Can magnitude be used to calculate exposure times?

[Kaptain Klevtsov]

I was pondering what to image yesterday (cloudy boooo) playing with the FOV option in starry night pro to check out what would fit where and as the potential targets all had magnitudes listed I could hazard a guess at how long the subs would be. It then occured to me that cameras use f/ numbers and ISO settings to work out the shutter speed given a light level known as EV. Could I work out, using magnitude instead of EV, how long to open the DSLR shutter for given that I know the f/ number and ISO setting, or is that silly?

I understand that the magnitude is related in some way to the angular size of the object, so that M31 has a magnitude of 4.5 but is still quite dim because of the sheer size of it. Anybody tried this?

Captain Chaos