Dedicated Focus Pixels (Focus Pixels): what it is, how it works and what it does

Definition

Dedicated focus pixels (also called Focus Pixels or dedicated PDAF pixels) are photosensitive cells within an image sensor whose primary function, or one of their functions, is to measure the phase difference of incoming light to determine subject distance and accelerate phase-detection autofocus (PDAF). Unlike conventional image pixels that capture luminance and colour, these pixels are optimised to extract optical position information.

Operating principle

Phase detection works by splitting the light beam arriving at each focus pixel into two laterally displaced sub-beams. A micro-lens and a phase filter (or diffraction mask) generate two slightly shifted images. The readout circuit compares the position of a reference pattern in both sub-beams and calculates the phase displacement, which translates directly into the distance the lens must travel to reach the sharp-focus point. This calculation is virtually instantaneous, unlike contrast-detection autofocus, which requires iterative trial-and-error.

Architecture and variants

The implementation of focus pixels varies according to the degree of integration with image pixels:

• Dedicated PDAF pixels (partial): a small percentage of the sensor (typically 0.5 %–10 %) is reserved exclusively for phase detection. These pixels do not contribute to the final image, slightly reducing effective resolution.
• Hybrid pixels (Hybrid Focus Pixels): each pixel serves a dual role: it captures luminance/colour for the image and simultaneously extracts phase information for autofocus. No resolution is lost, but signal processing is more complex.
• 100 % coverage: every pixel on the sensor participates in phase detection. This configuration maximises focusing speed and accuracy across the entire frame, as there are no PDAF «blind» zones.

In practice, manufacturers combine these variants across the different cameras of a device. For example, a main camera may offer 100 % focus pixels alongside optical image stabilisation (OIS), while the ultrawide camera incorporates Hybrid Focus Pixels at 48 MP with an f/2.2 aperture, and the telephoto (48 MP, f/2.8, 4× optical zoom, OIS) also integrates dedicated focus pixels.

Applications

• Rapid autofocus in smartphones: phase detection achieves the sharp-focus point in milliseconds, reducing focus lag in video and action photography.
• Subject tracking: with wide or full coverage, the system can maintain focus on a moving subject without jumps.
• Continuous video mode: dedicated PDAF enables smooth focus transitions without the hunting characteristic of contrast detection.

Scope and limitations

• Effective resolution: in the partial dedicated-pixel variant, image resolution is reduced proportionally to the percentage of pixels not used for capture.
• Low light: the phase signal degrades in dim conditions, so many systems switch to contrast detection or a low-sensitivity hybrid mode.
• Manufacturing complexity: the additional micro-lens and phase filter increase cost and assembly tolerance of the sensor.
• Signal processing: hybrid pixels require a more elaborate image pipeline to separate the colour component from the phase component in real time.

Interpreting the specification in a product

When a spec sheet mentions Focus Pixels, Hybrid Focus Pixels, or a percentage such as 100 % focus pixels, it is indicating the PDAF strategy of the corresponding sensor. A 100 % value means the entire active sensor area participates in phase detection; the word Hybrid signals that the pixels are dual-use (image + phase) with no resolution loss. The presence of this feature across multiple cameras in the same device (main, ultrawide, telephoto) indicates the manufacturer has extended phase detection to the entire photographic system, not just the primary sensor.

The mention of «Focus Pixels» alongside OIS and a 48 MP resolution in a telephoto with 4× optical zoom suggests a high-end sensor in which phase detection is integrated as a native silicon function, not as an external module.