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  • Writer's pictureIan Miller

Phase detection autofocus (AF)Contrast detection autofocus (AF)

If you are interested in learning more about autofocus systems in cameras, you might have come across terms like phase detection and contrast detection. What do these terms mean and how do they affect your photography? In this blog post, I will explain the basics of phase detection and contrast detection autofocus, as well as their advantages and disadvantages.

Phase detection autofocus (AF) is a type of autofocus system that uses a dedicated AF sensor to analyze the light coming from the lens. The AF sensor splits the light into two beams and compares them to see if they are aligned or not. If they are aligned, it means that the subject is in focus; if they are not, it means that the lens needs to adjust its focus distance. The AF sensor can also calculate how much and in which direction the lens needs to move to achieve focus, which makes phase detection AF very fast and accurate.


Contrast detection autofocus (AF) is a type of autofocus system that uses the main image sensor to analyze the light coming from the lens. The image sensor measures the contrast or sharpness of the image at different focus distances and tries to find the highest contrast possible. The higher the contrast, the more in focus the subject is. Contrast detection AF does not need a separate AF sensor, but it relies on software algorithms to find the optimal focus point.

Both phase detection and contrast detection AF have their pros and cons. Phase detection AF is generally faster and more reliable, especially in low light and for moving subjects. However, phase detection AF requires a separate AF sensor, which adds complexity and cost to the camera. Phase detection AF also suffers from calibration issues, which can cause the front or back focus problems if the AF sensor is not aligned with the image sensor.


Contrast detection AF is generally simpler and cheaper, as it does not need a separate AF sensor. Contrast detection AF also does not have calibration issues, as it uses the same image sensor that captures the photo. However, contrast detection AF is generally slower and less accurate, especially in low light and for moving subjects. Contrast detection AF also tends to hunt or oscillate around the focus point, which can cause delays and missed shots.

Some cameras use a hybrid autofocus system that combines both phase detection and contrast detection AF. These cameras have some pixels on the image sensor that act as phase-detection points, while the rest of the pixels are used for contrast detection. This way, the camera can use phase detection AF to quickly find the approximate focus point, and then use contrast detection AF to fine-tune it. Hybrid autofocus systems offer the best of both worlds, but they also have some limitations, such as reduced resolution and dynamic range due to the phase detection pixels on the image sensor.


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