On Tue, 26 Jan 2010, Nicolau Werneck wrote:

Hello. I have this very cheap webcam that I sent a patch to support on
gspca the other day. The specific driver is the t613.

I changed the lens of this camera, and now my images are all too
bright, what I believe is due to the much larger aperture of this new
lens. So I would like to try setting up a smaller exposure time on the
camera (I would like to do that for other reasons too).

The problem is there's no "exposure" option to be set when I call
programs such as v4lctl. Does that mean there is definitely no way for
me to control the exposure time? The hardware itself was not designed
to allow me do that? Or is there still a chance I can create some C
program that might do it, for example?

It looks like the camera has some kind of automatic exposure control. If
I cover the lens, and then uncover it quickly, the image is all white
at first, and then it gradually becomes darker. Should that give me
some hope of being able to control the exposure, or is it common for
cheaper cameras to have just an automatic exposure control that cannot
be overrun?


Having had some experience with cheap cameras, I would say that they come with all kinds of variations. Thus, this particular camera (with which I am _not_ experienced) could either have a built-in automatic exposure control, or it could require monitoring from software with periodic correction of various settings. It just depends.

Thus, one thing to do is to look carefully at some usbsnoop logs and try to see if there are occasional correction sequences in between big hunks of data. If there are, then the next thing is to figure out what those correction sequences are doing. Then build those correction sequences into the driver. An example of this kind of thing going on can be seen in lots of the camera drivers in gspca, but not in all of them.

Second thing, try to look for sensor setup commands which give initial settings. If there are such, then one or more of them might control exposure settings.

Third, if there are no occurences of either of the previous items at all, then either the camera does not self-adjust and can not be adjusted (i. e. really, really cheap), or the adjustment mechanism is completely built-in. In that case, I would say that you are probably up a tree about getting the exposure reset.

Hope this helps you.

Theodore Kilgore
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