Originally Posted by

**Panopeeper**
I have been thinking for a while about a fitting analogy. Here is one; I am not fully happy with it, but I don't have any better. Suggestions are welcome, we will share the royalty fees.

Let's imagine we have water in a container and we want to measure its volume.

First we measure it in gallons. If the amount is large, like in a swimming pool, we get a large number and a relatively good measurement. However, if there is much less water there, the resulting number is lower, and the difference between the measurement of two containes with almost identical amount of water is very small to zero. In order to get a more accurate measurement, we measure the leftover (less than a gallon) as well, in quarts. If the amount is even less, we have to measure the leftover after the quart, so we turn to a pint.

This way we can measure and compare small amounts as well. However, there is a downside: the smaller the unit, the less the accuracy of measurement, some measurements will be off.

A pixel well captures photons; these turn into measurable electric charge. The higher the exposure, the higher the difference in the charges of two pixels, which were not equally illuminated.

The closer we "look", the more fine measurement we can take, thus we can distinguish between like charges. At the same time, more and more measurements will be off.

This off-measurement is *one* source of noise.

Now, to the substitution of ISO gain by multiplication: if we measure everything in gallons and multiply the result by eight, then we get that very measurement in pints - but that's not the same, as if we had *measured* it in pints.