A carefully selected lens provides high-quality images from the objects. The image circle is formed when a light strikes a perpendicular target, i.e., the sensor, which forms a circle of light.
Below, Figure 1 shows the relation between the lens image circle and sensor diagonal. As a lens-related property, the image quality typically deteriorates towards border areas, and the images might suffer from shading or vignetting.
To avoid mechanical vignetting, choosing the right size optics with the sensor is necessary. If the lens image circle or lens mount is too small, the image will be heavily vignetted (lower-middle image).
Another source of vignetting, Cos4, is seen in the upper-middle image (above, Figure 1). If the light travels to the edges of the image from a further distance and reaches the sensor at an angle, it affects image quality; the light falloff is determined by the cos4(θ) function, where θ is the angle of incoming the light with respect to the image space’s optical axis.
The drop in intensity is more significant for wide incidence angles, causing the image to appear brighter at the centre and darker at the edges (Greivenkamp 2004). Since it is sometimes difficult to find inexpensive optics of the right size for the desired sensor (other parameters that depend on the target might limit the selection), the lens image circle can be oversized. In such cases, the images can be cropped and used, or the effect can be controlled by decreasing the aperture size.
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References
Greivenkamp, John, E. 2004. Field Guide to Geometrical Optics, Vol. FG01. SPIE, https://doi.org/10.1117/3.547461.