How do I know how 'strong' my ND filter is?
As pictured above, ND filters are 'measured' or characterized by at least one of three possible methods. All of these indicate precisely how dark (or strong) the filter is; how much the light reaching your sensor (film) is reduced.
As far as I know, B+W is the only brand that uses all of the three methods. The image above shows the darkest (or strongest) ND filter available on the market today. This filter reduces the light falling onto your sensor by a full 10 stops, which is equal to a filter factor of 1000x, which is equal to an optical density of 3.0.
The chart above shows you how all three methods of characterizing ND filters work.
B&W Neutral Density filters are available in the following strengths: 0.3 (2X) Reduces the light one f-stop. 0.6 (4X) Reduces the light two f-stops. 0.9 (8X) Reduces the light three f-stops. 1.8 (64X) Reduces the light six f-stops, and 3.0 (1000x) reduces the light ten f-stops.
A ND filter is darker (stronger) the higher the f-stop reduction, the higher the optical density and the higher its filter factor.
Some of the other manufacturers of ND filters only use one of the above methods to indicate the strength of their filters. For example, the Tiffen filter I own indicates ND 2.1 (which would be equal to 7 f-stops and a filter factor of 128).
The Filter Factor:
I frequently get students in my workshops who are disappointed with the effect or strength of their purchased ND filters. When they start using it they discover that it seems much 'weaker' than expected. Most often, they mixed up the filter factor with f-stops. The filter reads 8x and they expect 8 f-stops. Looking at the table above, you can see that, in fact, a filter factor of 8x is equal to 'only' 3 f-stops of light reduction.
Calculating exposure times with ND filters:
This is the 'million dollar' question, isn't it? Well not quite. It's actually really straight forward.
Say you are out shooting landscapes in full sunlight and instead of a 'regular' shutter speed of 1/250 (without the use of any filters) you want to blur the water and clouds in your composition using a long exposure. Using the table above, you want to calculate how your exposure changes using a 10 stop ND filter. This is the strongest single ND filter currently available on the market.
Here is how to do it:
1 stop down: 1/125s
2 stops down: 1/60s
3 stops down: 1/30s
4 stops down: 1/15s
5 stops down: 1/8s
6 stops down: 1/4s