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Phase-detection auto-focus (PDAF) is an auto-focus method that provides high focusing speed. PDAF eliminates the need for measuring contrast within a sensor field, making it possible to adjust the lens to move its focal point quickly and directly to a new position to achieve optimal focus.
The conventional auto-focusing method measures contrast within a sensor field until maximum contrast is detected. Thus, it takes longer to achieve correct focus. This makes it difficult to track moving objects without losing focus.
Images taken using the conventional AF adjustment
Single-lens reflex cameras divide the incoming light into two images with separator lenses and instantly measure the phase differences between the two images. The auto-focus unit moves the lens directly to a new position where the phase differences become minimal, giving optimal focus at high speed. A major drawback of this mechanism is that it requires an AF sensor and a pair of separator lenses. This makes it difficult to reduce the size and thickness of the auto-focus unit.
PDAF used in single-lens reflex (SLR) cameras
Toshiba offers PDAF CMOS image sensors that incorporate an AF sensor to eliminate the need for separator lenses. This helps to significantly reduce the size of the camera's internal mechanism. Toshiba's PDAF CMOS image sensors make it possible to realize small, thin cameras with quick auto-focus performance that are ideal for mobile applications such as smartphones and tablets.
The AF sensor for phase-detection auto-focus is embedded as part of the active pixels of a CMOS image sensor with alternate halves blacked out.
AF Sensor in a CMOS Image Sensor
Pixels with the right half blacked receive light from the right side of the lens, and those with the left half blacked receive light from the left side of the lens. The AF sensor has several pairs of these pixels in the active area of a CMOS image sensor to measure phase differences.
How an AF Sensor Receives Lights in a CMOS Image Sensor
The phase differences are used to determine in which direction and how much the lens should be moved to achieve optimal focus. Consequently, the PDAF CMOS image sensor provides high-speed auto-focus.
Images taken using the PDAF adjustment
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