In an attempt to achieve a shallow depth of field (DOF) with my Canon M-500 video camera, I purchased a Polaroid HD Multi-coated Variable Neutral Density (ND) Filter for that purpose. The concept of the variable ND filter is to reduce the light, forcing the camera to open the aperture, reducing the DOF.
For regular ND filters, this is easy to do. Put dark glass is put in front of the lens, cutting the light, the aperture opens, and the DOF drops. There is no adjustment with these filters. Really dark ND filters introduce a color shift, as the Hoya ND400 does, but it can be dealt with.
But with the variable ND filter, two polarizers are crossed and their relative position will determine how much the light is cut. That’s the theory. However, the variable Polaroid filter I chose makes the image blurry. And not just a little bit but a lot.
At first I tried testing with video, as the above YouTube clip shows. Once I realized the filter was making the video blurry. It was still difficult to tell the extent of the problem, so I switched the M-500 to still mode to compare. The problem was instantly evident. The two photos show the problem. Click on them to download and see for yourself. Wow.
So, now it’s either drop $400 on a Singh-Ray Variable ND or just get a 3-stop ND for the M-500. Considering the Canon video camera costs $400, it doesn’t make a lot of sense to purchase the Singh-Ray, as much as I want it. Buying a $1k camera with built-in NDs would be a better choice.
Here are some links to other discussions with the same problem:
There seems to be a lot of non-uniform feedback on the problem. This filter is something to be tested at the store prior to purchase if possible. There’s nothing worse than getting home and having things not work.