CLASSROOM: Here’s How Sensor Sensitivity Really Works
Let’s start at the very beginning. At the most fundamental level, light detected by a sensor is represented by a number of electrons sitting in each photosite. To put this in perspective, Fairchild’s LTN4625A sensor has a “full well capacity,” a maximum number of electrons per photosite, of over 40,000. It’s easy to assume that if a sensor has (say) 16-bit raw output, the 40,000 electron capacity would read as full scale, or 65,535, and no electrons would read as zero. At this stage, the sensor is pretty linear; double the light and you get double the electrons.
There are a couple of problems with that explanation, however. The first issue is that all sensors have a “dark current,” representing an electron count that might be read from a photosite regardless of whether any light fell on it or not. Also, they have “read noise,” which is the sum total of all the sources of inaccuracy throughout the entire device. The LTN4625A has an average dark current of 15 electrons, and an average read noise, in rolling-shutter mode, of 2 electrons.
Image credit: Vladimir Arndt