Proper Blind Exposure with a Gray Card

Started Nov 12, 2012 | Discussions thread
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Jack Hogan
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Proper Blind Exposure with a Gray Card
Nov 12, 2012

I would be interested in your opinion with regards to the following concepts on metering ‘blind’ with a gray card:

  1. that ‘blind’ exposure is defined in ths context as the exposure to be chosen with the sole help of a spot meter, without knowledge of the scene, time or place – it is the input quantity to the camera sensing medium
  2. that ‘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
  3. that brightness is an output medium dependent quantity typically not related linearly to exposure
  4. that for a given ISO the job of calibrated spot meters is to provide indications of shutter speed and aperture to achieve proper blind exposure
  5. that spot meters do not know or care what average scene reflectance or reference card is in front of them
  6. that in order to achieve proper blind exposure with most DSLRs it is best to use a standard Middle Gray card (that is one with 18% reflectance), which is what their spot meters are calibrated to
  7. that when metering off of and capturing such a target a DSLR will typically record digital raw values about 0.127 times the full scale reading of the sensor, subject to many provisos
  8. that if instead of a Middle Gray card a 12% reflectance card (or alternatively an 18% card with a ½ stop positive Exposure Correction) is used to set Exposure, the meter is fooled into raising Exposure by half a stop, reducing a DSLR’s designed highlight hard ceiling accordingly
  9. that if exposure is locked as per this last case, a Middle Gray tone will be recorded by a DSLR at raw values around 0.18 times the full scale of the sensor, so that a capture of an 18% Gray Card will record raw values of about 18% of Full Scale
  10. that other than a warm and fuzzy feeling there is no reason why those two percentages should be the same
  11. that we should all write an email to someone with whom I share a last name telling him that his blog entry on this subject is confusing a lot of peopleand does not stand up to his otherwise excellent standards

To help with the discussion I include this image:

Thanks for your input,

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