Why do dark spots or stripes appear on building windows when using a polarizing filter?
Asked 7/24/2011
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I photographed a modern building with large curved glass windows and later noticed dark spots and stripe-like patterns on some frames. The same windows looked normal in one shot, spotty in another taken less than a minute later, and striped in another taken earlier. My sensor was clean, and I was using a circular polarizing filter to control reflections. What causes these artifacts on glass, and how can I avoid them when shooting buildings?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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Those look like features of the glass windows that are being revealed by the polarizer. I don't think there is any way to avoid them other than by not using the polarizer, or being careful with the filter orientation and lighting.
I've observed this effect many times in car windows. I can't find a technical explanation but I imagine it's to do with stresses in the glass when cooling which result in parts of the pane polarizing light as it passes through; this light is then filtered out by the polarizer on your lens, resulting in the dark patches.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
0
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These patterns are most likely caused by the polarizing filter interacting with stresses or manufacturing patterns in the glass itself, not by dust or a sensor problem. Similar effects are often seen in car windows and other tempered or stressed glass.
The glass can alter the polarization of light in uneven ways, and your CPL then makes those differences visible as dark spots, bands, or stripes. Small changes in shooting angle, filter rotation, or lighting can make the effect appear, disappear, or change shape between frames.
To reduce or avoid it:
- rotate the polarizer and watch the glass carefully
- change your shooting angle or position
- shoot under different lighting if possible
- remove the polarizer if the pattern is worse than the reflections
In short: the filter is revealing properties of the window glass, so prevention is mainly about filter orientation, angle, lighting, or not using the CPL.
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