Fun with a variable ND filter

A little while ago I set out to do a bit of casual filming using my stabilised 18-55mm lens. I wanted to shoot wide open for narrow depth-of-field so I screwed on a ND8 filter (which blocks 8 stops of light), but at the maximum aperture of f/3.5 the picture was underexposed. The weather was a little overcast but mostly bright. So I swapped the lens for my 50mm f/1.8 and the picture was exposed enough, but I was shooting hand-held so the unstabilised 50mm wouldn't do.

Back on the computer, I ordered a variable ND filter. Comprising two polarisers that darken considerably when their orientations are crossed (see video at the bottom), these filters are great when you want fine control over exposure in bright environments. As soon as it arrived I tried some experiments in my back yard.

f/3.5 with variable ND filter

f/18 without filter

These two stills are grabs from video clips I shot with 1/50th shutter. In the first picture I set the aperture first, at the maximum of f/3.5, and then adjusted the filter so that the highlights were just below clipping. In the second picture the filter was removed, and then I adjusted the aperture to retain the highlights, which turned out to be at f/18. Quite a difference, which is demonstrated in the noticeably narrower depth-of-field in the first picture.

One other noticable difference is the colour tone. With the filter, the picture looks cooler with a blue tint, and possibly desaturated. I have found with cheap ND filters that the optical quality is not perfect and colour shifts can occur, usually pushing towards blue. The effect varies with the severity of the light loss, and the quality of materials used -- one time I tried a plastic filter and the effect was something akin to picture taken with a Lomo/Diana toy camera, or something tweaked with Instagram. In the case of this variable ND filter, the colour shift is not so bad that it can't be corrected during editing.

A short video to demonstrate: