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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:

enter image description here

A spotty shot 46 seconds later:

enter image description here

A striped shot 9 minutes earlier:

enter image description here

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?

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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.

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  • \$\begingroup\$ Cooling sounds like a valid theory, it was evening after a hot day. \$\endgroup\$
    – Imre
    Jul 25, 2011 at 7:25
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    \$\begingroup\$ @Imre I actually meant the original cooling when the glass is tempered, but change in temperature during the day probably contributes to the effect, and explains why the patterns changed over time. \$\endgroup\$
    – Matt Grum
    Jul 25, 2011 at 13:32
  • \$\begingroup\$ The change in patterns can also be a result of camera not being in the exact same spot (Assuming a tripod was not used or was moved between shots). As the glass is a reflective surface, but the picture is taken from a distance, the exact angle between the glass and the camera has a major effect. try to frame somebody in a mirror that is 100 ft from both the camera and the subject... \$\endgroup\$
    – Jahaziel
    Jul 25, 2011 at 15:26

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