#37. Exposure Range for Color Positives (Slides or Transparencies)
True or False?
The standard width of the photosensitive exposure range for color positives (slides or transparencies) is greater than the standard width of the photosensitive exposure range for monochrome negative emulsions.
As discussed in the answer to question #31, the standard width of a photosensitive exposure range for a photosensitive array in stops
Width of a Range in stops = 2 x [ log2(δ) + log2(M) ]
can be calculated the shift δ from the minimum usable exposure Hmin to the speed point exposure Hsp
δ = Hsp / Hmin
and from the shift M from the speed point exposure Hsp to the midtone exposure, Hm.
M = Hm / Hsp
Unlike the standards for monochrome and color negatives, which place speed point exposures fractions of a stop above the minimum usable exposures, the standard for color positives (slides or transparencies) place the speed point exposure halfway between the exposure associated with the minimum usable density plus 0.2 density units, Dmin+0.2, and the exposure associated with the minimum usable density plus 2.0 density units, Dmin+2.0. The midpoint of this section of the exposure range is Dmin+1.1 density units (where 1.1 = 0.2 + (2.0-0.2)/2). Since density units are logarithmic units, the speed point exposure for color positives is 3.67 (=1.1/0.30…) stops above the minimum usable exposure, and the speed point shift δ for color positives is 3.67 stops.
The midtone shift M is determined from the value of the reference exposures Ho given in the sensitometric (film speed) standards for emulsions using an equation implicit in the exposure meter standards.
M = qo K / Ho
The reference exposure, Ho, for monochrome negatives found in the definition of the photosensitivity
S = Ho / Hsp
is 10.00 lumen seconds per square meter, so the midtone shift is 0.84 using 12.70 candela seconds per square meter for the exposure meter constant K and using 0.21π lumens per candela for the conversion constant qo. The binary logarithm of the midtone shift is -0.24 stops.
Since the maximum usable exposure Hmax corresponds to the minimum usable density Dmin for color positives, the total from the minimum usable exposure Hmin to the speed point exposure Hsp to the midtone exposure Hm is 3.67 + 0.24 = 3.91 stops.
The standard width of the photosensitive exposure range for monochrome negatives is, therefore, twice 3.91 stops, that is, 7.82 stops–something quite different than the five stops that most photographers expect and somewhat greater than the 7.47-stop standard width of monochrome negative emulsions.
Calculating the midtone reflectance for a 7.82-stop exposure range as described in the answer to question #32, we find that monochrome negatives have a midtone reflectance of 6.7%. This is a lot different from the 18% midtone reflectance that most photographers expect.
The diagrams that support the calculation of the standard width of the photosensitive exposure range for various types of photosensitive arrays can be found in the book Photographic Exposure Calculations and Camera Operation.
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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