The following long exposures were taken during daylight hours using an ND400 and ND8 filter stacked together on the front of my Sigma 10-20mm lens. The combination of these two filters attenuates so much light that peering into the viewfinder reveals an inky blackness even during bright daylight conditions.
It’s therefore necessary to compose the image BEFORE screwing the filters onto the lens. It’s also necessary to adjust the focus manually since the auto-focus system has no chance of working under such conditions.
*** (Update October 2012) The introduction of “live view” to many DSLR cameras means that is’s no longer necessary to compose and pre-focus with the dark filters off the lens. I shoot all the time now using live view to peer through the combined darkness of an ND400 and ND8. The filters now stay on the lens which means there is a lot less fiddling around. If you’re looking for a guide to show you how to use an ND400 have a look at my recent blog post here.***
I found that if I set the ISO as low as possible, and set the aperture very small (i.e. a high number), then it was possible to obtain a shutter duration of about 3 minutes. Such long exposures during daytime create some interesting images. I’m looking forward to experimenting more with the ND400.