Posts: 6
Joined: Thu Oct 26, 2017 1:55 pm

exposure issues

Thu Oct 26, 2017 2:57 pm

I am using the bcm2835-v4l2 driver, Pi camera version 2, raspbian stretch, and lin_guider for an astrophotography guiding system project. Lin_guider is an application written to use the v4l2 API. I've got some strange exposure related issues that I hope someone can shed some light on.

I need long exposures, and want 1 second exposures at iso 800 if possible. I've found that if I set manual exposure mode either thru the ioctl in the application or the v4l2-ctl command, and exposure absolute=10000 (1 second) I get a dim image. But if I have the application start up in auto exposure mode (set with the ioctl), and 15 seconds later I use the v4l2-ctl command to set manual exposure mode, I get a much much brighter image. Although I can live with this, it is really weird. Why can't I start off in manual mode? It is not set anywhere else in the application.

Currently, I start my application in 640x480 format to get reasonable exposures. If I start off in 1920x1080 mode, I'm stuck in a dark image mode, even with the above mentioned workaround to set the 1 second manual exposure mode. Way too dark images. Any ideas? fps is set to 1 with v4l2-ctl before the application is started.

ISO seems to be stuck in auto mode. In looking thru the forums here, I found that others had the same issue.

v4l2-ctl -c auto_exposure=1 (this is defined as manual exposure)
v4l2-ctl -c exposure_time_absolute=10000 ( 10000 * 100us = 1 second)
v4l2-ctl -c iso_sensitivity_auto =0 (this is defined as manual mode)
v4l2-ctl -c iso_sensitivity=4 (this is defined as iso 800)
Pictures are not changed compared to
v4l2-ctl -c iso_sensitivity=1 (iso 100)

Thanks for looking.

Posts: 6
Joined: Thu Oct 26, 2017 1:55 pm

Re: exposure issues

Thu Oct 26, 2017 10:51 pm

I should have also mentioned that lin_guider is provided as source code only, and I have checked over the capture setup code in lin_guider as it compares to the v4l2 user space driver spec and sample code provided there. It looks to be quite ordinary with respect to the image capture setup portion.

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