CDAF vs PDAF: Autofocus Technology Explained

Contrast Detection AF (CDAF)

Basic Definition

Contrast Detection AF (CDAF) is an autofocus technology that calculates focus by analyzing the contrast level of an image’s pixels. It works by moving the camera’s lens (or image sensor) back and forth to find the position where contrast in the focused area is maximized—since a sharply focused subject has higher contrast (distinct edges between light/dark areas) than an out-of-focus one (blurry, low-contrast edges). CDAF is widely used in mirrorless cameras, smartphones, compact point-and-shoots, and camcorders, and is distinct from Phase Detection AF (PDAF), the primary autofocus method in DSLRs.

Core Working Principles

1. Contrast Measurement & Lens Movement

CDAF operates in a continuous loop of “sample → measure → adjust” until peak contrast is achieved:

Step 1: Initial Sample: The camera captures a low-resolution image of the focus area (e.g., a subject’s face) with the lens in a starting position.
Step 2: Contrast Calculation: The camera’s image processor analyzes the contrast of the focus region by measuring the difference between adjacent pixels (e.g., the edge of a subject’s eye vs. the background). Contrast is quantified using algorithms like the Sobel operator (detects edges) or Variance of Laplacian (measures blurriness).
Step 3: Lens Adjustment: If contrast is low, the lens is moved slightly (either toward or away from the subject). The camera repeats the sampling and measurement process.
Step 4: Peak Contrast Lock: The lens stops when contrast reaches its maximum value (the “peak” of the contrast curve). This position corresponds to a sharp focus on the subject.

2. Key Characteristics of the Contrast Curve

CDAF relies on the relationship between lens position and contrast, which forms a bell-shaped curve:

Out-of-Focus (Low Contrast): When the lens is too close or too far from the subject, the focus area has low contrast (flat region of the curve).
Approaching Focus (Rising Contrast): As the lens moves toward the optimal position, contrast increases steadily (rising slope of the curve).
In-Focus (Peak Contrast): The lens reaches the peak of the curve, where contrast is highest (sharp focus).
Past Focus (Falling Contrast): If the lens overshoots the optimal position, contrast decreases (falling slope of the curve), prompting the camera to reverse direction and lock onto the peak.

Key Characteristics

Feature	Description
Accuracy	CDAF delivers very high focus accuracy (often sub-pixel level) because it directly measures the final image’s contrast—critical for static subjects (e.g., landscapes, portraits).
Working Mechanism	“Closed-loop” system: Uses the final image sensor data (not separate AF sensors) to calculate focus, making it compatible with mirrorless/sensor-based cameras.
Low-Light Performance	Struggles in low-light or low-contrast scenes (e.g., a plain white wall, dark rooms) because low light reduces contrast, making it harder to detect the peak contrast point.
Continuous AF (AF-C) Limitations	Less effective for moving subjects: CDAF must “hunt” for the contrast peak, leading to slower focus adjustments and potential focus lag (blurred action shots).
Silent Operation	No mechanical AF sensors or mirror movement (unlike DSLRs with PDAF), so CDAF is quiet—ideal for video recording or silent photography (e.g., weddings, wildlife).
Compatibility	Works with all lens types (including manual lenses adapted to mirrorless cameras) and requires no dedicated AF sensors—cost-effective for consumer devices like smartphones.

Advantages & Limitations

Advantages

Superior Static Focus Accuracy: CDAF excels at focusing on stationary subjects (e.g., landscapes, macro photography) because it fine-tunes focus to the exact contrast peak.
Silent & Smooth Operation: No mechanical noise from AF sensors or mirror flips, making it perfect for video and quiet shooting environments.
Cost-Effective Implementation: Uses the main image sensor for AF (no separate PDAF sensors), reducing camera hardware complexity and cost (key for smartphones/compact cameras).
Compatibility with All Lenses: Works with any lens mounted on the camera (including third-party or adapted lenses), unlike PDAF (which requires lens calibration).

Limitations

Focus Hunting: In low-contrast or low-light scenes, CDAF may repeatedly move the lens back and forth (“hunt”) to find the contrast peak, leading to slow focus acquisition.
Poor Performance with Moving Subjects: CDAF’s “sample-measure-adjust” loop is too slow for tracking fast-moving subjects (e.g., sports, wildlife), resulting in missed focus or lag.
Slower AF Speed: Generally slower than PDAF (especially in bright light) because it requires multiple lens movements and contrast measurements to lock focus.
Dependency on Contrast: Fails to focus on subjects with no distinct edges (e.g., a uniform blue sky, a blank wall) since there is no contrast to measure.

CDAF vs. Phase Detection AF (PDAF)

Feature	Contrast Detection AF (CDAF)	Phase Detection AF (PDAF)
Working Principle	Measures contrast of the final image to find peak sharpness	Compares phase differences of light rays on dedicated AF sensors to calculate focus distance
Accuracy	Very high (static subjects)	Good (slightly lower than CDAF for static subjects)
Speed	Slower (requires lens hunting)	Faster (direct focus distance calculation)
Low-Light Performance	Poor (relies on contrast)	Good (works with low light/contrast)
Moving Subject Tracking	Poor (laggy adjustment)	Excellent (real-time tracking)
Hardware Requirement	Uses main image sensor (no dedicated AF sensors)	Requires dedicated AF sensors (DSLRs) or on-sensor PDAF pixels (mirrorless)
Silence	Silent (no mechanical AF sensors)	May have mechanical noise (DSLRs) or silent (on-sensor PDAF)
Typical Use Cases	Static photography, video, smartphones, compact cameras	Sports, wildlife, action photography, DSLRs

Hybrid AF: Combining CDAF & PDAF

To overcome CDAF’s limitations, modern mirrorless cameras and smartphones use Hybrid AF (a combination of CDAF and PDAF):

Step 1: PDAF for Speed: PDAF quickly calculates the approximate focus distance (using phase differences) to move the lens close to the optimal position.
Step 2: CDAF for Precision: CDAF then fine-tunes the focus by measuring contrast, ensuring maximum sharpness (eliminating PDAF’s minor accuracy gaps).
Benefits: Hybrid AF delivers the speed of PDAF and the accuracy of CDAF, making it ideal for both static and moving subjects (e.g., Sony’s Fast Hybrid AF, iPhone’s Smart HDR AF).

Application Scenarios

Macro Photography: CDAF’s high static accuracy is ideal for focusing on tiny subjects (e.g., insects, flowers) with precise detail.

Smartphones: All modern smartphones use CDAF (or Hybrid AF) for autofocus, as their compact design does not accommodate dedicated PDAF sensors (early smartphones relied solely on CDAF).

Mirrorless Cameras: Used in conjunction with PDAF (Hybrid AF) for stills and video (e.g., Fujifilm X-series, Sony Alpha series).

Compact Cameras/Point-and-Shoots: Budget compact cameras use CDAF for cost-effectiveness and silent operation.

Video Recording: CDAF’s smooth, silent focus adjustments make it the preferred choice for video (even in high-end mirrorless cameras).