What is the difference between using an ND filter versus 2 polarizers?

I know what an ND filter does. I know what a polarizer does. I also know what two polarizers stacked together and rotated properly do.

So the question: why should I use an ND filter to achieve a darker image at the input, when I can use 2 polarizers instead and rotate them to exactly as dark an image at the input as I want?

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Also asked by Julien Gagnet

Is it possible to use two polarised filters to create a variable ND filter?

I was reading that by attaching two polarised filter we could create a variable ND filter.

Has anyone done this? How was this done? Any drawback (colour cast, quality...)? What would be the strength in light filtering of such filter?

Answer

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  Polarizers are often more expensive than ND filters and you need two of them.

  
  


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  Stacking two filters can cause vignetting with wide lenses.

  
  


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  You have an extra glass surface with two polarizers which can cause flare and potentially loss of contrast/sharpness.

  
  


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  This arrangement can cause colour shift toward yellow (but so some ND filters).

  
  


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  Extreme wide angle lenses will exhibit uneven darkening due to the difference in incidence angle across the polarizers.