#18. The Accuracy of Incident-light Exposure Meters
True or False?
Incident-light exposure meters are more often accurate because they are not “fooled” by excessive highlights or shadows in a scene.
Incident-light exposure meters appear to be accurate when used to predict the exposures of repetitive scenes–perhaps scenes constructed in a studio. The photographer who knows what to expect can adjust the exposure compensation or the shutter speed to get an appropriate exposure. These meters work, but they don’t tell the photographer anything about the light reflected from the surfaces in the scene.
Incident-light exposure meters cannot accurately describe the distribution of tones in any scene. They assume that the scene has a low key distribution of tones with a (currently 3% reflectance) midtone exposure. (Is this an accurate representation of your scene? A knowledgeable photographer will judge and adjust.) This works because, when this representative luminance is matched to the midtone exposure of the photosensitive exposure range in use, there is plenty of room in the exposure range to capture highlights at the expense of shadows. Ignorance is bliss!
It is generally incorrect to say that an incident-light exposure meter is more accurate than a reflected-light exposure meter because a reflected-light exposure meter in the hand of a photographer who recognizes that a representative luminance from the actual scene is always matched to the midtone photosensitive exposure can always adjust for high and low key representative luminances.
To say that incident-light meters are more accurate is a descriptive stretch. To say that they are more consistent is a more accurate description. (The photographer who knows what to expect can adjust.)
Perhaps the question is moot because the photographer can and does adjust when using either type of exposure meter. Perhaps this question is moot because there is never an accurate or a correct exposure.
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.
Should you have any comments or questions about this web site, please contact me. Thanks.