#1. "RE: "Fader" variable ND filters" | In response to Reply # 0esantos Nikonian since 10th Nov 2002Mon 31-Dec-12 04:36 PM
A lot of us here use the models made by Singh-Ray Filters, the Vari-ND and Vari-N-Duo variable neutral density filters. They are used anytime you need to reduce the amount of available light in the scene to be photographed. It could be to slow down the shutter speed (to photographically show motion) or to be able to use wider apertures (to control depth of field). I would NOT use them to view a solar eclipse, but I guess you could use them to photograph one if 8 stops of neutral density filtration is sufficient. These types of filters are limited to 8 stops.
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#2. "RE: "Fader" variable ND filters" | In response to Reply # 0AMusingFool Registered since 30th Dec 2012Tue 08-Jan-13 02:27 PM
I have used mine for water photographs, and for shooting the Venus transit last year (wouldn't have minded a 10-stop filter for that, but putting the variable all the way to 8 worked reasonably well).
One thing I've noticed about them (my dad's Tiffen did this, as does my Singh-Ray), is that they don't give a completely flat level of darkness. That is, they seem to often (whenever closed all the way? I haven't experimented a lot) have a spot that is darker than the rest.
Here's an example (unretouched photo):
Having said that, I've still gotten some that I liked a lot (including a couple that same day). Here's one of my favorites: