What is a variable neutral density filter? The neutral density bit means it is a filter simply designed to block some of the light getting into a camera. The variable bit means it is variable - you can control the darkness of the filter just by twisting one part of it. A proper variable neutral density filter can cost £100 or more!
Why would you want to block light getting into the camera? In short; control. A fully manual camera can be controlled via exposure time, aperture size and film speed. Adding a variable neutral density filter adds control of the amount of light entering the lens too. This lets you increase the exposure time and/or aperture while using the neutral density filter to prevent overexposure.
What effects are possible? The main tricks with a neutral density filter are to get shallow depth of field (a wide aperture) or long motion blur (a long exposure) under bright lighting conditions. This makes it very handy for taking portraits or nature shots, where you have bright lighting but want a shallow depth of field, or capturing the feel of a public event while bluring out individual people as they move.
How does it work? This method uses the properties of polarised light, specifically that two parallel polarisers will block very little light but two at 90 to each other will block nearly all light travelling through them. Find out more about polarisation, including in photography, here. This variable neutral density filter is far from perfect, but great if you want to make one cheaply!
Step 1: Raw Materials
All you need is two circular polariser filters (of the correct size for your lens of course!). There is no point spending too much on these, I used an old slightly broken one and one I picked up for about £5 on Ebay.
Step 2: Dissasemble One Filter
Pick one filter and dissasemble it. Filters are normally easy to take apart; just unscrew the retaining ring from the front size of the filter. Most have a couple of small notches in the retaining ring which you can push round with a small screwdriver. Remember righty tighty, lefty loosey...
Be careful with this step - it is very easy to slip and scratch the filter. Make sure you don't do this with a polariser you care about!
Step 3: Flip the Filter and Reassemble
Flip the polarising filter and then reassemble the lens by screwing the retaining ring back in.
It is easy to check the correct orientation of a circular polariser if you get confused. Find a polarising light source, most LCD screens should work, and look through the filter while twisting it. For the normal orientation of a circular polariser if the filter gets darker and lighter then you are looking through the camera-facing side of the filter. If filter stays the same lightness but changes colour slightly (normally yellow to blue) then you are looking from the other side of the filter. For the reversed orientation of the circular polariser (for this step) the opposite applies.
Make sure you mark which filter has been flipped so you don't get confused!
Step 4: Final Assembly
Screw the two polarising lenses together making sure the normal filter is on the camera side and the flipped lens is on the other side.
If you don't plan on using the unmodified filter as a polarising filter on it's own then you can glue the two together to make sure you don't get confused. If you do want to glue the filters together be careful; to use the filter you have to be able to twist the front filter while the back filter remains stationary - don't jam the twisting mechanism with glue! Also avoid using cyanoacrylate-based glues (eg. superglue and krazy glue), their vapours can fog the glass.
Step 5: Usage
To change the darkness of the filter just twist the front polariser while keeping the back filter stationary. A twist of 90° will take you from maximum darkness to maximum clearness. My filters gave me about a 10 f-stop range, from ~4 f-stops darker to ~14 f-stops darker.
The light from the filters entering the camera is circularly polarised so should work with all digital camera autofocus and metering mechanisms. Unfortunately, because this method is based on polarisers, you will see some of the normal effects of polarising lenses - bear this in mind if you are photographing reflective objects such as glass or water. The filter construction is also quite thick so you might get more vingetting, especially at short focal lengths on zoom lenses.
Depending on the quality of the filters you might see some colour changes depending on the orientation of the filters, blue in one direction and yellow in the other. The blue tint can normally be countered with a fluorescent light white balance setting.
Step 6: Applications
This filter is useful any time you want to capture shallow depth of field or motion under bright lighting conditions, things you might want to try are:
A flower under bright sunlight - use a wide aperture and a dark neutral density filter to capture the flower, without overexposure, with a nicely blurred background.
People in movement - use an extremely long exposure time and a very dark neutral density filter to blur the movement of people through a public space.