Issues of auto_exposure with a logitech c905 webcam

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Issues of auto_exposure with a logitech c905 webcam

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I'm running on an ubuntu 10.04.3 with v4l2 and I'm using a logitech c905 as a webcam. I've got a simple problem :

I want to be able to disconnect and reconnect my webcam (or restart my computer) and to have still exactly the same configuration on the webcam, whatever will be in front of the webcam.

I used first the v4l2-ctl utils to stop auto_exposure and set an absolute_exposure. But a strange thing that I've checked is that I can't rely on the absolute_exposure. Because when the auto_exposure is turned on, this value won't change, but the image will, and when we will turn it off again, for the same value of absolute_exposure, we'll have a different real exposure. I'll try to summarize it by an example

1) Exposure_auto is off, absolute_exposure is at 300 and I've got an image a little bit dark.

2) I put something in front my webcam, hiding all lights from it, and then I turn on exposure_auto.

3) I turn off exposure_auto with something in front of the webcam.

4) I've got exactly the same values in all parameters than in 1), but the image is very shiny.

I tried to use a lot of tools like v4l2ctrl (-s to save the parameters in the file and -l to load them) but I've still this issue that the value I need for absolute_exposure depends off what was "seeing" the camera when I turned off the auto_exposure.

Has anyone heard of this problem or has an idea of a solution to this?