For a given film type, shutter speed and aperture, how do I calculate the light needed to get a good exposure?
For example, if I use a f/8 aperture, an exposure time of 100 ms and a ISO 800-film (just as an example) - how much light is needed (per cm^2 film or any other measure) to get a good exposure?
I have read wikipedia's explanation of how light conditions relates to exposure, but I don't really understand it. All help is much appreciated!
When you are calculating the "correct exposure" ("Correct exposure" being the amount of light necessary to achieve a negative with a given density) you are actually playing with shutter speed, aperture and film sensitivity. If you restrict those variables to a fixed value, the only things you can do is overdeveloping or underdeveloping the film after the shooting or 'guessing' (Or calculating) what light you need to get a correct exposure and then shoot only in those situations.
If you are in the second situation, I recommend you the f/16 rule. (for any given ISO sensitivity under direct, not very harsh sunlight, you need to shoot at the nearest speed to that ISO and f/16. e.g. ISO400, 1/500. f16). Or, if you have a light meter handy you can measure with those values and see if a given spot or area is within the acceptable lighting range for your settings.
If you actually want to calculate the actual 'amount of light' needed you will need the specific curves of the film and previous tests with the film, developer and fixer you are going to use, which takes time and (to be totally honest) is a royal pain in the ass. If you are curious the Ansel Adams trilogy has everything you need to do this (Book 2 talks about this procedure throughout the chapters but doesn't have one specifically for it), if you are looking for a practical approach to take photos without much hassle it will not be worth it.
If, as I guess, you are doing this to calculate the correct exposure on a pinhole camera, you should know that pinhole cameras are essentially cameras with fixed aperture. You can caulculate the aperture (f number) simply by dividing the distance from the pinhole to the film by the diameter of the pinhole. There are several other useful formulae you can use, like d=sqrt(2fw) [f:f number; w: wavelenght of the light you want to see sharper] for the actual pinhole diameter. You have plenty of resources for this online, and following them will be way easier than going to the brute-number approach.
Through @MichaelClark's help to clarify my question and @Achifaifa's hint to look at film curves, I found the answer to my question.
It turns out that answer lies in the film characteristics curve. It plots the density of the silver halide in the film (i.e. how opaque the film is) as a function of the amount of light the film is exposed to (measured in log[lux * seconds]).
The scale log[lux * seconds] is usually also translated into f-stops to make it more practical.
To get a "good" exposure, I probably need the amount of light that puts me in the middle of the curve. In the figure above that is -1.0 log[lux * seconds] which is the same as 0.37 lux*seconds. With a fixed exposure time of 0.1 seconds, I would need 3.7 lux.
Thank you for all help!
There is a standard concept in photography called "exposure value". The question and answers at What is the EV scale? go into this in detail, but the quick version is that this is a series of numbers representing the amount of light in different scenes. Different combinations of aperture, shutter speed, and ISO can correspond to the same exposure (see this answer if you're not familiar with this concept), and therefore the same exposure value.
You're basically asking how to work backwards from those known settings to the exposure value, and then what that exposure value actually represents. That's answered at What is EV, when used as an absolute measurement?, and one of the answers even gives EV equivalents in lux and foot-candles.
(That answer also gives a useful link to a Wikipedia note about light meter calibration constants, where you can find the answer to the concerns Michael raises in comments about what exactly "good exposure" means in this context — the short version is that it's subjective but roughly standardized.)
You may also find Wikipedia's "Tabulated Exposure Values" to be helpful in a less precise but more pragmatic way. This gives things like Typical scene in full or slightly hazy sunlight (EV 15), Bright street scenes (EV 8), and Home interiors (EV 5-7).
