/diy/ - Do It Yourself

Camerafag here, the 556 circuit is sitting in a pile of dysfunctional wires waiting to be re-breadboarded, but that's not what I'm talking about here. I went through the definitions of F-stop and ISO and found myself a general equation for a well exposed shot:
>I = F^2/(ISO * T)
Where I is the effective relative intensity, F is the F-stop which is proportional to the inverse of aperture diameter, ISO is the sensor sensitivity, and T is exposure time. It makes sense because rearranging it 1/ISO = I/F^2*T since 1/F^2 is a unit of area. So I guess ISO has units of inverse joules, or perhaps volts/joule or volts/photon, but that's also not particularly relevant.

What I want to make is some small light sensor that gives me a reading (checking with a multimeter is fine) that I can convert into an ISO, exposure time, and F-number to tell me how to get a well exposed shot, primarily for multi-hour exposure star photography.

So I calculated this intensity value for a series of well exposed shots under different lighting conditions, where I also taped an LDR to the top of the camera and measured its resistance. For relatively normal lighting conditions (curtain up to curtain down, camera pointed at opposite wall) I got a pretty good relationship between LDR resistance and intensity. Pic related. However once I darken the room further (inside at night time with a small dimmable LED), the resistance went up past 60MΩ (the maximum my meter can read) and my camera started to tell me it was too dark for its maximum non-bulb exposure time coupled with the largest aperture size. I think I can compensate for the latter problem by cranking up the ISO, but the LDR is still a problem. Anybody else tried detecting very low light levels? I can't seem to find much documentation, besides advertisements for $100 avalanche photodiodes.