Image artifacts on Alexa at slow shutter speeds

 Setting an Alexa to slow shutter speeds to capture a low light timelaps comes with a warning. Fixed pattern noise artifacts. This is an anomaly that is hard to avoid with long exposure times. The Alexa manual states  “Note: Be aware that combination of long exposure times (small sensor frame rate with large shutter angle) and high exposure indexes can lead to image artifacts! “Now the image above is a frame grab enlarged of worst case settings. 1fps, 358deg shutter, 3200ei, rec709 out, so your shots won’t be this bad. The Artifact has everything to do with the length of time the sensor is exposed. To avoid it try a smaller shutter angle and compensate with exposure index.  I found that 1fps at 11deg shutter worked well, and the shutter can go all the way down to 5deg. At 1fps, 7.5deg is equivalent to 180deg at 24fps . Anything you can do to decrease the exposure time will greatly reduce the artifact. I rifled through the Arri Forum and this seems to be consensus on how to work around the problem.

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