A photosite filling up
The path from light to a number, as a well that fills: photon rate and exposure time set how many electrons are collected, gain converts them to a voltage, and the converter rounds that to an integer. The well has a ceiling, and hitting it is how a highlight is lost for good.
Try this
Set a bright photon rate and lengthen the exposure until the well clips. Then drop the bit depth and watch the same scene lose the levels that told two dark tones apart.
Where this lab comes from
From Photons to Counts: What a Raw Number MeansA photosite collects electrons, a converter turns them into an integer, and that integer is all any algorithm ever sees. Measured on eighteen exposures of one scene: the count doubles with the shutter to within a few percent, then sticks near 3875 DN — and the ceiling the file declares is not the ceiling the sensor has.
More in the image itself
- One colour per photosite — Throw away two thirds of the colour, then guess it back.
- Hitting the colour a meter measured — Drag the gains and watch ΔE against 22 metered patches.
- Sixteen by twelve real pixels — Hover a pixel, read its index and its three numbers.