What's a Stop?
We're photographers so we all understand the concept of a stop. In terms of exposure, it simply refers to doubling or halving of the amount of light striking the sensor by changing the shutter speed or the lens' aperture. With digital, we can also change the sensitivity of the sensor - also measured in stops - by doubling or halving the ISO value.  Simple, right?

But what happens when we get our images onto the computer.  What's a stop of brightness in an RGB or Lab color space? Recently I got sucked down the rabbit hole of exposure and grayscales to sort out a few answers for myself. What I found was revealing and a bit surprising, at least to me.

It started when I was watching an online video by a well-known photographer and author. He displayed a grayscale like the one above. He explained that in the Lab color model 50% is considered middle gray and then defined a "zone system" using zones 0 through X as shown. All of that made sense and I'm sure we've all ...