Phase Autofocus vs. Contrast Autofocus

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The Difference Between Phase and Contrast Autofocus

Autofocus is without a doubt one the most indispensable features in modern photography, but don’t take it for granted. Understanding the focusing system in your camera can mean the difference between making or missing the shot. Today, we’re going to take a look at phase detection and contrast detection autofocus systems; what they are; how they work; and how they can help you get critical focus in your photography.

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In SLRs, light passes through the lens, reflects up from the mirror into a pentaprism and out to your eye in the viewfinder. The simplicity of the design has been celebrated by photographers for years.



Phase Detection Autofocus

Phase detection autofocus works by employing a sub-mirror behind the regular mirror. Light is focused into two distinct beams for determining subject range on an autofocus sensor. Distance is calculated, and a motor drives the lens to bring these beams into “phase”. This type of AF system requires tight alignment between the AF sensor and the lens.

Many advanced Single Lens Reflex Cameras offer the user micro adjustments to calibrate specific lenses with the AF Sensor. On modern mirrorless systems, Phase Detection works much the same way except there are no mirrors in the mix. The reading is made right off of the image sensor.

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The process varies from manufacturer to manufacturer. Many companies include pixels on the sensor that only receive light from one direction of the lens and compare this data with light readings from the other side of the sensor. When the information matches up, the camera knows that the subject is in focus.

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Canon’s Dual Pixel CMOS AF system splits each pixel on the camera’s sensor into two photodiodes. One diode gathers light, the other serves up phase detection AF. This makes for incredibly fast autofocus in both photo and video applications.

When we’re talking about phase detection, we’re really talking about speed. It’s the best type of autofocus system for fast moving subjects. Think sports, wildlife, children at play. One thing to keep in mind, though, is that phase-detection systems can fall out of alignment or perform inconsistently with different lenses. This is why it is sometimes important to fine-tune your focusing system to each specific lens in order to achieve the best performance.

Contrast Detection Autofocus

Contrast Detection AF analyzes pixels on the camera sensor using contrast calculations to obtain focus. This works by pushing focus back and forth on the subject to identify the point of highest contrast.

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In practice this feels a little strange – your lens focuses on the subject, goes beyond the initial focus and then pulls back. Despite this “hunting”, the system is highly accurate. Mirrorless cameras have employed Contrast Detection AF for years boasting its high precision, and some DSLRs use contrast-detection focusing when shooting in live view.



Comparison

Compared to Phase Detection AF, the process is usually slower, and it’s not the best way to capture a moving subject. However, in portraiture, landscape, still life photography, really any application where the subject isn’t moving very much, Contrast Detection AF can be valuable because of its accuracy and potentially larger array of AF points. Additionally, contrast-detection systems are not affected by front and back focusing issues the way that phase-detection systems can be.

In Conclusion

Autofocus systems make a huge difference in the way your camera and lenses perform from subject to subject. If you’re an action shooter that requires tracking and continuous AF, Phase Detection autofocus systems offered in SLRs and high-performance mirrorless cameras can help you make the shot.

On the other hand, contrast-detection systems offer greater precession and flexibility of choosing a point in more places of the image frame. Some of the latest mirrorless camera systems actually employ both methods to give photographers a nice middle ground for both speed and accuracy.

Do you have any other information about autofocus? Left your ideas and questions in the comments section below! and please don’t forget to follow kuulphoto.com on Twitter @kuulphoto

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