Auto Focus
Source：Shenzhen Kai Mo Rui Electronic Technology Co. LTD2026-06-15
1. Zoom
1) Working Principle
Zoom adjusts focal length by shifting the position of lens elements inside the lens assembly, which alters the field of view and image size to achieve zoom-in or zoom-out effects.A longer focal length corresponds to a narrower field of view, capturing fewer scene elements and making subjects appear closer. A shorter focal length delivers a wider field of view, framing more scenery and making subjects look farther away.
2) Zoom Types
- Optical ZoomThe physical focal length is adjusted. If the target object remains stationary, magnification increases proportionally with focal length without any loss of image clarity, similar to binoculars.
- Digital ZoomThe focal length stays unchanged; the system enlarges pixel areas of the captured frame. Image quality degrades progressively as magnification rises, just like manually zooming photos in a mobile phone album.
2. Focus
1) Working Principle
Focus adjusts image distance by repositioning the entire lens unit (instead of individual lens pieces). It calibrates the gap between the lens and imaging plane so the distance from the optical center to the imaging plane matches the theoretical image distance, projecting a sharp image onto the photosensitive chip (film, CCD or CMOS sensor).The process of adjusting the camera to form a sharp image of the subject is known as focusing. A perfectly sharp image forms when the imaging plane sits between 1× and 2× the lens focal length and aligns precisely with the CCD/CMOS sensor plane. If the projected image shifts off the sensor plane, the frame turns blurry, which is called misfocus.
2) Focus Modes
- Manual Focus
- Auto Focus
- Multi-point Focus (also named Zone Focus): Applied when the target subject is not centered in the frame; common layouts include 5-point, 7-point and 9-point arrays.
3. Auto Focus (AF)
1) Core Operating Principle
- Automatically identify the shooting subject
- Measure the distance between the subject and the camera’s image sensor
- Drive a motor to move the lens focusing unit to the position matching the measured distance
2) Two Main AF Categories
Active AF (Laser AF, TOF AF, etc.)
Active AF relies on distance detection technology: it measures the space between the lens and target, then calibrates lens position for accurate focus lock.
Passive AF (PDAF, CDAF, etc.)
Passive AF emits no energy or signals toward the subject; it only analyzes incoming light and captured image data to adjust focus.
① Laser Detection Auto Focus (LDAF)
Also known as ranging AF. A dedicated infrared laser sensor emits low-power infrared beams to the subject, receives reflected signals, calculates target distance, then triggers the focus motor to reposition lens groups to complete focusing.
- Advantages: Performs well in dim light and flat solid-color surfaces with faint texture
- Disadvantages: Limited effective ranging distance; poor focusing performance for faraway subjects
② Time-of-Flight (TOF) Auto Focus
The sensor transmits infrared light, then calculates subject distance by measuring the time or phase difference between light emission and reflected signal reception.
- Advantages: Completes focus calculation in a single pass, reduces processor load, and resists ambient light interference
- Disadvantages: Requires high-performance physical hardware and ultra-precise time measurement components
③ Phase Detection Auto Focus (PDAF)
Most widely adopted in DSLR cameras. It calculates focal offset by detecting phase differences between adjacent image pixels to realize fast, precise focusing. After measuring phase deviation, the lens drive module shifts lens groups into alignment for focus lock, consuming less time.
④ Contrast Detection Auto Focus (CDAF)
Also called contrast AF. It uses an iterative, peak-seeking convergence process analogous to manual focusing: blurry → sharp → blurry, gradually tuning to the sharpest position.Workflow: Calculate contrast value → shift lens and recheck contrast → repeat until peak contrast is reached. It judges subject distance via pixel value variance across the image sensor to achieve auto focus.
- Advantages: No extra hardware required, zero impact on native image quality, no risk of focus shift, supports focusing at any position across the full frame
Image contrast, also referred to as sharpness, is a metric that quantifies fine detail resolution and overall image clarity. Contrast-based AF uses sharpness values to evaluate focus accuracy; higher sharpness values mean closer alignment to the optimal focus plane. Sharpness values are computed via sharpness evaluation functions, also called focus functions. A high-quality focus function generates a single-peak focus curve, where the peak coordinate corresponds to the perfectly focused lens position.
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