There is only one level of light on the film/sensor that is correct, but there are several combinations of shutter speed and aperture that will give you the correct amount of light.
In your example f8 @ 1/60th gives a correct exposure, if you change to f5.6 then 1/120th would give the same correct exposure. What we have done is twice the amount of light for half the time, nett result the same level of light on the cameras sensor. So you could also use f4 @ 1/250th, or f2.8 @ 1/500th.
You can continue right across the shutter speed range the problem is the lens will run out of wider apertures.
Another alternative is to use a higher ISO, literally make your camera more sensitive to light. So instead of increasing the aperture by one stop you can go from 200 ISO to 400 ISO, with the same result. There will be more noise, but it will be OK until you get to the point of having to use high ISO such as 1600 ISO, then noise will certainly be present.
I am guessing here, but I bet you are using a Canon camera, what is not generally realised is Canon's old technology Cmos sensors are at least 2 stops less sensitive than any DSLR with a CCD sensor. What is 1/60th of a second on your Canon will be 1/250th of a second on a Nikon, Pentax or any other camera with a CCD sensor. The new Cmos sensors from Nikon and Pentax are about 1.5 stops faster than Canon.
Chris
