A lens does not paint a rectangle. It paints a circle. The sensor is a rectangle you place inside that circle. Image circle is the diameter of the usable drawing. If the circle is smaller than the sensor diagonal, the corners go dark, soft, or both.
In one sentence. Image circle is the lens saying how large a sensor it is willing to light. Format is the sensor saying how large it is. They have to agree.
The only comparison that matters
Take the sensor diagonal in millimetres. Take the lens image circle in millimetres. The circle should be larger. How much larger is taste and tolerance: a little spare room helps against holder's decentering and against the ugly last percent of the glass.
On Camemaker the lens filter is Image Circle (mm). The sensor filter is Optical Format. You still do the millimetre check. Nicknames are for sorting. Millimetres are for coverage.
What failure looks like
- Hard vignette. Corners fall to black. A robot loses the edge of the aisle.
- Soft corners. The circle is just large enough on paper, but MTF has already collapsed.
- Colour shading. CRA and the circle run out together. Faces go magenta in the corner.
People blame the sensor. The chip is doing its job: counting the light that arrived. The drawing stopped short.
The coin-on-a-postcard test
Put a postcard on the table. That is the sensor. Put a drinking glass on it. That is the image circle. If the glass sits inside the card and leaves the stamps uncovered, you have a 1/2.5" lens on a 1/1.8" chip. No ISP mode called "digital zoom" fixes the stamps. You cropped. You did not cover.
How to use the library
Choose the sensor format first. Convert it to a diagonal when you can. Then in the lens library raise the image-circle floor until the circle is larger than that diagonal. Only then pick EFL for FOV. A beautiful 2.1 mm that does not cover is not a wide-angle lens. It is a spotlight.
If a lens page is quiet on image circle, do not guess from the thread. M12 is a screw. It is not a coverage spec.