Camera Basics #11: Phase Detection AF
Phase detection AF (also known as phase-difference detection AF or phase-difference AF) is the autofocus system used in viewfinder shooting on DSLR cameras. Its main feature lies in its rapid autofocusing speed. In the following, we will explain more about phase detection AF, and how Canon’s Dual Pixel CMOS AF utilizes the latest AF technology to enable phase detection AF even in Live View. (Reported by Tomoko Suzuki)
Phase detection AF is the AF system used in viewfinder shooting on DSLR cameras
- Rapid autofocusing speed.
- Requires a mirror mechanism that separates the light entering the lens, along with a separate AF sensor.
Phase detection AF is the AF system used in viewfinder shooting on DSLR cameras. It works by splitting the light entering the lens into two so that it forms two images. Based on the difference in the focus point position between these two images, the camera calculates the required direction (towards the camera, or away from the camera) and the amount (the distance) to move the lens in order to achieve focus, and moves the lens accordingly.
Phase detection AF enables autofocus be established swiftly, since the camera knows exactly how much and in which direction to move the focusing lens. However, this form of AF requires a dedicated AF sensor along with a mechanism that separates light between the AF sensor and the image sensor, which converts light entering the lens into an image. This makes it difficult to make the camera body compact.
Phase detection AF is able to focus rapidly, since it knows the distance and direction from the subject
An example of phase detection AF
An example of contrast AF
To understand better, let’s imagine a situation where you have to split a watermelon. Phase detection AF would be like trying to get to the watermelon without a blindfold. You already know the distance and direction to the watermelon, and this knowledge enables you to move to it quickly.
Meanwhile, contrast AF would be like trying to get to the watermelon on blindfold. Since you are not able to know the distance and direction to the watermelon, you have to move around in order to try and locate its position. This is why contrast AF requires more time to achieve focus on a subject as compared to phase detection AF.
Related Concept 1: Line sensor and cross-type sensor
Line sensor and cross-type sensor layout
The cross-type sensor has a high accuracy in autofocus
A: detects the horizontal line of the subject
B: detects the vertical line of the subject
On the AF sensor of a DSLR camera, there are two kinds of sensors: the line sensor and cross-type sensor. Line sensors are either vertically-oriented or horizontally-oriented, hence, they can only detect the horizontal or vertical line of a subject. However, cross-type sensors, which are made of line sensors positioned in a cross, are able to detect both the vertical and horizontal lines of the subject, and as a result, have a higher accuracy in achieving focus.
Entry-level cameras such as the EOS 1300D usually have only one cross-type AF point positioned in the centre. However, newer camera models such as the EOS 77D and EOS 800D come equipped with cross-type sensors for all their 45 AF points. This design is suitable even for advanced users who want to be able to arrange their composition such that their subject can be placed anywhere in the frame. With more cross-type sensors, focus can be achieved quickly even if the subject is located near the edges of the frame.
Related Concept 2: Dual Pixel CMOS AF
On cameras that come equipped with Dual Pixel CMOS AF, phase detection AF can be used in Live View within the wide area marked in red, without the use of a separate AF sensor. Rapid and accurate focus can be achieved even on moving subjects.
Dual Pixel CMOS AF is the latest AF system proudly engineered by Canon. It enables phase detection AF to be used in Live View and video shooting, whereas it could only be used in viewfinder shooting on previous DSLRs. As all the pixels on the image sensor of Dual Pixel CMOS AF are equipped with phase detection sensors, it does not require a separate AF sensor, which is why it can be implemented in mirrorless cameras, such as the EOS M series. Along with its smooth and rapid focusing, it is able to establish focus easily even on moving subjects.
The following clip shows how Dual Pixel CMOS AF benefits video shooting - some points also apply to photography!
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A monthly magazine that believes that enjoyment of photography will increase the more one learns about camera functions. It delivers news on the latest cameras and features and regularly introduces various photography techniques.
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After graduating from the Tokyo Polytechnic University Junior College, Suzuki joined an advertisement production firm. She has also worked as an assistant to photographers including Kirito Yanase, and specializes in commercial shoots for apparels and cosmetic products. She now works as a studio photographer for an apparel manufacturer.