Is there a formula for that?? That would be a good thing to memorize and practice...
just remember that your tripod of exposure (iso, shutter speed and aperture) are all dealing with stops. One stop down half's the amount of light. One stop up doubles the amount of light.
The difference between 800 and 1600 ISO is one stop. 800 is half as sensitive to light as 1600. The difference between f/5.6 and f/1.8 is almost 4 stops. So that's 8 times as much light at 1.8 than it is at 5.6 (not quite, but close, 1.8 isn't on a full stop, its a third of a stop and I haven't had coffee yet this morning so im not going to attempt that math).
A good rule of thumb for beating the blur is you want your shutter speed to be twice the (effective) length of your lens. So for instance if you are shooting at 50mm on a crop-sensor camera (I think stu is the only one around here without a crop sensor to my knowledge), that is effectively an 80mm lens (1.6 x 50mm), so you want to make sure your shutter speed is 1/160 or faster to beat the blur. If you are using a 105mm lens, that is effectively a 168mm lens (1.6 x 105) so you would want about 1/350 or faster.
For more information about stops, check out this link: http://www.uscoles.com/fstop.htm
Its a lot of information and don't worry if it doesn't all make sense, but its all good info.
