Sony FCB camera block
When a target moves within the frame, the lens frequently exhibits the “hunting” phenomenon: the image oscillates repeatedly between blurriness and sharpness. By the time the lens finally achieves focus, the moving target has already left the frame.
Many people wonder: in 2026, why is fast autofocus still hard to achieve?
It is not that the technology is incapable. Instead, we need to understand the fundamental differences between two autofocus solutions: Contrast‑Detection AF (CDAF) and Phase‑Detection AF (PDAF).
I. Contrast‑Detection AF (CDAF) and Phase‑Detection AF (PDAF)
1. Contrast‑Detection AF (CDAF)
Also known as contrast AF, CDAF is the mainstream focusing solution for embedded integrated camera modules, security bullet cameras, and live‑view mode of mirrorless cameras.
Working Principle
The lens motor drives the lens group back and forth. The ISP analyzes the edge contrast (luminance gradient) within the AF window in real time. The sharper the image, the higher the edge contrast value. Focus is confirmed once the contrast peak is detected.
Advantages
– No additional phase‑detection pixels or dedicated AF hardware modules are required, keeping hardware costs under control.
– Focus judgment is based directly on images output by the imaging sensor, delivering high focusing accuracy with virtually no focus shift.
– Custom AF regions can be set at any position across the frame.
Inherent Limitations
– It cannot tell whether the focus is front‑focus or back‑focus at the initial stage. A trial‑scan‑retreat iteration is mandatory, commonly referred to as focus hunting.
– It tends to fail in low‑contrast, solid‑color and low‑light environments, and generally takes longer to focus than PDAF.
2. Phase‑Detection AF (PDAF)
PDAF is widely adopted in DSLR cameras and consumer smartphone CMOS sensors.
Working Principle
Paired phase‑detection pixels built into the sensor receive incident light from the left and right sides of the lens respectively. By calculating the phase difference between the two sets of signals, the required direction and displacement of the lens are directly computed, driving the lens elements to the in‑focus position in one step.
Advantages
Fast focusing response, suitable for tracking moving subjects, and greatly reduces focus hunting.
Disadvantages
Dedicated phase‑detection pixels occupy part of the photosensitive area, increasing module size and hardware cost.
Focus accuracy degrades under low‑light and low‑contrast conditions.
Its complex hardware architecture is not suitable for OEM integration of compact industrial‑security integrated camera modules.
Analogy:
CDAF is like climbing a mountain blindfolded. You can only judge whether you have reached the summit by foot feel. Only after passing the peak and sensing the downhill slope do you turn back.
PDAF is like navigating with GPS. It tells you exactly how far to go, and you reach the destination in one move.
| Comparison Item | Contrast‑Detection AF (CDAF) | Phase‑Detection AF (PDAF) |
|---|---|---|
| Detection Method | Iterative search for image contrast peak | Direct calculation from phase difference |
| Hardware Requirement | Standard CMOS + ISP algorithm | Phase‑detection pixels / dedicated AF module |
| Focus Speed | Relatively slow, requires iterative scanning | Fast, completed via single calculation |
| Focus Accuracy | High accuracy, almost no focus shift | Good accuracy, usually requires contrast fine‑tuning |
| Module Compatibility | Suited for compact, low‑cost industrial‑security modules | Large form factor and high cost; unsuitable for small integrated modules |
For these reasons, integrated camera modules for security and machine‑vision applications (including Sony FCB series and mainstream domestic zoom modules) mostly adopt the CDAF architecture. This compact solution has no light‑blocking phase‑detection pixels and fully meets practical OEM integration requirements.
II. Why Do Sony Zoom Cameras Deliver Superior Focus Performance Despite Using CDAF?
The performance bottlenecks of CDAF lie in three aspects: lens scanning steps, contrast‑evaluation frame rate, and search algorithm strategy.
Sony zoom cameras (e.g. FCB‑ER9500, FCB‑EV9520L) achieve outstanding focusing performance under the CDAF architecture through coordinated optimization of hardware, algorithms, optics and driving systems. There are four core factors:
1. Sony Proprietary High‑Speed AF Search Algorithm
Built‑in DSPs on Sony zoom cameras run self‑developed intelligent focusing algorithms. Based on zoom magnification, aperture value and the trend of frame contrast changes, the algorithm predicts the in‑focus range, skips large numbers of invalid scanning strokes, effectively reduces hunting steps, and eliminates unnecessary reciprocating movement of lens elements.
2. High‑Frame‑Rate ISP + Low‑Latency Signal Feedback
Sony zoom cameras are equipped with custom Sony ISP and STARVIS™‑series CMOS sensors:
1. High signal‑to‑noise ratio: lower image noise under identical lighting conditions, enabling more reliable contrast calculation.
2. Fast sensor readout speed and low image‑data latency, shortening the iteration cycle of AF algorithms.
Compared with conventional cameras using low‑sampling‑rate and software‑polling implementations, FCB modules can complete focus determination much faster.
3. High‑Precision Stepping Motor and Deep Lens Matching
Sony zoom cameras adopt high‑quality zoom lenses and motor‑drive circuits with full‑system co‑tuning, supporting fine control with tiny step sizes. When the algorithm sends a stop command, the motor brakes rapidly to prevent the lens from overshooting the contrast peak, reducing secondary retreat‑and‑correction cycles. This hardware advantage cannot be replicated merely by replacing the image sensor.
4. Intelligent Scene Adaptation and Multi‑Zone AF
Sony zoom cameras support a 6×8 grid multi‑zone AF window. It can prioritize faces and high‑contrast target regions. In scenarios such as traffic monitoring and UAV dynamic inspection, it lowers the probability of re‑search after defocus, improving focusing stability for dynamic scenes.
Ordinary CDAF cameras “find focus by trial‑and‑error step‑by‑step”. Sony FCB modules leverage algorithmic prediction to intelligently skip invalid strokes. The synergy of hardware, optics and algorithms delivers superior focusing experience within the same architecture.
Summary
Sony zoom cameras do not switch to PDAF. Instead, they implement end‑to‑end deep optimization on the CDAF framework. Faster focusing response, smooth zoom‑tracking capability and stable performance under complex scenarios make them the mainstream choice for high‑end security surveillance, UAV electro‑optical pods and machine‑vision acquisition projects.
FAQ
Q1: Can firmware upgrades further improve the CDAF focusing speed of Sony zoom cameras?
A: Firmware can optimize AF search strategies and adjust focus‑sensitivity parameters to improve performance in some scenarios. However, the upper limit of focusing speed is determined by hardware including motors, sensor readout rate and optical components. Firmware cannot break physical hardware limits.
Q2: Why do Sony zoom cameras still exhibit focus hunting on solid‑color walls and in low‑light environments?
A: CDAF relies on frame contrast for judgment. When a scene lacks edge textures and overall contrast is extremely low, the algorithm cannot obtain valid peak signals and triggers wide‑range scanning. The issue can be mitigated by adjusting AF sensitivity, enabling interval focusing, or using external auxiliary lighting.