Raspberry Pi HQ camera exposure time jumps at 1 second

Question

I am trying to build a timelapsing program in python, which analyzes the picture and determins the exposure, based on which it corrects it's shutter speed using a polynomial regression.
But I got stuck when the exposure is somewhere around 1 second, where you can see in the graph below, the brightness jumps

The orange line is the expected curve, the blue dots are actual measurements.

Digging deeper, I found out, that the actual exposure times differs from the shutter speed I set.
With cam.exposure_speed I found out, that at cam.shutter_speed = 1000000 (-> 1s) the actual exposure time was 7997813 (-> 8s) where as at cam.shutter_speed = 990000 (->0.99s) the exposure time was 989976 (-> 0.989s).

Here is an example code to recreate the effect:

from picamera import PiCamera
import time

cam = PiCamera(sensor_mode=2)
cam.framerate = 1/8
cam.iso = 100
cam.shutter_speed = 1000000
cam.resolution = (1600, 900)
time.sleep(5)
cam.capture("test.jpg")
print(cam.exposure_speed)
cam.close()

So far I tested:

• Diffrent ISO values (needs to fixed ISO)
• diffrent resolutions
• removing the sensor_mode argument
• diffrent framerates

I am running

• Python 3.7.3
• Raspberry Pi 4 with 4GB RAM
• picamera 1.13

Any help on this will be helpful, I will try to edit missing information asap.

Answer 1

I also struggle with exposure time settings on a regular basis. I even get sometimes different behavior with the same code. (Maybe some internal parameters in the camera might be modified behind the scene).

However, I have good experience by combining consistent values for shutter_speed and frame_rate and setting exposure_mode to 'off'.

E.g. the following code yields a correct exposure time (at least here for me).

from picamera import PiCamera
import time

with PiCamera(sensor_mode=2) as cam:
    exposure_time = 1.0 
    cam.framerate = 1/exposure_time
    cam.iso = 100
    cam.shutter_speed = int(exposure_time*1000000)
    cam.resolution = (1600, 900)
    cam.exposure_mode = 'off'
    time.sleep(5)
    cam.capture("test.jpg")
    print(cam.exposure_speed)