# Photographic Myth Buster #40

#40. The Standard Photosensitive Exposure Range of ISO Equivalents

True or False?

The standard width of the photosensitive exposure range for a solid-state photosensitive array is less than the actual width of the photosensitive exposure range for ISO Equivalent photosensitivities.

True.

As noted in the explanation in Photographic Myth Buster #39, “digital” cameras produce eight-stop ranges of exposure data associated with each ISO Equivalent photosensitivity.

As discussed in the explanation in Photographic Myth Buster #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

The ISO standard for reporting the photosensitivities of solid-state photosensitive arrays, ISO 12232:2006, does not explicitly identify a speed point shift δ for solid-state arrays, however, it is very explicit that “ISO speed ratings for a DSC [digital still camera] should relate directly to the ISO speed rating for photographic film cameras.” Since the standard uses a reference exposure that matches the reference exposure used in the standard for color positive emulsions (questionable as this may be), it seems most practical and most appropriate to use the speed point shift δ used in the standard for color positive emulsions, that is, 3.67 stops between the minimum usable exposure Hmin and the speed point exposure Hsp.

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.

The standard width of the photosensitive exposure range for monochrome negatives is, therefore, twice 3.67 stops plus -0.24 stops, that is, 6.86 stops–something close to the seven stops that most photographers expect for monochrome negatives. The photosensitive exposure range for solid-state arrays is similar to that of color positive emulsions with the speed point exposure near the midpoint of the range and with the midtone exposure to the left of (less than) the speed point exposure. Nevertheless, this standard width of 6.86 stops is less than the eight-stop width of the eight-bit (“cooked”) data downloaded from digital cameras for a particular ISO Equivalent photosensitivity.

Calculating the midtone reflectance for a 6.86-stop exposure range as described in the explanation Photographic Myth Buster #32, we find that monochrome negatives have a midtone reflectance of 9.3%. 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.