Lens image circle vs. sensor size calculator
Lens image circle too small? The corners go dark — see the largest usable crop. Too large? You lose field of view and lens resolution — see how much reaches the sensor.
Details
How it's calculated
A lens produces a round image, the image circle. The rectangular sensor is only fully illuminated if the image circle's diameter is at least the sensor diagonal:
sensor diagonal = √(width² + height²) · full coverage when image circle ≥ diagonal
Image circle too small: the corners stay dark (vignetting). We compute exactly what share of the sensor area lies inside the circle, and the largest fully lit crop in three variants: same aspect ratio (scaled by image circle / diagonal), full sensor width (height = 2 · √(r² − (w/2)²)) and full sensor height. In practice brightness and sharpness fall off before the nominal edge of the image circle.
Image circle too large: everything is lit, but part of the image falls beside the sensor. The lens spreads its rated resolution over its own image field, and only the area share of it lands on the smaller sensor:
lens MP on sensor = lens MP × (sensor diagonal / image circle)²
If that is below the sensor's pixel count, the lens limits the usable resolution.
All results are planning estimates based on simplified models and manufacturer data, without guarantee. Please verify critical values against the manufacturer datasheets before you make a design decision.
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