I recently bought a neutral density fader also called a neutral density variable filter. It’s a nice tool to get some long exposure but it’s not a plug and play tool. You need to learn how to use it.
My first trials at long exposure with my new fader were just too dark, after a few time I found that a I need to apply a correction of the exposure of +0.7 when I use it close to its maximum. I am not sure if it does apply to all fader but I definitely in my case.
The focus was my second problem and something you need to adjust before setting the fader. When set at the maximum, it’s just too dark and the auto focus does not work. You then need to switch to manual but even manually it is difficult to set the focus right. What you need to do is set the fadder to a lighter tone, set the focus and then adjust the fadder where you want it.
Very important : If you are doing long exposure, remember to block you visor in order to stop the light entering there, for that I use the black electrical tape. It does the job.
The other result that a fader can produce is to give you a wider aperture for the same speed. If you cannot shoot below a certain speed and wants a wider aperture for a shallow depth of field, the fader will help you achieve that. Unfortunately I do not have a good example of that.
So here are two of my first trials at long exposure
Sunset on the river
Nikon 18-55 mm at 18 mm, 100 ISO, F/22 at 15 seconds, corr +.07 with ND fadder
This one is interesting, the sky was cloudy and a few seconds after I pressed the button, a few rays of sun popped for a few seconds giving a shinny orange color to the water. I took many shots that day and only this one has those shinny orange spots.
Nikon 18-55 mm at 55 mm, 100 ISO, F/36 at 25 seconds, with ND fadder (HDR)