My webcam gives me very dark image, so I tried to find out how to adjust.

v4l2ucp - Video4Linux Universal Control Panel

A GUI control panel. It's very easy to adjust.


v4lctl of Xawtv

This program is part of Xawtv, a command-line tool. You can do with it like:
$ v4lctl -c /dev/video0 list
attribute  | type   | current | default | comment
-----------+--------+---------+---------+-------------------------------------
norm       | choice | (null)  | (null)  |
input      | choice | tv8532  | tv8532  | tv8532
bright     | int    |     399 |     399 | range is 1 => 767
contrast   | int    |   32767 |   32767 | range is 0 => 65535
$ v4lctl -c /dev/video0 bright 80%
$ v4lctl -c /dev/video0 contrast 75%
$ v4lctl -c /dev/video0 list
attribute  | type   | current | default | comment
-----------+--------+---------+---------+-------------------------------------
norm       | choice | (null)  | (null)  |
input      | choice | tv8532  | tv8532  | tv8532
bright     | int    |     613 |     399 | range is 1 => 767
contrast   | int    |   49151 |   32767 | range is 0 => 65535
You can man v4lctl.

Unfortunately, my webcam has only small change on brightness and totally no difference on contrast.