Posts: 7
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: 7
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.

Return to “Camera board”