[vga50]

Кстати, попалась на глаза вот такая фраза:

When an exposure is made according to the above setting, an 18% gray card has a voltage or pixel level 18/106 of full scale. In an 8-bit color space encoded for display at gamma = 2.2 (sRGB, Adobe RGB, WGRGB, etc.), the corresponding pixel level is 114 (from the formula, pixel level = 255*(18/106)(1/2.2) ). Of course this is an ideal value; pixel levels in real cameras may vary, especially when a tonal response "S" curve is applied.

Вот тут, во вкладке "ISO speed".