How to calculate very long exposures

We are all so used to the fact that our camera is doing all the thinking for us, focus, exposure time, aperture etc…, that most of us will simply set the camera to the automatic modeP mode on my Canon EOS 30D, or worse use of the Program modes or those among us, me for example, who strictly use the Av, TV, and M modes, even if we decide about the shutter speed, aperture and/or ISO sensitivity, we greatly rely on the camera’s light meter to get the other variable.
But what about situations where the exposure time is beyond the capability of our camera like, for instance, very long exposure times that are greater than 30seconds?

Well even in such situations we can rely on the light meter of our camera but we will have to do some thinking on our own too.

1) Boost your camera ISO to the highest ISO possible (typically 1600 or 3200 on most DSLRs)
2) Switch your camera to Av (Aperture Priority mode)
3) Set your lens to the desired aperture
4) Check the proposed exposure time of your camera, if you are still getting a blinking 30 then you should skip to plan B
5) Now is calculation time, let say you have your camera set to f/16 and ISO 3200 and the camera proposed a 2sec exposure time.
Set back your camera to ISO 100, from ISO 3200 to ISO 100 that’s 5 stops100 200 400 800 1600 3200 than you should reduce your shutter speed by 5 stops also that’s 2sec, 4sec, 8sec, 15sec, 30sec, 1min.
Plan B
If in step 3 you set your lens to the desired aperture and in step 4 the exposure time is still more than 30Sec, then start by gradually open your lens until the 30sec is no longer blinking.
e.g. Camera set to ISO 3200, f/16 the 30sec is still blinking we start by opening the lens aperture. The blinking stops at f/11 so that’s 1 additional stop we should compensate for.
In summary we should compensate for 5 stops in ISO difference from 3200 to 100 and 1 extra stops from f/16 to f/11. that’s a total of 6 stops. So if we got 30sec at ISO 3200 f/8. This would be 32 min at ISO 100 and f/16.

If you need to understand more about the ISO, Aperture couples please refer to this article.