Published: June 29, 2020 | Last Updated: September 19, 2024
Phase detection, dual pixel, and hybrid autofocus are the best technologies for continuous autofocus in video and filmmaking.
This article explains the technologies behind the different types of autofocus and the pros and drawbacks of each system.
Autofocus vs. Continuous Autofocus
There’s a big difference between autofocus for stills and continuous autofocus for video. Autofocus for stills must focus on a target to quickly snap a photo. However, autofocus for video needs to track a moving object and quickly switch between objects continuously.
So what we want in a video camera is good continuous autofocus.
You might need to use your camera’s continuous autofocus in certain situations. For example:
- When you’re a single-camera operator doing interviews or corporate videos
- When you’re filming using a gimbal
- When you’re filming dolly shots (because keeping focus on the subject can be a difficult and preoccupying task as the camera-to-subject distance changes).
You can see the cameras with the best continuous autofocus on the market today here.
The four types of Autofocus Technologies: Contrast Based, Phase Based, Dual Pixel, and Time of Flight
There are four leading autofocus technologies: contrast-based, phase-based, dual-pixel, and time-of-flight (or laser) autofocus.
The two most widespread technologies are contrast-based autofocus and phase-based autofocus – or a hybrid between the two.
Laser autofocus is rare outside of some types of phone cameras and DIY experimental autofocus systems. However, there are indications that we might soon start to see time of flight in prosumer cameras.
Hybrid Autofocus Systems
Camera manufacturers often combine two or more systems to create a hybrid autofocus system. Fx Panasonic has begun combining phase-detect and contrast-based autofocus, combining the best of both worlds.
Furthermore, these systems can be combined with features such as eye-tracking for even better accuracy.
The best autofocus systems available on the market today consist of these technologies:
- Phase detection + contrast-based + eye detection (fx Panasonic)
- Phase detection + eye detection (fx Sony)
- Dual Pixel autofocus (Canon)
Below, you can read more about these in more detail.
Contrast Based Autofocus
Contrast-based autofocus is a technology that has been around for a while. It mimics what you would do when manually searching for focus: When you look through your viewfinder, you bring the subject into or out of focus by moving the lens in or out—you know you’re in focus when your subject edges are sharply defined. Contrast-based autofocus is excellent for still photography but not good for video.
Your camera does essentially the same thing with contrast-based autofocus. It looks for micro-contrast along edges in a small image section. If the edges surrounding the subject are out of focus, the dark and light pixels are blurred together, and there is low contrast. When the subject is brought into focus, the lights and darks are not blurred, and there is higher contrast.
Drawbacks of contrast-based autofocus
Contrast-based autofocus can do a great job of bringing your subject into clear focus and even tracking it to a degree. However, there are drawbacks.
When hunting for focus, you must gradually adjust the lens in and out, searching for the sweet spot. Your camera does the same thing. The autofocus does not tell the camera whether the lens is front-focusing or back-focusing. Consequently, the camera will move the lens back and forth, bringing the subject into focus, overshooting, and then readjusting to optimize.
This can result in subtle pulsing in the backgrounds as the camera continuously refocuses.
Another drawback to contrast-based autofocus is that it can take longer for the camera to focus on the subject than other technologies. This can be an issue when shooting video with lots of camera movement in the frame.
The relative weakness of contrast-based autofocus when shooting video and the fact that it can remain very effective for focusing even in settings with a shallow depth-of-field or shooting through telephoto lenses means this technology is better suited for keeping track of moving subjects in still photography.
Phase Detection Autofocus
The second major autofocusing technology is phase detection autofocus, which uses dedicated focusing sensors. Phase Detection Autofocus is excellent for video.
Phase detection autofocus works in stereo. A sensor receives signals from two separate diodes (the light-sensing bits in your camera). A microprocessor then analyzes the similarity in the signal. If light is focused on the same place in each sensor, then the image is in focus. If not, the microprocessor calculates how far and in what direction the lens must be moved to bring the subject into focus.
In some cameras, certain pixels are used to focus rather than capture the image. The missing pixels are interpolated from the surrounding image. In the dual-pixel autofocus technology, pixels can perform focusing and image capture tasks. But more on that later.
The nice thing about this system is that the camera knows whether to move the lens forward or backward and just how much. This makes phase detection autofocus significantly faster than contrast-based autofocus.
This is a boon for video shooting, as it means you can smoothly and continually maintain focus on your subject, even during complex camera maneuvers or a rapidly moving subject.
Drawbacks of phase-detect autofocus
Unfortunately, due to the quantized nature of the sensors, your camera computes the exact lens distance with a degree of rounding. Usually, this causes no discernible effect, but for exact photography with a very shallow depth of field or when shooting with a telephoto lens, your camera might be slightly less in focus compared with contrast-based autofocus.
Dual Pixel Autofocus
Dual pixel autofocus is a relatively new technology. Dual Pixel autofocus is great for video.
At its core, dual-pixel autofocus is a form of phase detection autofocus. However, whereas traditional phase detection relies on pixels that capture the image or focus on the subject, dual-pixel autofocus does both simultaneously.
This is achieved by focusing the light input in each pixel onto two photodiodes (the material that senses light). Two input feeds per pixel are then sent to a microprocessor. The microprocessor analyzes the light data and sends a signal to reduce the phase shift between signals.
Because pixels can capture images and determine focus distance, this technology eliminates the need to interpolate the missing pixels. Also, since each pixel can be used to determine focus distance, it is easier to focus on subjects in any area of the frame.
Dual-pixel autofocus has the same advantages as phase detection technology. There is no need to hunt for focus; it’s quick to focus and keeps the subject focused, allowing you to adjust focusing speed and sensitivity.
This technology is also beneficial for tracking and maintaining focus on a moving subject or shifting focus between different subjects.
Time of Flight Autofocus
Time-of-flight or laser autofocus is a less widespread technology that works similarly to sonar. Few cameras, outside phone cameras, and a few DIY solutions are equipped with this autofocus technology. Still, it can’t hurt to understand how this technology works, so let’s take a quick look at it.
As mentioned, time-of-flight autofocus works a lot like sonar. The camera sends a beam of visible or infrared light, which bounces off the subject and back to the sensor. The camera measures the time it takes for the emitted light to return to the sensor.
Because the speed of light is constant (thank you, Einstein!), the distance between the camera and the subject can be calculated. The camera can then adjust the lens to the appropriate distance to bring objects at that distance into focus.
Time-of-flight autofocus is not widespread, but there are rumors that it might soon be available in consumer and professional video cameras.
Summing Up
Contrast-based autofocus systems can give you the highest focus on a subject, even in shallow depth-of-field situations. This technology can help shoot stills, even when using a tele lens.
Phase detection autofocus systems eliminate the need to hunt for focus, making it quicker and cleaner to keep your subjects sharp during a shoot. Evolving tech, such as dual-pixel autofocus, continues to refine this system, making it a powerful tool for a videographer.
Time-of-flight autofocus shows promise but hasn’t been integrated into professional and consumer cameras. That might soon change.
Up Next: Understanding the different autofocus settings available on video cameras.