofocus lenses are and how they work, you’ll find much to learn, including the many differences between AF lenses and manual ones.

Having a better understanding of your autofocus lens mechanics will give you the tools you need to take your photography game to the next level. Whether you have a lens you’re not sure how to use or you’re in the market for a new one, you’ll want to know whether it uses phase or contrast detection, as well as how many sensors it uses. Keep reading to get to know your camera.

What Are Autofocus Lenses?

The first autofocus camera was released in 1977 by Konica, with Polaroid releasing the first AF lens in 1978. Lens technology has advanced significantly since modern SLR and DSRL cameras were invented. Developing blurry photos is practically a problem of the past with autofocus lenses. Designed using periscopes developed for military strategy, AF lenses rely on dual points that aim toward the desired object to deliver a more accurate, or clear, shot.

Autofocus is an algorithm or system that takes the pressure off when you want a perfectly focused photo. With this software, your camera does all the work to figure out how to focus on the subject in your frame. The lens scans the scene and automatically identifies the subject on which to focus.

How Do Autofocus Lenses Work?

Active AF System vs. Passive AF System

Autofocus lenses have two types of systems: active and passive. Passive is the standard and most efficient type of lens software, and it is used in DSLR and smartphone cameras. Using phase or contrast detection, the software uses mirrors or algorithms to compare a set of images and determine where they match up. This method tells the camera where to move the focusing ring, so you get a consistently clear photograph.

Phase detection tends to be better at tracking fast-moving subjects because the system works quicker. With phase detection, light bounces off mirrors that quickly adjust to keep the image in focus. Still, contrast detection uses algorithms that probe for edge detail, which can take longer in some cameras. It works best for high contrast photos, in which the algorithm scans the pixels in the camera’s sensor.

With the active system, the lens shoots a beam of red light at the subject which bounces back to the camera to inform the device of the object’s distance from the lens. The camera uses this information to adjust the lens and focus the image. This method is useful in dimly lit environments where standard (passive) autofocus is not working. A downside to active AF lenses is that you can only use them for stationary photography. Active autofocus is also best used for photographing subjects near you.

AF Sensors

Sensors are one of the things that make autofocus lenses work. Manufacturers build AF lenses with auxiliary sensors that detect light coming from opposite sides of the lens. The number of sensors in a lens varies by model and by the number of autofocus points in the camera. Manufacturers place at least two sensors on each point, either horizontal, vertical, or cross-type. The quality of a lens’ autofocus improves with the quality of its sensors.


Your autofocus will work best if you are photographing something boldly different from its background. The autofocus function tends to give up when you take pictures of plain blue skies or blank walls. Typically, autofocus chooses to focus on the object in the middle of the frame and fades everything else into the background. Some AF lenses can pick out eyes and faces and choose to focus on these details. In general, autofocus gravitates toward edge detail, meaning the camera will bring the most striking features into focus.

Autofocus vs. Manual Focus Lenses

Manual Focus Lenses

With manual lenses, you must guess or measure the distance between you and your subject to make sure you’re using the right lens and adjusting the focus correctly. If someone moves, you might have to start all over again. Autofocus lenses give you more freedom to move around or allow your subject to move around without constantly messing with the lens. Filmmakers still prefer manual focus lenses, which work well in low-light situations. For instance, if you want to take photos of the night sky, you can manually alter the contrast and bring out light that your lens wouldn’t be able to pick up if using autofocus.

Autofocus Lenses

Whether you are an amateur photographer or seasoned at your craft, you will find times when autofocus lenses come in handy. Autofocus technology guarantees a clearer shot when you are photographing moving objects, and it is good at picking up edge detail. Portrait photographers, DIY bloggers, and landscape photographers use autofocus to get clear photos from the first click. Autofocus can prevent you from having to edit images later because the shot you took will appear focused and professional.

The internal focusing system quickly keeps up with the changes in light and distance to produce a sharp image when your subject is on the move. With an AF lens, you can avoid blur lines and streaks when taking photos of subjects in motion. Some incredible shots you could capture with an autofocus lens include:

  • Running horses
  • Birds in flight
  • A baseball player swinging a bat
  • A rockstar singing
  • A dancer performing
  • Waves crashing

Autofocus lenses are an essential part of any photographers’ toolkit, right along with camera bags and tripods. The second most important thing to finding a lens that works for your project is finding one that fits your mount. Whether you already know what autofocus lenses are and how they work or are beginning your photography journey, a Rokinon autofocus lens will make all your images look sharp and professional. Rokinon sells lenses for popular mounts manufactured by brands like Sony, Nikon, and Canon. Pay close attention to your camera’s model before choosing a lens that works for you.

Autofocus Lenses: What They Are and How They Work

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