Dr. Michael G. Prais Profile

Dr. Michael G. Prais

Consulting Theoretician and Numerical Analyst

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Proceedings Article | 24 January 2011 Paper

Improving the sensitometric and OECF standards: recognizing the photosensitive exposure range

Michael Prais

Proceedings Volume 7876, 78760T (2011) https://doi.org/10.1117/12.871549

KEYWORDS: Standards development, Holmium, Solid state electronics, Reflectivity, Safety, Photography, Absorbance, Cameras, Binary data, Optoelectronics

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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(H_max) + log₂(Hmin)]/2. The reference exposure H_o = S/H_sp 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, H_max and H_min. Knowledge of H_max and H_min (or H_mid and Δ) makes the reference exposure H_o, the speed point exposure H_sp 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 R_mid for standard photosensitive arrays that are much less than oft-touted 18% making most references to 18% inappropriate.

Proceedings Article | 19 January 2010 Paper

Reviewing the exposure meter constant: improving the exposure equation

Michael Prais

Proceedings Volume 7537, 75370X (2010) https://doi.org/10.1117/12.844933

KEYWORDS: Cameras, Photography, Calibration, Holmium, Photosensitizer targeting, Standards development, Camera shutters, Hassium, Reflectivity, Einsteinium

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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 K_s 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 H_o = Hsp/_S and the midtone shift M = H_mid/H_sp). It also concludes that the sensitivity and the common exposure equation ultimately depend on the midtone photosensitive exposure target H_mid. 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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