Where does the light go when a lens with a larger image circle is projected on a smaller sensor?

Question

I am using a full frame lens on an APS-C body.

What happens with all the light, that doesn't reach my sensor, but would in a full frame camera?

Do manufacturers take special measures so that light does not scatter onto the sensor from the sides, like coating the inner parts of the camera as dark black as possible? (when I take a look inside my camera it doesn't really seem to be that whay..)

Same thing for lens adapters: I am using a Canon EF lens on a non-Canon APS-C Body. The two mounts on the adapter are not the same size, so there has to be light hitting the walls of the adapter? Those parts do look quite "shiny" to me"

Answer 1

In general cameras do use darker materials to construct light boxes and the interior surfaces of lenses. Many also use materials, such as the flocking material found on the inner surface of higher end lens hoods or plastic that is molded with a textured surface, that scatter what little light is reflected.

Even when using a camera with a full frame sensor a significant portion of the image circle projected by a full frame lens falls on areas outside of the rectangular imaging sensor. The light is either absorbed by the materials/coating of the areas it strikes or is reflected. With materials that scatter the reflected light the portion that may wind up falling on the sensor is so diffused and evenly spread out that it will normally not affect the image captured by the sensor in any meaningful way.

It will be much like what happens when we take a long exposure photo and people who cross the field of view are not visible in the image recorded. The minimal light their presence cast on the sensor for a few short moments is overpowered by the light from the same areas of the scene behind where they moved that shone on the sensor for all of the exposure except those fleeting moments.

Remember, the light going directly through the lens and striking the sensor directly will almost always be proportionally brighter that diffused reflections of the light from the edges of the image circle.

There are cases where extension tubes, telescope adapters, etc. are not coated with dark, light scattering materials. Sometimes they do cause reflections that wind up in the final image. This is most often the case if the overall scene is very dark but there are small, very bright light sources in the scene.

The ring on Jupiter was likely caused by a T-mount adapter with a shiny interior between a telescope and camera:

Answer 2

Lens produces circular image (image circle), which projected on rectangular matrix. So question is not only about full frame lens on a crop sensor.

AFAIK black box (mirror box) is specifically covered in material that absorbs light well. I remeber that in older cameras (my film Nikon F90) this space is covered in fuzzy (furry?) material so that light bounces between hairs and not into camera. I didn't stick fingers in my newer DSLRs, but from pictures it seems that mirror box is covered in porous material. It works same way as "bumpy" materials that line walls of sound-proofed rooms.

Answer 3

The interior of the camera is coated with a light absorbent paint plus the surroundings are shaped like the insides of a bellows. The accordion shape has sharp angles that reduce internal reflections. The surrounds of the image sensor / film area consists of a flat black mask. All this mediates internal reflections. Otherwise, internal reflections bring about flare. Flare is devastating, it reduces image contrast.

Answer 4

The light scatters around in the camera and reduces contrast by a diffuse light background.

Often telescope tubes are fitted with black velvet, so that light is absorbed. This is reduced diffuse scatter and increases contrast.