Originally posted by isusdfr if i am getting 1/20 at f6.3 what would i get at F2.8? Is there a rule that can be followed for shutter speeds at different apatures??

The F-stop notation means the width of the aperture is the focal length of the lens divided by the number. So at 200mm, f/2.8 means the aperture is 200/2.8 = 71mm; and f/6.3 means the aperture is 200/6.3 = 32mm. So f/2.8 is 2.25 times as wide as f/6.3. When you are comparing two F-stops, the focal length cancels so that's can be worked out more simply as 6.3/2.8.

Because the lens is a circle and lets in light over its entire area, a lens twice as wide lets in four times as much light. So f/2.8 lets in 2.25*2.25 = 5 times as much light as f/6.3. The sensor responds in a linear way, so to receive the same amount of light, the shutter needs to be open for 1/5th as long. So going from f/6.3 to f/2.8 should reduce the shutter time from 1/50s to 1/250s to give the same exposure.

Some people prefer to think in terms of stops, and they know all the magic numbers, f/1.4, f/2.8, f/4 etc, and they know the effect of moving up a stop or down a stop to the next magic number. I find it helps to know what the numbers mean and then just do the maths. The numbers aren't really magic. They arise out of the geometry, and are mostly based on the fact that the square root of 2 is 1.4. So in going from f/2.8 to f/4 you are multiplying by the square root of 2, which means letting in 2 times as much light, and halving the shutter speed.