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Smoked Glass, Gray Theater Gel, Tinted Plastic... You need some sort of grayish substance to do the actual neutral density bit. That smoky gray tinted glass could be used, as long as you have the proper supplies to cut it to shape. Gray gel used for theater lighting could also be used, if perhaps a bit harder to get your hands on depending on your available resources. If you can find good quality transparent gray plastic, this might also be an easy way to make one. Whatever you use, make sure that there is enough of it clear of scratches to do the job.
As you can imagine, the darker the glass you choose, the less light that will be let through the lens, so test the glass before purchase to make sure it's as dark or light as you want it to be.
Also, make sure that the material is gray—hence neutral density filter. Any glass that is slightly colored will, well, slightly color your image. This may in itself be a cool effect, but it's not what we're covering in this DIY article.
Alternatively, if you have an old film camera handy, you can always expose film to varying degrees and use that for a filter medium.
Cutting Utensil. Depending on the material you chose for the filter itself, you'll need a different cutting utensil. If you're using glass, then a glass cutter is really your only solution to get a clean cut. Otherwise, if you're using plastic or theater gel, a box cutter will probably do the trick.
Sandpaper. Chances are, your edges won't be too clean, and they'll need to fit your filter mount exactly.
Lens cloth. You'll need something to clean off the future filter during the construction process to keep it clean.
Glue. You'll need something strong that won't puff up too much when it dries, as Gorilla Glue will. Hot glue if you're using plastic materials will do especially well.
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Your first step is to create a filter mount. There are a number of ways in which you can do this, which are described in this DIY filter mount article.
Once you've got the medium (as described in the supplies section) and the filter mount done, then it's time to put them together. Apply glue around the edges of the piece and snuggle it within the holder so that it is perpendicular at all sides, that is, perfectly vertical. Scrape off any excess glue—you don't want that around the edge of your shots! Also, make sure that filter holder remains vertical as it dries and that the tinted medium does not slip to either side.
If you want a few neutral density filters at several levels of darkness for different lighting situations, you can just repeat the process with different pieces of glass or gel to have a full set.
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Another easy way to make your own neutral density filters without relying on cutting glass and the like? Just put two polarizers together! Think about it for a moment: a polarizing lens cuts the light intake from one angle, which is how it reduces shine off of metal and clouds and the like. Put two of them together, and you can reduce light from two different directions. Just play around with spinning the topmost of the two filters to vary the amount of light intake—just remember that once you have the filters at 90 degree angles from each other, that's pitch black and zero light intake. This creates the ND effect while minimizing the number of lenses you have to lug around with you, hence variable ND filter.
Check out this article for a great detailed guide on how to use polarizing filters for ND effects.
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How To Use ND Filters
Keep in mind that even the best store-bought filter lenses will degrade the quality of your photos somewhat: even if you make your own neutral density filter with the best materials you can get your hands on with the best possible build job, it will still cause a bit of image degradation.
Filters can confuse autofocus mechanisms, especially ones that aren't clear—like ND filters. So, be prepared to use manual focus while using these filters.
For that matter, you'll need to use manual exposure and f-stop while using ND filters as well due to the artificially altered lighting conditions to get exactly the effect you want. You'll probably be doing longer exposures, so using a tripod or some other stabilizing device might be necessary.