Hi Tim,
They go a lot darker than that! B+W have a on their ND filters, the ones I have are 103 and 106, meaning that if stacked they give a reduction of about 9 stops, so i can do 3,6 or 9. This is generally enough for me - but they also make 110, 113 and 120 filters - the 120 gives 20 stops of reduction. Sadly some of the higher values seem to be no longer made by B+W - but you could possibly find one second-hand. Heliopan do them as well - see page 7 of their . The stronger ones are surprisingly difficult to find, PM me for a source of ND1000.
The higher values are typically used in astronomy - e.g. very long exposures of the night sky - for these you need a tripod plus a rotating platform (unless the aim is to make a star trails photo).
Some pics with the ND filter: Witches' Cauldron, Loch Arkaig, Scotland; in this one, the sun was obscured, and the dynamic range is just about bearable:
The following picture has two problems - firstly droplets of water from the falls have settled on the lens making everything slightly out of focus, and secondly, the sun has come out, this was shot on slide film, and the dynamic range of the film just isn't great enough to capture the scene properly - the highlights are blown out, and the quartertones are all plugged.
Actually there is another problem now - saving this as .jpg has created some nasty banding in the darker areas of the picture. I've got both side by side - web and Photoshop and the Photoshop image doesn't have the problem.
The dynamic range problem makes me wonder whether this type of photography would also benefit from HDR (high dynamic range) treatment.
Same waterfall, with a rainbow:
Finally an old favourite of mine - Allt a'Chumhann near Kinlochleven, Scotland:
The water is nice and clean, and the long exposure tends to make it invisible - the picture is mostly of the stream bed - except where there are some air bubbles in the flow, those areas show up white-ish. The blue tint to the water isn't Photoshopping, by the way - reflections of the sky over a long enough period of time make the water slightly blue where it is at the right angle. Some of the foliage which appears out of focus probably isn't - over a long enough period even on a fairly still day you will get some motion blur as leaves move around. The professional would pitch a tent and hang around till the wind died down, given that this is in Scotland, it could be a while
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I don't have details of how long the exposures were, I'm afraid... I'm not that kind of photographer. I know some people keep detailed notes, when I have tried that, I just lose the notes, or I can't pair them up with the photos afterwards, or something. Actually, I was shooting on slide film until a couple of months ago, when I gave up the unequal struggle and went digital - so now the camera does keep track of stuff like this for me - exposure lengths and amazingly it even seems to know what the aperture was - even for 3rd party lenses, which amazed me the first time I saw it.
By the way, I'm viewing these on a Mac, and I know the PC uses a different gamma... I'm hoping they still look OK on a PC, I don't have one I can use to check (must get a bumper sticker saying "my other computer is also a Mac").
I hope this encourages anyone who is interested to try an ND filter (hint - if you have several lenses requiring different filter sizes, just buy the biggest size one and use a step-up ring for the smaller lenses. Just watch for vignetting if you're lucky enough to own a super-wide lens). It's great fun and if you're using digital, you don't have to worry about wasting loads of film if it goes wrong.
ND8's are great, not too expensive, and depending on the minimum aperture of your lens, at f/16 you are probably exposing for 1/15th or 1/30th, and if your camera will go to f/22 then you have 1/8th and if it will go to f/32 then you have 1/4. At 1/8th or 1/4sec you are starting to get some nice motion blur in water. On a slightly overcast day, you can get more - say 1/2sec to 1sec.
ND64s are more expensive, but on a nice sunny day you can make an exposure of perhaps a second, or two if overcast, and if you stack an ND8 with an ND64 you can get 9 stops, maybe up to 8 seconds. ND113 will give about 30 seconds of exposure. Another hint - if you've got an ND8 and you're depserate to get a longer exposure, you can stack it with a polariser for an additional 2-stop typical reduction. Yes, I have done this...
Will stop now as I am going on too long... I'll leave you with one more - not sure whether this really works as intended, I probably wanted an even longer exposure, but it's fun to experiment: