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

Question

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?

Answer

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

• Stacking two filters can cause vignetting with wide lenses.

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

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

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

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