Thanks for the correction of the 18% grey value.
C.H.Ling
----- Original Message -----
From: "Johnny Johnson"
> At 10:46 PM 1/29/2009, C.H.Ling wrote:
>
>>There should be a standard to measure the ISO for digital cameras, I
>>believe
>>it check the grey rendering. For example, the grey card should read 128
>>(?)
>>if properly exposed.
>
> IIRC the number for a Kodak gray card is 117, not 128. It has to do
> with the gray card having an 18% reflectance and viewing the file in
> a 2.2 gamma color space. A Google search would probably turn up the
> formula.
>
> There's also been talk for years of Canon (and some others) using 12%
> gray instead of 18% for meter calibration. Here's a link to an old
> article by Tom Hogan on the subject:
>
> <http://www.bythom.com/graycards.htm>
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