#21. Hand-held Exposure Meters
True or False?
Hand-held exposure meters are more accurate than exposure meters found in modern cameras.
The most important characteristic that can be used to compare hand-held and camera-held meters is the measurement range in EV (Exposure Values). The second most important characteristic that can be used to compare meters–if you can find it–is the precision to which the measurements are made or, perhaps, displayed. It is sometimes called the brightness difference. The photosensitivity (ISO) range, the aperture range, the shutter speed range, and the compensation range are used in calculations that are made by the exposure meter after the measurements. They have nothing to do with accuracy.
The measurement range in EV is the range of luminances that the meter can accurately measure. The range in EV is usually offered for some photosensitivity–typically ISO 100. The photosensitivity S along with the Exposure Equation
2EV = A2 / t = Ls S / K
allows the photographer to calculate the luminance range of the meter. Using binary logarithms, log2(), this range can be calculated in stops using ISO 100 and K = 12.70 lumen seconds per square meter.
EV = log2(Ls) + log2(S) – log2(K)
log2(Ls) = EV – log2(S) + log2(K)
log2(Ls) = EV – 6.67 + 3.67 = EV – 3.00
Given the number of stops in luminance as three stops less than the Exposure Value (EV – 3.00), the photographer can use this number of stops as an exponent of two to get the luminance in units of lumen seconds per square meter.
Ls = 2 log2(Ls) = 2 stops(Ls)
Camera-held meters typically have a measurement range of something like 0 to 20 stops. In spot metering mode, camera-held meters need a little more light to operate at the low end of the range and have a measurement range something like 2 to 20 stops.
Hand-held meters have measurement ranges that are wider, comparable, and narrower than that of camera-held meters. You seem to get what you pay for. The Sekonic L-758 appears to have the widest ranges (-2.0 to 22.9 stops in wide-angle and 1.0 to 24.4 stops in spot), while its cousin, the Sekonic L-398M, appears to have the narrowest range (4 to 17 stops). Most hand-held meters have measurement ranges similar to the measurement ranges of camera-held exposure meters.
Camera-held meters use segmented photosensitive sensors, while hand-held meters use one or two photodiodes or photocells as sensors. Camera-held meters compete with medium-priced hand-held meters and often appear to lack some of the luminance comparison and calculation features offered by hand-held meters.
While exposure meters, in general, display brightness and EV differences in whole-, half-, and third-stop increments, some offer that they measure these differences in tenth-stop increments. While it might be better to use twelfth-stop increments to precisely represent third-stop as well as half- and whole-stop increments, it appears that manufacturers are using typical voltmeter circuitry to produce tenth-stop increments that are used in their calculations of shutter speed or aperture number.
The size of the measurement increment is an indication of the precision of the measurements and has nothing to do with the accuracy of the measurements. Accuracy of measurement can only be judged by comparison of the measurements by meters of a standard light source. Manufacturers offer nothing to indicate accuracy and leave the photographer to expect that the manufacturer has designed and found sufficient accuracy to cover the third- or tenth-stop precision across the whole measurement range. Unless a magazine or a photographer is willing to test the accuracy and precision of several meters, there is nothing that can be said about the relative accuracy of hand-held and camera-held exposure meters used within their measurement ranges except that they appear to have similar accuracy.
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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