One full "stop" means that you've either reduced the light exactlly by half -or- you've exactly doubled the light (depending on which direction you are shifting.) In the case of ND filters (which obviously only cut the light) each "stop" means you cut the light in half. Two stops means you cut the light in half a second time ... 1/2 of 1/2 is 1/4. etc.
In the Lee system (and B+W uses the same notation) the filters are sold based on "density" values.
Each "0.1" worth of "density" is equal to 1/3rd of a stop in exposure. That means each 0.3 density value is 1 full stop.
Lee sells practically every conceivable density up to 0.9 (3 full stops) and then skips a bunch of stops until they get to a filter they call the "little stopper" which is actually 6 stops (because if you wanted 5 stops you'd slide in a 0.9 (3 stops) and a 0.6 (2 stops) to get 5 stops). They also sell a filter called the "big stopper" which is 10 full stops (ND 3.0).
You'll find this is typical. Most filter makers do not actually sell every conceivable stop since you can stack filters. I carry a 2 stop, 3 stop, and 10 stop filter in my bag. This pretty much covers everything.
When you get to 10 stops it's usually because you're trying to take long exposures to strongly blur things like the motion of water (that's how you get those silky/creamy looking waterfalls.) BUT... 10 stops is SOOO dark that you'll find you are not actually able to focus the camera nor even see anything to frame up the shot. When using 10 stops you have to pre-frame and focus the camera with the filter out... then slide the filter in to take the shot. If you were trying to shoot a waterfall outside in mid-day sun and no shadows, then the sunny-16 rule would apply.... at ISO 100 and f/16, 1/100th sec would be the shutter speed. But a 10-stop filter would let you shoot at ISO 100, f/16, and a full 10 second exposure (plenty of time to create a lot of blur.) In less than full mid-day sun (a stop or two lower) and the exposure quickly turns into a 20 or 40 second long exposure. During the golden-hours when light may be chanigng rapidly you could easily exceed a few minutes. A 10-stop filter is "strong juju!" 😉
You can go to the Lee filters website to learn more about the system and you'll also see lots of "grad" filters (clear on one half... dark on the other) and you can get these in "hard" edges and "soft" edges (which is how fast it transitions from light to dark.)
Singh-Ray makes "reverse grads" -- I've not seen those from anyone else. Reverse grads are clear on one half (like other grads) but then they are ESPECIALLY dark in the middle and then fade to a weaker density as you get to the far end (most soft grads would have weak density in the middle and get strongest toward the end). These filters are mostly made for sunset shots... the idea is to heavily cut the brightness of the sun when it's on the horizon, not cut the foreground landscape at all, and then apply less of a cut to the sky high above the sun.
The filters and holders are interchangeable but they do come in sizes. E.g. Lee's standard size is 100mm wide (and the square NDs are 100x100mm, but the grads are longer (I think they're 100x150mm) so that you can adjust where you want the grad tinting to star in your scene (they're just held in with friction). In the Singh-Ray filters, they come in standard sizes measured in inches but it turns out 100mm = 4" for purposes of sizes (technically it's 3.93 inches but the holders fit because it's so close.)
If you go to the Lee filters website you'll see the filters, filter holders, etc. and get a much better idea for how the system works. You cannot buy the filters from them directly... you have to go through a dealer. This is the same company famous for their flash gels (and also theater gels) used to color the lighting.
Tim Campbell
5D III, 5D IV, 60Da
