Question
A few days ago I took photos of a nice curvy business center in another city. Later I discovered that many frames had weird artifacts on the glass, shown below as 100% crops.
An almost normal shot:
A spotty shot 46 seconds later:
A striped shot 9 minutes earlier:
All crops show the same windows.
I did check the sensor, it was clean. I used a CPL filter to control reflections. Unfortunately the artifacts are too small to notice on camera LCD without zooming in, so I did not notice them and did not experiment to lose them on spot.
What could have caused those artifacts and how could I prevent them in future?
Answer
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.
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