DISCLOSURE: AS AN AMAZON ASSOCIATE I EARN FROM QUALIFYING PURCHASES.
THIS POST MAY CONTAIN AFFILIATE LINKS, MEANING, AT NO ADDITIONAL COST TO YOU, I EARN FROM QUALIFYING PURCHASES. AFFILIATE LINKS ARE MARKED WITH #ad. "I" IN THIS CASE MEANS THE OWNER OF FILMDAFT.COM. PLEASE READ THE FULL DISCLOSURE FOR MORE INFO.
When it comes to shooting video, especially with DSLR cameras, the mantra is usually “always use manual focus.”
And while it is true that manual focus gives you the most control over your project, autofocus technology has come a long way, especially in recent years.
There are certain situations in which you might need to utilize your camera’s autofocus. 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).
So in this article, we are going to take a look at the technologies behind different types of autofocus.
Also, we will examine which methods work best for shooting video and which are best for still photography. Because there’s a big difference between autofocus for stills and continuous autofocus for video.
It is called continuous autofocus because the camera will continuously adapt to the moving subject while filming. So what we want in a video camera is good continuous autofocus
Hopefully, after reading this article you will have a better idea about when and how to use the different autofocus settings on your camera.
The four types of Autofocus Technologies: Contrast Based, Phase Based, Dual Pixel, and Time of Flight
First let’s talk about the main ways autofocus works for your camera.
There are four main technologies; contrast based, phase based, dual pixel and time of flight (or laser) autofocus.
However outside of some types of phone cameras, and some DIY experimental autofocus systems, laser autofocus is actually quite rare. However, there are indications, that we might soon start to see time of flight in prosumer cameras as well.
The two most widespread technologies are contrast based autofocus and phase based autofocus.
Contrast Based Autofocus
Contrast based autofocus is a technology that has been around for a while. It mimics what you would do when searching for focus manually.
I.e. when you look through your viewfinder you bring the subject into or out of focus by moving the lens in or out – and you know you’re in focus when the edges of your subject are sharply and clearly defined.
With contrast based autofocus, your camera does essentially the same thing. The camera looks for micro contrast along edges in a small section of the image.
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.
Contrast based autofocus can do a great job bringing your subject into very clear focus, and can even track the subject to a degree.
However there are drawbacks.
You know how when you are hunting for focus you have to 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 a bit longer for the camera to focus on the subject than other technologies. This can be an issue when you are shooting video, and there is lots of movement of the camera or in the frame.
The relative weakness of contrast-based autofocus when shooting video, along with 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 we will look at is phase detection autofocus. This method works by using dedicated focusing sensors.
In some cameras, certain pixels are used for focusing rather than for capturing the image. The missing pixels are interpolated from the surrounding image.
In the newer dual pixel autofocus technology, pixels can actually perform both tasks: focusing and image capture. But more on that later.
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 to 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.
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.
For shooting video this is a boon, as it means you can smoothly and continually maintain focus on your subject, even during complex camera maneuvers or a rapidly moving subject.
Unfortunately, due to the quantized nature of the sensors, there is a degree of rounding when computing the exact lens distance by your camera.
Normally this causes no discernible effect, but for very precise photography with a very shallow depth of field, or when shooting with a telephoto lens your camera might end up being slightly less in focus than compared with contrast-based autofocus.
Phase Detection Autofocus is great for video.
Dual Pixel Autofocus
Dual pixel autofocus is a relatively new technology.
When Canon introduced it they touted it as a revolutionary new innovation. Whether or not the product lived up to the hype is debatable, however, it is becoming a widespread feature as numerous companies seek to introduce models equipped with similar technology.
At its core, dual pixel autofocus is a form of phase detection autofocus. However, whereas traditional phase-detection relies on pixels that either 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). There are then two input feeds per pixel that are sent to a microprocessor. The microprocessor then analyses the light data and sends a signal to reduce the phase shift between signals.
Dual pixel autofocus gives you the same advantages as phase detection technology. No need to hunt for focus, quicker focusing and keeping subject in focus, and the ability to adjust focusing speed and sensitivity.
Because pixels can both capture image and determine focus distance, this technology eliminates the need to interpolate the missing image 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.
This technology is also extremely useful when it comes to tracking and maintaining focus on a moving subject, or even shifting focus between different subjects.
Dual Pixel autofocus is great for video.
Time of Flight Autofocus
Besides contrast based and phase shift autofocus there is one more major technology we will take a look at. Time of flight autofocus or laser autofocus is a less widespread technology that works similar to sonar.
Few cameras outside of phone cameras, and a few DIY-solutions, are equipped with this sort of 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 out 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, the distance between the camera and 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 at the moment, but there are rumors that we might start to see it soon in prosumer and professional video cameras.
The technology and quality of built in autofocus has come a long way in recent years and can be a useful tool in many situations.
Whether you are shooting stills and need to track a moving subject, or filming video and need to continuously refocus there are advantages and drawbacks to each technique.
Contrast based autofocus systems can give you the highest level of focus on a subject even in shallow depth of field situations. This technology can be useful for shooting stills, even when using a telefocus lens.
Phase detection autofocus systems eliminate the need to hunt for focus. This makes it a quicker and cleaner method of keeping 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.
At the time of writing, the two best technologies for continuous autofocus for video and filmmaking is phase detection autofocus and dual pixel autofocus.
Time of flight autofocus shows promise but hasn’t been integrated into professional and prosumer cameras. But that might soon change.
Next stop is to understand the different autofocus settings available on video cameras.
But whether you are focusing by hand, or utilizing your cameras built in features what matters is your vision. So stay creative and keep shooting.
Cade Taylor is a filmmaker and writer based out of Los Angeles. Originally from Seattle, he continues to work as the Outreach Coordinator for the Bigfoot Script Challenge, where he helps connect up-and-coming writers with industry professionals. When he’s not working on his own projects, helping out with Bigfoot, or covering desks, Cade loves to share what he knows with other filmmakers and promote great content.