Definition
The Maximum Image Circle (MIC) is the largest circular imaging area formed by a lens on the image plane, and its diameter is referred to as the Image Circle Diameter. The MIC is used to characterize the lens's coverage capability over the image sensor and is a critical optical parameter for matching the lens with the sensor.

In practical engineering, the MIC typically refers to the effective coverage area that meets the system's imaging requirements, rather than merely the area illuminated by light. Although light may reach a larger area, regions beyond the MIC often fail to meet imaging requirements due to illuminance falloff, increased aberrations, or degraded image quality. For conventional imaging systems, the MIC should cover the entire active photosensitive area of the sensor; otherwise, issues such as vignetting, field of view loss, or image cropping may occur.

Matching MIC with the Sensor Format
The image field formed by a lens is typically circular, while the image sensor is rectangular, resulting in different coverage relationships between the image circle and the sensor.
Let:

• $\Phi$: MIC diameter
• $W$: Sensor width
• $H$: Sensor height
• $D$: Sensor diagonal length

Based on the relationship between $\Phi$ and $W$, $H$, and $D$, the matching between the MIC and the sensor format will exhibit the following different states:

Selection and Design Requirements
When matching a lens with an image sensor, the MIC must fully cover the active photosensitive area of the sensor. For conventional imaging systems, the MIC is typically no less than the diagonal length of the sensor format to ensure effective imaging across the entire field of view.
In practical design, the MIC is usually larger than the sensor diagonal dimension, with a certain coverage margin reserved to compensate for imaging shifts caused by sensor mounting eccentricity, lens assembly tolerances, OIS displacement, and temperature variations.
Therefore, MIC selection requires not only considering the sensor size but also conducting a comprehensive evaluation in conjunction with system tolerances and other factors.

See Also
视场角_fov, 均匀性_shading