08-24-2016 02:28 PM
For the Moon, you can use an exposure rule call the "Looney 11 Rule". No, I did not make that up.
In simple terms, the rule says that as long as your f-stop (Aperture value or "Av" setting) is set to f/11, then... the shutter speed is simply the INVERSE of the ISO setting.
So at ISO 100, you use a shutter speed of 1/100th.
At ISO 200, you use a shutter speed of 1/200th.
At ISO 400, you use a shutter speed of 1/400th, etc.
The direct inverse relationship only works correctly (for the Moon) at f/11.
If you learn how exposure settings work (e.g. get a book such as "Understanding Exposure" by Bryan Peterson or pick up Scott Kelby's "Digital Photography" series (which I think is no up to 5 volumes but the primer information is in the first volumes if you choose not to buy them all) then you'll realize that you can learn to "trade" stops of aperture for stops of shutter speed.
In other words you can use other f-stops (you don't just have to use f/11) if you trade stops of aperture offset by stops of, say, shutter speed.
Here's a video that helps explain this "exposure" concept: