Will two CPL make an ND?

Capturing
1

Would two quality CPL serve as a quality variable ND until I can afford nice ND filters?

2

Short answer: No.

  1. These filters do not reduce light by a fixed stop. It changes depending on the angle the filter is turned.
  2. You’ll be introduced to the effects of what a polariser is actually supposed to do.

Using a second one will:

  1. Double the problems mentioned above,

All in all you’ll end up with conditions that you cannot control/predict.

Alternative:

If you want to simulate the effect of an ND filter you might want to look into averaging (many) multiple exposures.

I’m sure there are more examples/explanations to be found online how to do this, but this one came to mind first, an article written by Pat David:

3

If it’s not two polarizers, how do variable ND filters then work?

4

Iirc, variable ND filters are made of two polarizers, so in theory your idea might work.

But:

  • using two CPL filters in their normal orientation will not work: a CPL filter is a linear polarizer fowwolwed by something called a quarter-wave plate (see wikipedia. This is needed for digital cameras to ensure light metering and autofocus work correctly. But putting a second CPL filter behind the first will only lower the intensity of the light… (*)
  • even if you could mount the second filter the wrong way around (so you have the two polarizers facing), you wouldn’t get a good variable ND filter. There’s a reason those are very expensive (cheaper ones tend to give colour shift that vary across the image). And I’m not sure of the effect of the second quarter-wave plate (which is now the first element in the full stack).

*: the polarizers used before digital became common were “simple” linear polarizers. Combining two of those would be a lot easier, but you may run into problems with metering and autofocus. Besides the colour problems, that is…

2 Likes
5

Yes your idea may work. I have done this myself in the past with two polarizers. It may have some undesirable side effects, but it did work. However, I cannot state that I used CPLs since I don’t remember. Just try it if you have two filters. BTW, I bought a variable ND filter in the past and it has proven not to be buyer’s regret. I am more likely to use my fixed value ND filters if I need an ND filter. Maybe other variable ND filter owners have more positive reviews.