britstanger
Posts: 1
Joined: Tue Aug 15, 2017 8:42 am

exposure_speed drops to some fixed value

Tue Aug 15, 2017 9:17 am

Hello
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):
  • 666
    991
    4956
    8239
    32942
Does anyone know what significance these stable values have, and how we can predict which other values are going to be stable?
Also is this a bug or are we trying to set the exposure time in the wrong manner?

Leigh

habropoda
Posts: 10
Joined: Sun Apr 30, 2017 6:19 pm

Re: exposure_speed drops to some fixed value

Wed Aug 23, 2017 12:07 am

Possibly a bug. I was seeing something like this and the code in this post was a workaround:
viewtopic.php?f=43&t=156159#p1020690
(hint: pootlepics does not import. It works fine without it. He's hung up on poots.)

I posted about a similar problem using raspistill and 6by9's workaround fixed it.
viewtopic.php?f=43&t=187976&p=1184955#p1184693
I don't know if that bug was ever fixed.

6by9
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 5671
Joined: Wed Dec 04, 2013 11:27 am
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Re: exposure_speed drops to some fixed value

Wed Aug 23, 2017 10:34 am

britstanger wrote:
Tue Aug 15, 2017 9:17 am
Hello
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):
  • 666
    991
    4956
    8239
    32942
Does anyone know what significance these stable values have, and how we can predict which other values are going to be stable?
Also is this a bug or are we trying to set the exposure time in the wrong manner?
Hmm, there is a rounding calculation done as the exposure is set as a multiple of a line length, and is then converted back out to a time value for metadata. It's possible that the conversion back to time results in a number that ends up rounding down on a subsequent request to convert it back to a number of lines. If "camera.exposure_mode = 'off'" then it shouldn't recalculate things at all, but I'd need to check that.
habropoda wrote:I posted about a similar problem using raspistill and 6by9's workaround fixed it.
viewtopic.php?f=43&t=187976&p=1184955#p1184693
I don't know if that bug was ever fixed.
Unrelated, and fixed the next day. That was that the frame length calculation was being clipped incorrectly.
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herbert570
Posts: 1
Joined: Mon Jul 16, 2018 8:35 pm

Re: exposure_speed drops to some fixed value

Mon Jul 16, 2018 8:43 pm

I found similar behavior for raspberry pi cam v2. The stable exposure_speed after exposure_mode is set to 'off' is
9 or 2363*n - 9
where n = 1,2,3,...
The first couples of such numbers are 9, 2354, 4717, 7080, 9443, 11806, 14169, 16532, 18895, 21258.

I think this might be a bug in the hardware/firmware.

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