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Re: [OM] ISO may not be accurate

Subject: Re: [OM] ISO may not be accurate
From: Andrew Fildes <afildes@xxxxxxxxxxxxx>
Date: Sat, 31 Jan 2009 08:04:25 +1100
It may be why DxO was looking for a more standardised approach?
This all sounds pretty mathy - I always assumed that somewhere  
between 12-18% was used simply because it was more likely to render  
Caucasian and East Asian skin tones well, not to mention average sand  
and vegetation greens. The ultimate mid-tones.
Andrew Fildes
afildes@xxxxxxxxxxxxx



On 31/01/2009, at 7:39 AM, Johnny Johnson wrote:

> 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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