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Photographic Speed Ratings :
By Henry A. Fowler and Lloyd E. Varden
Agfa-Ansco Cor poration, Binghamton, N. Y.
Part III
Practical Limitations of Speed Ratings
Aside from the difficulties in the measurement of film speeds and the perfection of a system of speed numbers. the practical application of even a so-called “perfect” rating system would be limited by several consequential factors. No system of speed rating can possibly be expected to account for personal preferences in judging negatives, to allow for personal errors in the use of exposure meters, to take into consideration mechanical variations in exposure meters and camera shutters, to allow for peculiarities of divers lenses, or to be in accord with any of several other possible causes of exposure error. If such such errors are accumulative it is highly probable that they can cause deviations as great as 300 to 400 per cent from the calculated exposure. A great many of these are shown in the accompanying chart. Fig. 18.
Personal Factors
No factors in determining exposure properly are so difficult to systematize as personal ones. However, it should not be thought that this is the result of individual stubbornness, for (if we may be philosophic) “the truth may
* < Reprinted through courtesy of the 1940 American Annual of Photography)
be spoken without the fact.” Should one person be able to perceive the number 14 when making a reading w ith an extinction type exposure mefter, it does not violate a lower reading, say 12, of another person. To both these individuals, the truthful reading is what they perceive, and since the readings are based on visual interpretation, fact, as such, does not exist.
Personal factors such as the preference for either thin or dense negatives are just as difficult to rationalize, and so we can never hope to discover a system of rating film speeds which will not require some individual adjustments to account for these personal idiosyncrasies.
In addition to these strictly personal factors, there are other considerations which may be classified under this same heading, although they apply to all individuals to a greater or lesser degree. In judging light intensities, it is often thought that experience enables one to become quite expert. On the contrary, dependence on judgment in this instance is a most efficient method of experiencing considerable difficulty. The human eye is a marvelous mechanism and a part of its function is to adapt the individual to a wide range of illumination levels.
The iris and retina of the eye function to make it more or less sensi
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