Picture of Variable Neutral Density Filter
[UPDATE: thanks to Zephyris, which has reminded me with his good Instructable that you first have to reverse one of the two CPL filters, of course I've found his Instructable shortly after publish mine, as often happens to me, and in this case I've also forgotten I already commented his instructable two years ago...]

A Neutral Density (ND) filter is a cool gadget, you should always carry it (at least daytime). You can find easily ND2, ND4, ND8 filters, these would darken your scene of 1, 2 or 3 stops respectively. When daylight is very strong it's difficult to keep a very large aperture, so you couldn't have low deep of field or nice bokeh. In these cases an ND filter should be very useful. I also use it to keep very long exposures in daytime. You can obtain unique effects, with cars and people moving as ghosts.
Of course you'll reach a better quality with traditional ND filters, but if you have two polarized filters you can experiment a really funny phenomenon. I suggest circular polarized filters (CPL) rather than linear ones, because with CPL you can keep automatic focus of your camera, whichever is the rotation angle of the filter on the lens. You can buy CPL filters on eBay, or at your favourite photography shop, you can find every diameter.
ghavel8 months ago

Wouldn't limo dark window tint on a UV filter work similar to ND filter?

yellowcatt2 years ago
"Making toast pass through two barbecue grids" is a bit over simplified and does not explain why if you have two crossed plane-polarisers blocking most of the light and put another in-between them at 45 degrees you get will get more light passing through.
reptedjess2 years ago
Oh my god! How is it I've never though of this? Here I am using 4 different ND filters, when all I needed was this!
andrea biffi (author)  reptedjess2 years ago
you should try this, but don't give your ND filters away, because their quality is probably much and much better ;-)
efabric2 years ago
Isn't it possible to aproximate the same effect without a filter, at 100 iso, with a cheap dark lens and 16' aperture ?
andrea biffi (author)  efabric2 years ago
Hi, typical exposure values with cloudy weather are 100 ISO, f8, 1/125 s. If you want to reach about 2-3 seconds of exposure time you need to move of 8 stops, you can't stop down your lens enough (neither with 50 ISO and f22).
simondburt2 years ago
What if...

What would the outcome be if you were to take the glass out of the one and put it into a 'regular filter' ring? that way you could have a non-rotating base. This surely would make it much easier to then adjust the ND level on the fly so to speak?
andrea biffi (author)  simondburt2 years ago
Yes, you're absolutely right, I've already thought that, but...
To semplify adjusting darkness you need to block the "regular" one and leave the "reversed" one rotating, but doing that you'll lose the possibility to use the unmodified one alone, as a polarized filter.
magicexp2 years ago
Is it really a ND effect or is it a polarizing effect mod? PL are supposed to cut back reflection. Where a ND is to darken the lens preventing over exposure reflection still get's through a ND filter. There are occasions you need to slow down the shutter so you get a blur effect but you have too much light to do that. in some cases you might want to keep the reflection such as in a waterfall photo.
AJMansfield2 years ago
Making toast pass through two barbecue grids? How close together are you placing the grids anyway? I've never managed it with grids farther apart than the toast height no matter how I align them.
Interestingly, if you do this with 2 normal (unmodified) CPL filters, what you get is a filter that adds a slight colour shift, whose hue varies from yellow->pink->blue.
mrayw2 years ago
Good topic.
One note: The outer filter can also be a linear filter with no ill effects, as long as the filter closer to the camera is a CPL. If you do that, you won't have to worry about reversing a filter.
andrea biffi (author)  mrayw2 years ago
Yes, but probably (I may be wrong) your autofocus shouldn't work anymore... I tested it with my filter and it works also with very dark image.
Your autofocus will still work if you use a linear polarizer for the *outermost* filter, as long as the second one (the one closer to the camera) is a CPL.
andrea biffi (author)  mrayw2 years ago
Oh, that's interesting, thanks I've to try that.