As you will soon understand, I am not a photographer. For research in a completely different field, I am interested in estimating, even with a large approximation, the brightness of the environment in a series of pictures.
I won't enter the lux vs illuminance vs luminance topic - let's say I am OK with anyone of these. So, as far as I understand:
EV to Lux
There are tables like this that, maybe with huge uncertainty, let you pass from EV to Lux, and you can get EV from exposure time and f-number. Fine!
1. What about ISO?
These tables work with
ISO = 100. From my understanding, if I have
ISO = 200, I just need to add 1 to my EV, thus looking at the row below. Is this correct?
2. What about pictures not exposed correctly?
If what I said in
1 is correct, I should be able to infer Lux by simply using ISO, exposure time and f-number. However, I could take two pictures, varying a single parameter between the two, and I would get two different Lux values even if the scene was the same.
For my application, I think another parameter should come into play, which might be the average brightness of the picture (the mean of all pixel values), that numerically stands between
Automatically exposed pictures tend to have average values near
128. My wish is to take that into account, and, if my average value is, say,
150, add, say,
1 to the EV because the scene is presumably brighter.
Does this, approximately speaking, make any sense? Is there a relation between a single stop and the consequent variation in the average picture brightness?
Note: of course this logic breaks if the picture is very underexposed (has some 0s) or very overexposed (has 255s) because then you can't use the average brightness information. Let's say all pictures have average brightness between 80 and 200.