Proper Blind Exposure with a Gray Card

Started Nov 12, 2012 | Discussions thread
Jack Hogan
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Re: The Crux of the Matter
In reply to panos_m, Nov 12, 2012

panos_m wrote:

Jack Hogan wrote:

panos_m wrote:

If I meter a 18% card with the GF1 for example it will render it as 12.5%. If I want to render it as 18% I will have to add 3.0 - 2.47 = 0.53 stops of positive exposure compensation.

Why would you want to record a middle gray at raw values around 18% of full scale

Isn't it that your "proper blind" exposure?

No, "‘proper’ blind exposure is defined in this context as the exposure that allows a camera to best record on its sensing medium a subset of the relative distribution of photons reflected from the scene; the full set is the relative distribution of photons that would be perceived by the human visual system in the same setting as the camera"

Camera manufacturers and the standards believe that a mid-tone should be stored at a raw value of less than 1/7.8 of full scale to 'best record' the natural scene. For a number of reasons, storing middle gray at raw values equal to about 18% of the maximum that the sensor can record has been determined to be too high, it may cause some important highlights of the scene to be clipped.

ISO calibration is on processed JPEGs at standard settings. We are talking raw values here. Don't we?

Yes, and ISO has (mostly) nothing to do with it. A meter will produce an exposure on the sensor of

Hm = 10 / S lx-s

Saturation for a sensor is Hsat = 78 / S lx-s

Therefore with regards to full scale (saturation), the metered exposure is

Hm / Hsat = 10 / 78 = 1 / 7.8 or 12.8% of saturation, so according to the standards it should be recorded as raw values representing less than 12.8% of full scale, independently of ISO.

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