How can I test whether scratches on a CPL filter are affecting image quality?
Asked 9/20/2010
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I dropped my circular polarizer on rocks and it now has some scratches. In a quick look at my photos I don't see any obvious problems, but I’d like to check more carefully. Are there simple tests to tell whether the scratches are affecting the polarizing effect, sharpness, or causing flare?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
9
You can test the polarizing effect by placing a non-circular polarizing filter in front, you should be able to place it at an angle that almost eliminates light transmission. However I highly doubt that the scratches will have an effect on it's ability to filter polarized light!
As already stated scratches on the front of a lens typically have a minimal impact on image quality: here's a very good example.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
0
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Minor scratches on a CPL usually have little to no visible effect, because the front of the lens/filter is so far out of focus. The main things to check are flare and any slight loss of clarity.
Useful tests:
- Compare shots of the same static scene on a tripod with and without the filter.
- Include a bright light source in the frame or near the edge of the frame; scratches are most likely to show up as extra flare, streaks, or ghosting.
- Stop down to a larger f-number, which can make defects easier to notice.
- To test the polarizing function itself, place another linear polarizer in front and rotate it; at one angle, light transmission should drop strongly.
If you can’t see a meaningful difference in normal photos, the filter is probably fine to keep using. Scratches are unlikely to harm the filter’s ability to polarize light unless the damage is severe.
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