This article demonstrates that and how sensitometric and opto-electronic characteristic function (OECF) standards
should be changed: The sensitivity S of all photosensitive arrays is--and, in standards, should be--determined by the
midtone photosensitive exposure of the array Hmid, the binary logarithm of which is [log2(Hmax) + log2(Hmin)]/2. The
reference exposure Ho = S/Hsp of a photosensitive array is--and, in standards, should be--determined by the width of the
photosensitive exposure range Δ, which is determined by the measured minimum and maximum usable photosensitive
exposures, Hmax and Hmin. Knowledge of Hmax and Hmin (or Hmid and Δ) makes the reference exposure Ho, the speed point
exposure Hsp and the safety factor σ unnecessary, and these extraneous quantities should be eliminated from the
sensitometric standards. In addition, the sensitometric standard for solid-state arrays, ISO 12232-2006, and the OECF
standard, ISO 14524-1999, should be changed because they use a photosensitive exposure range which is inappropriate
for solid-state arrays. Finally, this article shows that all current standards establish midtone reflectances Rmid for standard
photosensitive arrays that are much less than oft-touted 18% making most references to 18% inappropriate.
A careful mathematical analysis of the meaning of variables and equations used in standards for exposure meters and the
determination of sensitivities S demonstrates that standards and authors of many photographic texts have erred in
interpretations and applications of the common exposure equation. This article concludes that it is inappropriate to use
the exposure meter constant Ks as an exposure meter calibration constant because it is essentially a label for a product of
characteristics of the photosensitive array employed (the reference exposure Ho = Hsp/S and the midtone shift M =
Hmid/Hsp). It also concludes that the sensitivity and the common exposure equation ultimately depend on the midtone
photosensitive exposure target Hmid. This midtone exposure equation can be generalized by including a shift to a
arbitrary (non-midtone) photosensitive exposure target in its derivation. This more general exposure equation includes
the exposure compensation and eliminates the need for exposure indices. Analysis of the exposure equation for incidentlight
exposure meters shows that these meters avoid the vagaries of the current scene by calculating exposure for a
potentially very different standard scene and often can be, in effect, less accurate in exposure calculations than reflectedlight
exposure meters.
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