Proper Blind Exposure with a Gray Card

Started Nov 12, 2012 | Discussions thread
Jack Hogan
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In reply to JDinOregon, Nov 13, 2012

JDinOregon wrote:

Lsat/L=78/(k*q) or Lsat is 3 stops higher than the metered L value for q=pi/4.

Saying it with different words, the metered exposure recommended by this standard (Hsat=78/Ssat comes from ISO 12232) should be recorded at raw values around 3 stops below (or 0.128 times) the maximum recordable raw value.

To characterize a DSLR all we need is the Hsat of its sensor, which we have thanks to DxO, and an indication of where it stores s spot metered exposure along the full scale of raw values. My D90's spot meter puts it at about -3.4 stops (10% of saturation), and in my opinion it does not underexpose. Should I use a 10% Gray Card? Not!

IMHO there is a lot of confusion on this issue going back 15 years because even normally competent people like my namesake confuse the reflectance of a reference gray card with where a tone metered off of it and captured should be stored along the scale of available raw values to ensure that if the reference gray card is Middle Gray it will result in the 'proper' compromise of captured information. The two (raw levels and reflectance) have very little to do together, as Exmor class sensors are showing us ever more clearly with every new generation.

Cheers,

Jack

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