Every Lens Causes Shading

Shading is a well-known optical problem. Essentially, pixels farther from the optical axis on the sensor are imaged darker, resulting in a gradual darkening toward the edges of the image. This effect is symmetrical around the optical axis. Closing the aperture can increase the usable image area affected by this shading. Lens manufacturers have continuously improved to address this issue.

Shading Caused by the Camera

A surprising discovery often occurs when using modern CMOS industrial cameras, such as SVS-Vistek’s high-resolution hr342 with a Sony IMX342 sensor or those with a Canon 120MXSM sensor. Even with high-quality optics and advanced sensors, shading can become visible—a phenomenon not typically observed in less sophisticated cameras.

Cause: CRA

A CMOS pixel consists of a light-sensitive area and an amplifier area that is insensitive to light. Modern sensors incorporate microlenses above each pixel to direct incident light onto the sensitive part, enhancing sensitivity and reducing noise. However, this design requires light to enter within a specific angular range (chief ray angle or CRA). Light outside this range hits the insensitive areas of the pixel, causing intensity reduction and shading.

The Importance of CRA and Lens Compatibility

Light incidence is perpendicular on the lens axis for most sensors. But with entocentric lenses, pixels near the sensor edges experience oblique light entry. If the incident angle exceeds the sensor’s CRA, shading occurs.

  • Telecentric lenses avoid this issue by maintaining a perpendicular beam path across the entire sensor surface.
  • MFT lenses are often telecentric but not suitable for CMOS sensors with pixel shift design.

Proper alignment of lens and sensor is crucial to minimize shading-related loss in image quality.

Expert Advice Saves Time and Testing

CRA depends on lens design rather than focal length, making careful selection vital when working with sensors featuring small or inconsistent CRA values. The beam path on the sensor side is a key factor influenced primarily by the position and size of the exit pupil. SVS-Vistek has decades of experience in camera solution design and can assist selecting lenses tailored to your specific needs.

Contact us for expert support.

Last Updated: 2025-09-05 03:19:33