Letters

A question about exposure value-based metering

I was reading the article by Sanford Weiss in Evidence Technology Magazine (“Digital Low-Light Photography”, Vol. 7, No. 5). Under the heading Exposure, he talks about numeric exposure and how it represents a com-bination of shutter speeds, apertures, and ISO sensitivities. He makes an example as follows: EV of -1 at ISO 100 corresponds to a shutter speed and aperture combination of 15 seconds at f/2.8, or 4 minutes at f/11.

If possible can you explain the formula or mechanics to arrive at the example answer? I thank you in advance and hope you have a great New Year.

Detective Carlos R. Gutierrez
Hawthorne Police Department
Hawthorne, California

Author Sanford Weiss replies:

I hope you will agree when I say the computer age in general has made many things quicker and easier, but some of the tricks and gadgets camera manufacturers utilized in the earlier days of photography had great success and utility. Fifty years ago, many cameras, especially those made in Germany for Kodak, Agfa, and Zeiss, had aperture and shutter-speed rings that locked together after an initial mechanical adjustment. After telling the meter the ASA (ISO) of the film it carried, the camera meter would yield an exposure reading (light intensity) number in EV (exposure value) rather than a recommended shutter speed and aperture combination as built-in computers do today. The user would then adjust either exposure variable until the matching EV number was opposite a reading reference. Once done, the ring of exposure variables, when turned, would always set appropriate shutter-speed and aperture combinations to produce proper exposures on film. Some modern exposure meters (for example, the Luna-Pro by Gossen) are still calibrated this way. In the article I picked two random shutter-speed/aperture combinations matching an EV of -1 on the Luna Pro because -1 is a light intensity reading the photographer will often encounter in shooting situations.

The photographer can easily and quickly decide which combination of settings would be appropriate for the subject and transcribe the information into the built-in camera meter. When the photographer has no hand-held meter, the camera computer will perform exactly the same operations—changing either the shutter speed or aperture manually will cause the camera to change the other in order to maintain proper exposure. It is just not as easy for the photographer to visualize all the possible combinations at once.

Another common location for EV numbers on modern cameras is in the camera manual. The manufacturer may supply an EV reading range for the camera so the user can tell how dim or bright a situation should be in order to be successfully metered on automatic by the camera’s built-in meter.

Sanford Weiss
West Chicago, Illinois
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ORIGINALLY PUBLISHED:
"Letters to the Editor"
January-February 2010 (Volume 8, Number 1)
Evidence Technology Magazine
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