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I'm new to filters. I am considering buying a circular polarising filter. I would probably want to use it most on my Nikkor AF-S 24-70 f3.5-4.5 VR, which has a 72mm thread and by Nikkor 20mm f2.8 AF-D, which has a 62mm thread.

I was thinking I would therefore buy a 72mm filter and a 72 to 62 step down ring.

Is there any reason not to do this? (e.g. vignetting? the 20mm is already quite bad for that)

I am unlikely to buy any more lenses in the future that I would want to use this filter on.

Thanks in advance.

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    The biggest problem might be the fact that you're going to use polarizer filter with wide-angel lens, as it's so wide, different parts of the sky are going to be "polarized" differently. – Andrew Jun 21 '14 at 10:37
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You got it backwards. What you need is a step-up ring. This lets you use the largest size of filters on lens with smaller thread diameter.

It works just as expected and is that is exactly what I do most of the time. There are two downsides to doing this:

  1. Lens hood no longer fits. This is really the big one, particularly for polarizers which are generally used in bright light.
  2. Lens cap no longer fits. Easily fixed by buying an extra lens-cap of the right filter size. It works for all except the slim filters which are recommended for ultra-wide angle lenses.

What I did is standardize on 77mm and 62mm filters. I have full sets with both sizes and step-up rings for 72->77 & 67->77, plus 55->62, 52->62, 49->62 ones. My ultra-wide lens is coincidentally 77mm so it does not need a step-up ring. This is the only case I can imagine there might be some vignetting potential. Sadly, I standardized too soon and had to buy a few 86mm filters when I got a larger lens.

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The potential problem is that the step up ring will build too much, so that the filter comes so far to the front that it will obstruct the corners of the image.

If you have the opportunity to test this before buying the step up ring, you would set the lens to f22 (the smallest possible on that lens) and take a picture. With a small aperture the lens edge will be sharpest, at a larger aperture the lens edge may only be visible as a faint vignetting. You should take a picture rather than just looking through the viewfinder, because most viewfinders covers less than 100% of the image.

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