Basics of CMOS Image Sensors and Lens Selection
A practical guide to CMOS sensor fundamentals and lens selection for industrial and AI camera applications. Covers sensor size, resolution, shutter type, low-light performance, and lens focal length / F-number.
Basics of CMOS Image Sensors and Lens Selection
Introduction
In AI camera and industrial camera development, CMOS image sensor and lens selection is the single most critical decision affecting product performance. This article provides a practical overview of CMOS sensor fundamentals and key lens selection criteria.
What is a CMOS Image Sensor?
CMOS (Complementary Metal-Oxide Semiconductor) image sensors convert optical signals into electrical signals on a semiconductor chip. Known for low power consumption, high integration, and fast processing, they are used in virtually all modern digital cameras.
Key CMOS Sensor Parameters
1. Shutter Type
Rolling Shutter
- Exposes rows sequentially
- Suitable for static or slow-moving scenes
- Low cost, widely adopted
- Fast-moving objects can produce distortion and artifacts
Global Shutter
- Exposes all pixels simultaneously
- Ideal for capturing fast-moving subjects
- Essential for visual inspection, robot vision, and advanced automotive systems
- Higher cost than rolling shutter
2. Resolution
Determined by pixel size and total pixel count, expressed in megapixels (MP). Higher resolution captures finer detail.
3. Frame Rate
Measured in frames per second (fps). Higher frame rates suit fast-moving subjects — critical for automotive cameras and sports capture.
4. Pixel Size
Measured in micrometers (µm). Larger pixels improve low-light performance but, for the same sensor size, reduce resolution. This is a fundamental trade-off.
5. Sensor Size
Sensor size decisively influences image quality:
- Pixel Size Impact: Larger sensors can accommodate larger pixels, capturing more light, improving dynamic range and color depth
- Depth of Field: At the same aperture, larger sensors produce shallower depth of field (more background blur)
- Angle of View: Sensor size changes the effective focal length. The same lens has different equivalent focal lengths on different sensor sizes
- Image Quality: Larger sensors generally deliver lower noise, wider dynamic range, and higher color accuracy
Optical format conversion: Diagonal (inches) = Diagonal (mm) / 16
| Sensor Size | Diagonal (mm) | Typical Use |
|---|---|---|
| 1/4” | ~4.0 | Compact embedded |
| 1/3” | ~6.0 | Surveillance |
| 1/2.5” | ~7.1 | Industrial |
| 1/1.8” | ~8.9 | High sensitivity |
6. Dynamic Range
Measured in dB (decibels), defining the range between the darkest and brightest areas the sensor can distinguish. Wide dynamic range preserves detail in extreme lighting conditions.
7. SNR (Signal-to-Noise Ratio)
The ratio of image signal to background noise. High SNR means clearer images with less random noise — especially critical in low-light environments.
8. Output Interfaces
| Interface | Characteristics | Typical Use |
|---|---|---|
| USB 2.0/3.0 | USB 3.0 faster than 2.0 | Consumer cameras, PC equipment |
| MIPI CSI-2 | High data rate, D-PHY/C-PHY | Mobile, embedded |
| GigE | High-speed network, long distance | Industrial cameras, remote monitoring |
| Camera Link | High bandwidth | Industrial vision, high-speed/high-res |
MIPI CSI-2 Sub-specifications:
- D-PHY: ~2.5 Gbps per lane (up to 4 lanes)
- C-PHY: ~7.5 Gbps per lane (up to 4 lanes)
9. Output Image Formats
| Format | Characteristics | Use Case |
|---|---|---|
| RAW | Uncompressed, maximum information | Professional photography, scientific |
| JPEG | Lossy compression, small file size | Consumer applications |
| YCbCr 4:2:2 | Chroma subsampling | Video transmission/storage |
| Raw Bayer | Sensor native output, 1 color per pixel | Professional/scientific post-processing |
Industry-Specific Sensor Requirements
Security & Surveillance
- High resolution, high frame rate, excellent low-light performance, wide dynamic range
- Recommended: 1/1.8” or larger sensor, Sony IMX series
Automotive Electronics
- Very wide dynamic range, vibration resistance, fast response
- Recommended: Global shutter, high frame rate, HDR support, OmniVision OV series
Medical & Aesthetic Imaging
- Very high resolution, accurate color reproduction, low noise
- Recommended: High-resolution sensor, RAW output, USB 3.0 or GigE interface
Industrial Inspection
- High precision, high reliability. Global shutter essential
- Recommended: High resolution + low latency + industrial standard interfaces
Lens Selection Fundamentals
Focal Length and Angle of View
- Wide-angle lenses: Short focal length, wide FOV. Ideal for surveillance, conference rooms, smart home devices
- Telephoto lenses: Long focal length, narrow FOV. Suitable for distant subjects, sports, wildlife observation
FOV calculation example: A 10m-wide conference room with a 4mm lens + 1/2.5” CMOS sensor → approximately 90° FOV covering the entire room.
F-Number (Aperture)
F = Focal Length / Effective Aperture Diameter
- Small F-number (bright lens): f/1.4, f/2.0 → Strong in low light. Shallow depth of field (background blur)
- Large F-number (dark lens): f/16, f/22 → For bright conditions. Deep depth of field (everything in focus)
Installation Distance & Subject Size
For monitoring a 2m-wide robotic arm from approximately 3m in a factory:
FOV = 2 × arctan(w / (2 × L))
- w = sensor width, L = focal length
- 1/2.5” sensor (width ~10mm), FOV = 90° → focal length = 5mm
Camera Module Selection Checklist
- Define the use case: Monitoring, inspection, AI inference, or night vision
- Choose interface: USB (UVC) for direct PC connection, or MIPI CSI-2 for embedded
- Low-light requirements: If needed, verify IR sensitivity and low-light characteristics
- Motion capture: Moving subjects require global shutter
- HDR: Required for backlit or high-contrast environments
- Lens: Determine focal length from installation distance and field of view
- Platform compatibility: Linux, Jetson, RK3588, Raspberry Pi support
How CSUN Can Help
CSUN Technology has over 15 years of industrial camera module development experience, providing:
- Optimal sensor and lens selection recommendations for your application
- PoC evaluation kits (from 1 unit)
- Embedded development, AI inference, and WebRTC streaming technical support
- End-to-end support from prototype to mass production
Related Links
- Camera Module Selection Tool — Select the right camera by application
- Products — Evaluation kits and production platforms
- Contact — Technical consultation and quotes