#30. The Actual Width of a Photosensitive Exposure Range
True or False?
The actual width of a photosensitive exposure range is determined by the minimum usable exposure and the maximum usable exposure of the range.
The actual or usable width of a photosensitive exposure range falls between the minimum usable exposure and the maximum usable exposure of the range. Within this range a photographer can expect:
A small difference in the quantity of light (two tones) creates a small recognizable difference in darkening or in voltage from the array.
This recognizable difference in darkening or in voltage occurs consistently over a set range of exposures.
For quantities of light outside of this exposure range, there is not a recognizable difference in darkening or in voltage with a difference in quantity of light reaching the photosensitive array.
The sensitometric (film speed) standards define only one of the usable exposure extremes (that is, minimum usable exposures associated with minimum usable densities for negatives, maximum usable exposures associated with minimum usable densities for positives, and minimum usable exposures associated with signal-to-noise ratios for solid-state). These are standard values that manufactures associate with their products in order to communicate with photographers. It is useful to determine the actual width of a chosen photosensitive array.
The actual width of a photosensitive exposure range is fairly easy to determine:
• Set your camera to the (a) photosensitivity offered by the manufacturer.
• Select a moderate aperture, such as f/8.
• Place your camera in manual (calculation) mode.
• Mount a color checker card (or simply mount black and white cards side by side) where it (they) will fill most of the viewfinder of your camera. (I prefer to work in the shade on a sunny day.)
• Use your exposure meter to get the shutter speed that gives your card(s) a midtone exposure.
• Note the shutter speed, place it on a scrap of paper in your “scene”, and make an exposure.
• Start with a shutter speed that is five stops higher (or lower) and make exposures changing the shutter speed by one third or one half or one whole stop until you reach five stops lower (or higher) than the value for the midtone exposure.
• Compare the brightness of the surfaces in your sequence of exposures either visually or in a photo editor looking for the exposures (shutter speeds) at which the change in brightness between exposures is much less pronounced that it is around your midtone exposure.
• The difference in the two shutter speeds in stops at which this happens (one at low and one at high exposures) is the actual width of your photosensitive exposure range. (The precision to which you know this value depends upon whether you changed the shutter speed by one third or one half or one whole stop.)
Once you know this width, you know which surfaces will appear distinguishable in tone and or color in your images and how many stops from midtone will texture be apparent.
This procedure is similar to the Ansel Adams inspired procedure for determining the Exposure Index, that is, the value of the photosensitivity that puts your minimum usable exposure four stops below the midtone exposure. This process differs from Adams’ in it goal of finding rather than assuming the number of stops below the midtone. This difference is important when using photosensitive arrays other than monochrome negative emulsions. It also differs in assuming that the manufacturer’s photosensitivity is usable rather than assuming that the best photosensitivity is the one that places the minimum usable exposure four stops below midtone. This assumption is sensible with the knowledge that the photosensitivity points to the midtone exposure (rather than the speed point exposure). Its goal is to find the number of usable stops in exposure below (and above) the midtone.
Copyright 2008 Michael G. Prais, Ph.D.
For a readable but in-depth analysis of this concept along with many other concepts associated with photographic exposure, take a look at the book Photographic Exposure Calculations and Camera Operation. This book provides insight into the equations that govern exposure, exposure meters, photosensitive arrays (both solid-state and emulsion) and the Zone System as well as concepts associated with resolution, dynamic range, and depth of field.
The book is available through Amazon.com (ISBN 978-1-4392-0641-6) where you can Search Inside!™.
Check http://michaelprais.info under Photography for the table of contents, an extensive list of the topics and subtopics covered, the preface describing the purpose of the book, and a diagram central to the concepts in the book.
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