We have been using the RasPiCam V1.3 with Python-picamera, to try and take some consistent brightness images of a known brightness source. We have used the command camera.shutter_speed = x to set the exposure time, where x is in µs, and fixed the analogue gains with camera.exposure_mode = 'off'. We are retrieving raw information for the images , so no digital processing should be done to the signals before we save the data.
However when taking images we noticed that the measured brightness of the source was dropping by a consistent amount each frame until it reached some stable level. we discovered that on the first frame the property exposure_speed was equal to the property we set;
shutter_speed, but then for each subsequent frame we captured exposure_speed reduced by 30 µs until it reached a certain value. The values we have found where the exposure_speed is happy to operate without dropping are (in µs):
Also is this a bug or are we trying to set the exposure time in the wrong manner?