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 13, 2012

panos_m wrote:

I don't understand the above definition. Can you provide a practical example? How do you achieve "proper blind" exposure with your camera and your reflectance card? (which is 18%?).

Look at the image I provided at the top of the thread.  The horizontal scale is varying light from the scene (exposure on top of film/sensor), the vertical scale is how the sensing media store this light: film stores it in varying density and sensors in raw values. The scale for storage is inverted (lots of light is at the bottom of the scale and dark is at the top) because the original film chart comes straight from Fujifilm (thank Fuji!) and it is a negative.  More light/exposure = more density/raw value, less light/exposure = less density/raw value as characterized.  Film's response to varying light/exposure is non linear, sensors' is linear.

As we said, a spot meter indication will always produce the same exposure on top of the sensing medium, in this case at the exposure indicated as -1 on the horizontal axis (at ISO100 the metered exposure of Hm = 10 / S = 10 / 100 = 0.1 lx-s = -1 on the film manufacturer’s log10 scale).  This maps to a density of 1 on the negative, which leaves about three density stops to record brighter tones than the metered exposure from the scene.  Since the density curve is not linear, you can see that in those three stops of density this film is able to record information of 3+ stops/Zones from the scene, albeit highly compressing/distorting the top of the range.

This performance (i.e. 3+ stops/Zones of storage above the metered 10/S exposure) is not casual.  Millions of years of evolution of the human visual system have ensured that more often than not the points of interest in a scene are near Middle Gray.  After extensive testing with a large number of people and different kinds of film it was decided that more or less this ratio of stops/Zones above and below Middle Gray from the scene was best to capture on the limited dynamic range of film the wider dynamic range of a natural scene - and therefore film's sensitivity (S, alias ISO) was defined accordingly: the metered exposure should more or less always fall around the same spot in a similar film curve ensuring that the 'proper' balance of shadows, midtones and highlights are recorded (special films' curves that have substantially different shapes require in fact special instructions with regards to exposing them 'properly').

So back to us.  As long as we have a reflected spot meter that provides accurate values for aperture and shutter speed in the given luminance at the set ISO (we really do not care how it is calibrated as long as it is accurate) we would present a Middle Gray card to it to ensure that the whole scene is 'properly' exposed, as meter, film and standard manufacturers determined. If you use a different reflectance than Middle Gray to meter off of, the tones from the scene will not be recorded according to the well proven standards.

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.

Let's now go through the same thought process as above for DSLRs.  Things were much easier here (hey their curves are linear, no distortion - see the D7000 depicted in the image above), but there is a hard cut-off at sensor saturation, where film had a gentle roll-off.  So the industry went about trying to determine how to store the best compromise of 'proper' balance of shadows, midtones and highlights for the quite different characteristic curve of modern sensors.  They came to the conclusion that Middle Gray from a scene should be stored in raw values of around 1/7.8 or less of full sensor scale for the reasons quoted just above.  They pretty well left the approximate ratio up to individual manufacturers, who know their camera systems best.  But the exact number is immaterial in this context because the spot meter is paired to the sensor (we do not need S to tie sensing medium and meter together), so the metered exposure is always going to be stored in raw values at the properly designed proportion of full scale.  You may not agree with it as a result of your experience, just like you may not agree with the ISO speed printed on a certain film's case, but that's what the camera and standard designers thought was the best compromise.

So how do we achieve the exposure from which Middle Gray from the scene is mapped to the 'proper' raw levels, ensuring that the rest of the dynamic range in the scene is recorded in the designed 'proper' compromise by our DSLRs?

Why, simply present Middle Gray to your spot meter and lock exposure.  Middle Gray gets then recorded in the 'proper' raw values and the rest of the scene falls nicely into place, linearly as desired.

Cheers,

Jack

PS Where was 12% reflectance ever mentioned in this discussion or any of the relevant standards?  Pull it, Thom!

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