WHY CANON’S LATEST CAMERAS FOCUS SO QUICKLY
Dual Pixel CMOS AF explained.
Ask any photographer what his top considerations are when selecting a camera, and chances are you’ll nd autofocus (AF) speed and accuracy right up there. After all, all the resolution in the world won’t help if the image is out of focus.
This is only more evident now when Interchangeable Lens Cameras are increasingly being used to take videos as well as stills, and nobody wants footage that is out of focus. And that’s where Canon’s Dual Pixel CMOS AF stands out, as it promises fast AF even during Live View and Movie Recording mode.
First, an AF primer. AF methods for today’s cameras generally fall into two methods – Contrast Detection AF, and Phase Detection AF.
Contrast Detect AF works by searching for the point of maximum contrast, as that’s when an image is perfectly in focus. So, the camera essentially moves the lens backwards and forwards, measuring the contrast at each point. When the camera has detected that it has just passed the point of highest contrast, it tracks back a step and takes the picture. That’s why you see the camera hunting for focus when you’re trying to focus in low light or on subjects that have low contrast– like sheep in dense fog for example. Obviously, that’s not ideal for video work when all that shifting back and forth gets recorded live.
With Phase Detection AF though, every Phase Detect
AF point will have a corresponding pair of sensors located on a Phase Detection AF module that’s typically located below the reflex mirror. When you trigger AF; light from the lens bounces off the reflex mirror down to the sensors below. One sensor will read what’s captured from one side of the lens, while the other will read what’s captured from the other side, and the images on both sides will be compared.
If the images are in phase, then the object is in focus. Conversely, if the image is not in focus, the difference in phase tells the camera just how much and in which direction it needs to adjust for perfect focus. That’s why Phase Detection is faster, because the camera just needs to get one set of readings instead relying on a continuous process of trial and error.
Typical DSLR (Digital Single Len Reflex) camera systems use the reflex mirror to redirect light both to the viewfinder and to a separate Phase Detect AF module. This works fine with stills, but when the reflex mirror is raised for Live View mode or Movie recording mode, that same Phase Detect AF module is taken out of the equation because the reflex mirror is raised out of the way.
This means the only options left is to either use the Imaging sensor itself to perform Contrast Detection AF or to add separate Phase Detection AF sensors on the Imaging sensor – what’s commonly known as Hybrid AF.
However, adding separate Phase Detection AF sensors eats into the amount of space the Imaging sensor has to gather light, so the amount of coverage possible on Hybrid AF systems has generally been small, limiting their effectiveness.
What Canon has done with Dual Pixel CMOS AF is to take an area on the sensor that’s about 80% of the width and 80% of the height of the entire image plane, and cover it with Phase Detection AF pixels, each of which comprises two independent photodiodes.
When the camera is focusing, each photodiode captures light separately and hence the pair acts as a Phase Detect AF sensor, getting the necessary information to let the camera know how much and in which direction to adjust focus. When you press the shutter to take the shot, this light information is then combined again and the Phase Detection AF pixel acts like any regular pixel, so no light is lost for imaging.
This means no more footage of your camera hunting back and forth to get focus as you won’t have to rely on Contrast Detect AF, and also that focus shift can be done more seamlessly. With almost every part of the image having a corresponding AF point, switching focus is simply a matter of choosing which AF point to use - the corresponding Phase Detection AF pixels will do the rest.
In essence, Dual Pixel AF gives the best of both worlds when using the camera to capture stills and video with the mirror up.
Hybrid autofocus is so termed because it combines contrast and phase-detect AF for different tasks. However, traditionally, phase-detect and imaging pixels were independent of each other on a sensor.
In Canon’s Dual Pixel CMOS sensor, each pixel is split into two independent light collection diodes, which functions for both phase-detect AF and imaging.
During focusing, light collected in each diode is compared for phase-detect AF calculations. When exposure is taken, the light on both sides are combined and treated as one image pixel.