Exposure Book Preface


Photographic Exposure Calculations and Camera Operation
by Michael G. Prais, Ph.D.

I expect that you will find the material in this book both controversial and important. The material offers a unique and critical perspective on core operations in the practice of photography. This practice as an empirical pursuit can and certainly does function effectively without an accurate theoretical foundation. Nevertheless, I believe that the profession, especially its educators, should be grounded with the most consistent and, thus, accurate conceptual foundation.

I come to photography as a trained and experienced physical chemist and applied mathematician. I have enjoyed art all my life alongside of science and mathematics, but I did not find the chance to focus on art—particularly photography—until a dozen years ago. My training, experience and inclinations sounded alarms about theoretical consistency and accuracy of photographic exposure determination soon after I started reading and learning about it. My offering here is the result of my research into and analysis of the determination of photographic exposure.

I hope and expect that the results that I present in this book will have a significant impact on the education of photographers. I have held and critically read just about every book that addresses determination of camera settings for optimal exposure currently available for publication as well as many others published in the last eight decades. Except for a contradiction in C.B. Neblette, Photography, Its Principles and Practices (Van Nostrand, 1962, p138), described later in this manuscript and a very brief comment in Ctien, Post Exposure (Focal Press, 1997, p29). every author blindly offers inaccurate information about exposure meters and, particularly, the use of an 18% reflectance card. I was pleased to see Ctien’s comment about problems in this area, but his directive to see ANSI PH3.49-1971 is of little help. This standards document, which is addressed in this book, is itself inaccurate and does not offer the solution.

In this book I provide a thorough examination of exposure calculations in support of composition, photosensitivity (film speed) of both emulsion (film) and solid-state (digital) media, the photosensitive exposure range, spatial resolution, dynamic range, exposure compensation, exposure meters, the myth of 18% as the midtone, the Zone System, the Sunny f /16 Rule, APEX calculations, ISO/ANSI standards documentation and depth of field calculations. In addition to explanations of terms and their connections, this book offers a very general Exposure Equation, tables of representative and exact stop values, extensive diagrams for and examples of exposure calculations, very general depth of field equations and extensive depth of field tables that make it useful as a field guide. The book uses basic algebra, trigonometry and logarithms, but explanations, word equations and diagrams accompany symbolic equations.

This is the first time that material of this depth about exposure is presented. All generally available textbooks, handbooks and other photography texts present material based on simplified illumination, exposure and depth of field equations. These commonly appearing equations and the understanding of the quantities that appear in the equations originate with initial analyses developed in the early part of the twentieth century using some unnecessary assumptions. The use of these traditional, simplified equations is symptomatic of the weak level of presentation, if not understanding, of the underlying photographic theory. The equations and explanations presented in this book are no more complex, but are more complete, and thus, provide a better understanding of exposure.

I see the audience for this work as teachers and textbook writers who have used the traditional explanations for exposure in their work. I provide a critical analysis of the exposure process and recognize several commonly held and taught misunderstandings associated with illuminance and exposure meters. Photography can certainly be practiced without this additional, critical analysis, but I, and perhaps others, have found understanding photography more difficult with the traditional, shoddy analysis of its theoretical basis. This manuscript could and should be used as a supplement to existing textbooks. The technical student of photography should also find this manuscript illuminating.

The development of the Exposure Stop Equation in this book is used as a foundation to expand the Zone System through the explicit observation and consideration of the set of colors and tones in the photosensitive exposure range of the media. The Zone System generally promotes the use of a spot meter to indicate camera settings for exposure as an 18% reflected light midtone and the shift in the camera settings to handle other tones from the scene that are not midtone. This manuscript explains and promotes the explicit recognition that the photographer is not simply reducing the exposure based upon the scene but is also matching a variety of colors and tones in the scene with colors and tones of the photosensitive exposure range of the media around the midtone.

This manuscript points out that among other roles the exposure compensation can be used to index these colors and tones around the midtone. Unlike the recommendation of many photographers that the photosensitivity (film speed) of the media be adjusted to put a selected tone at the midtone of the photosensitive exposure range, I show how using the manufacturer’s photosensitivity and becoming familiar with the tones of the various colors at a variety of stops along the photosensitive exposure range is valuable. This approach requires the recognition of the extent in the stops of the photosensitive exposure range (for instance, seven stops for monochrome negatives), and the manuscript demonstrates how to calculate the extent for all emulsion and solid-state media.

The Exposure Stop Equation is used to show in this manuscript how cameras handle various media equivalently by focusing on a midtone target of the photosensitive exposure range and by letting the photographer handle the extent of the photosensitive exposure range. One of the core concepts developed and promoted in this book is that modern exposure meters point to the midtone of the photosensitive exposure range—not to the midtone of the scene as most photographers are incorrectly taught and assume.

This manuscript is the only one that explains and treats monochrome negative, color negative, color positive and solid-state media equivalently. In the manner presented, this equivalent presentation emphasizes the similarities of emulsion and solid-state media and serves to help the audience make a simple transition between media. The manuscript can be criticized for not treating the development of emulsion media, but that criticism would also demand treating digital editing equivalently. To treat both extensively extends the scope of the work significantly, deviating from its purpose of influencing the proponent of traditional exposure theory and taking away from its potential as a field guide for exposure.

I hope that you find reading this manuscript as valuable as I have found writing it.


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please contact me. Thanks.