18 January 2010 Reviewing the exposure meter constant: improving the exposure equation
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Abstract
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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Michael G. Prais, Michael G. Prais, } "Reviewing the exposure meter constant: improving the exposure equation", Proc. SPIE 7537, Digital Photography VI, 75370X (18 January 2010); doi: 10.1117/12.844933; https://doi.org/10.1117/12.844933
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