Нашёл, правда в англоязычном варианте.
he Canon Digital SLRs (350D included) have a 12-bit A/D converter for each sensor, which means that each sensor will produce a digital number in the range 0–4095 when shooting in RAW mode. The mid-point of this range is 2048, and therefore a Median of 2048 is the target value for the Flat Light exposure. One way to determine the Median value of an exposure is to use IRIS' stat command.
Thus, using the steps outlined in the Processing section below, load the Flat Light into IRIS, convert it from CFA to RGB, and run the stat command. In the Display window, IRIS will report several statistics about the image on a per-color basis, one of which is the Median value. Increase/Decrease the exposure time until the Median is in the neighborhood of 2048 — a tad higher or lower won't hurt, though my preference is for a tad higher rather than a tad lower. See also Verify Proper Exposure of your Flat Lights for some additional explanation on this.
At risk of beating this topic to death, there's one more point that needs to be made about exposing Flat Lights. Do not use the histogram indicator on the camera's LCD screen to determine the proper exposure of your Flat Lights! In that screen the camera is reporting the histogram of the exposure after applying a Gamma (contrast stretching) function to the image data, and therefore is very much not reporting the histogram of the "linear" image data captured directly into the RAW file. In fact, through experience and through using IRIS as recommended above, you'll find that the proper exposure for your Flat Lights appears massively overexposed if you use the LCD screen on the camera as an exposure guide. This is normal, and should not deter you from properly exposing your Flat Lights.
http://www.saratogaskies.com/articles/cookbook/index.html#Acquire_Flat_Lights
