Will a variable neutral density filter produce similar results to a fixed neutral density filter?

Question

I've decided to look at experimenting with neutral density filters and I found this variable neutral density filter on Amazon here in the UK.

It's a lot cheaper than buying a set of filters of different densities, so I was wondering if it would produce similar results to a fixed filter when set appropriately? I realise I have the added complication of setting the filter to the desired density, but as this would be my first foray into this area I don't want to spend a lot of money if I'm not going to use it a lot or get serious with the effect.

Answer 1

I have just trialled a variable ND filter and have written a review about it

Yes, it will produce similar results. It's very handy in photographing things like waterfalls, as you can dial in whatever effect you need.

The drawback is that it acts like a polarising filters, so if you have large expanses of sky, the sky will not be uniform - the polarising effect is such that parts of the sky will be darker than others. With a fixed ND filter, you should get uniform darkening across the image.

Also with some of the less expensive variable ND filters, you can get color casts or dark bands in the image. I didn't experience much of that under most operating conditions - you will tend to see those at very wide angles (i.e. under 18mm DX) and at maximum density. You can also get color casts with some brands of fixed ND filters, especially when stacked with other ND filters, or with polarising filters.

Answer 2

You can get good results with a variable filter but I have to question the necessity of having that much control. When you're stopped right down you can usually adjust the aperture by a stop without compromising DOF, likewise you can go from ISO 50 to ISO 200 without a noticeable change in noise.

Here's an example scheme:

10 stop    f/22   ISO50    512s
10 stop    f/22   ISO100   256s
10 stop    f/16   ISO100   128s
10 stop    f/16   ISO200   64s
6 stop     f/22   ISO50    32s
6 stop     f/22   ISO100   16s
6 stop     f/16   ISO100   8s
6 stop     f/16   ISO200   4s
no filter  f/32   ISO50    1s
no filter  f/22   ISO50    0.5s
no filter  f/22   ISO100   0.25s
no filter  f/16   ISO100   0.125s
no filter  f/16   ISO200   0.0625s

So here with the same lighting conditions you can have 1/16s to over 4 minutes, and everywhere inbetween (with one tiny gap ;) with only two non-variable filters. So you don't have to compromise on quality/suffer polarization effects to get flexibility if you get your camera to help you out!

Answer 3

I have not used a variable ND filter that was advertised as such, but my experience with two polarizers to create a variable ND filter has been a significant colour shift toward the blue end.

Perhaps filters intended to be used as variable ND filters have overcome this problem, or perhaps the polarizers I was using for this effect were defective in some way.

In short, a variable ND filter is making a normally invisible aspect of light visible, which may have any number of unintended effects, whereas a "real" ND filter should only reduce the amount of light passing through it -- of any polarization.