Picamera: set shutter_speed with RAW acquisition

Question

I'm using a RPi zero W with a camera V2 to acquire RAW images (I used this). However, I would like to set the shutter speed; I used this to do that, but that doesn't work:

import time
import picamera
import picamera.array
import numpy as np
from time import sleep

def get_pic(fname, sspeed):
    with picamera.PiCamera() as camera:
        with picamera.array.PiBayerArray(camera) as stream:
            # Set shutter speed
            camera.shutter_speed = sspeed
            camera.exposure_mode = 'off'
            print("sleeping")
            sleep(10)
            print("Exposure speed = {}".format(camera.exposure_speed))
            t1 = time.time()
            camera.capture(stream, 'jpeg', bayer=True)
            print("Time to capture = {}".format(time.time()-t1))
            # Write original stream to output
            output = stream.array
            with open('{}.data'.format(fname), 'wb') as f:
                output.tofile(f)

The printed exposure speed varies, while the time interval is always the same (around 30000, i.e. 30 microseconds), even if I set sspeed = 5000000 (i.e. 5 seconds), which is the shutter speed I need. What am I doing wrong?

Answer 1

From the docs:

"the shutter_speed attribute is constrained by the camera’s framerate so the first thing we need to do is set a very slow framerate"

Try this when you instantiate your PiCamera: picamera.PiCamera(framerate=Fraction(1, 6))