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What a lens actually does

It draws a circle. The sensor is a rectangle you place inside that circle.
​ 12 September 2026 by

A lens has one job: take the light from a scene and draw it as a circle on the sensor. Focal length sets how much of the world fits in that drawing. F-number sets how much light gets through. Image circle sets whether the drawing is big enough for the chip. Everything else (mount, distortion, CRA, IR-cut) is how honestly that drawing is made.

M12 lens on an optical bench forming a circular image on a card
If you remember one picture, remember this: the lens paints a circle. The sensor is a rectangle sitting in that circle.

In one sentence. The lens draws. The sensor counts. If the drawing is too small, too dim, or too bent, no amount of megapixels will save the picture.

The few numbers that actually matter

On a Camemaker lens page the useful order is not the same as the catalogue order.

  1. Image circle. Must cover the sensor format, or the corners go dark.
  2. EFL, effective focal length. Together with sensor size, this is field of view.
  3. F-number. Light, depth of field, and diffraction.
  4. Projection. Rectilinear and fisheye do not share a FOV meaning.
  5. CRA. The ray angle at the edge must fit the sensor's taste.
  6. Mount, TTL, BFL. Whether the glass will even sit on the holder.
Board camera module with an M12 lens on a workbench
Most Camemaker lenses live on a holder like this. The glass is short. The mistakes are not.

A doorway is enough

Stand in a corridor and look at a door at the end. A short focal length shows the walls and the door. A long one shows only the handle. Same door. Different drawing. That is EFL.

Now dim the lights. A faster F-number keeps the handle. A slow one turns it into noise. That is aperture.

Now put a large sensor behind a tiny circle of glass. The handle is sharp in the middle and the walls fall into a dark vignette. That is image circle versus format.

What a lens is not

It is not a FOV. FOV is a marriage of EFL and sensor size. Quote a lens as "120 degrees" without the sensor and you have quoted a rumour.

It is not "HD" or "8 MP" as a quality grade. A lens resolution line on the page is an MTF claim, not a sensor megapixel count. We will come back to that.

It is not interchangeable just because the thread is M12. Pitch, back focal, CRA, and the IR-cut decide whether the picture is usable.

How to use this on Camemaker

Pick the sensor first, or at least pick the format. Then open the lens library and keep only the image circles that cover it. Then set the FOV you need and the F-number the scene can live with. Mount and IR-cut come after the picture is possible.

The rest of the lens series is that list, one mechanism at a time.

What an image sensor actually is
A grid of wells that catch light. Everything else on the page describes those wells.