The range of light of the scene is broken up into Zones. Zone 0 is black and devoid of any detail. Zone X is pure white also devoid of any details. In the middle is Zone V or middle gray. This is important to remember, as every properly working light meter will measure brightness and indicate an exposure that will place that brightness in Zone V.
For This Discussion Spot Metering Is Used
The basic or starting foundation for exposure is determined thus:
Basic f/stop = 1/square root of the ISO
Basic Shutter Speed = 1/number of foot candles
This will place the part of the scene metered in Zone V. This is an inherent characteristic of light meters. Changing either shutter speed or f/stop will place that part of the scene in a different Zone, darker with less exposure and brighter with more.
So, if we are shooting a scene using ISO 125. The basic f/stop is f/11 (11^2=122, which is close enough)
Spot metering a part of the scene indicates that the brightness is 250 ft-candles. For this example, the basic shutter speed and f/stop combination would be 1/250@f/11.
If we were to change the shutter speed to 1/500 that metered part of the scene would now be placed in Zone IV. Similarly if we were to change the f/stop to f/16, again the metered part of the scene would then be placed in Zone IV.
With black and white film, the idea is to spot meter and place the darkest part of the scene in Zone III and see where the brightest part of the scene falls. This will determine the amount of time needed to develop the film to achieve the necessary tonal range of the negative to match the paper on which the print is to be made.
With digital, the situation is just the opposite. After determining the individual camera's capability to capture highlights and still maintain details in those highlights, one can spot meter the brightest part of the scene and increase the exposure to place that part of the scene in Zone VIII or IX, thus maintaining some details in the brightest part of the image.
I posted three articles exploring the basics of exposure and why a triangle is not the optimum way to think about the relationship of shutter speed and f/stop combined with ISO. You can find them here.
Exposure and How It Works - A Beginner's Guide
http://www.uglyhedgehog.com/t-484554-1.htmlMore Exposure and How It Works - A Beginner's Guide
http://www.uglyhedgehog.com/t-484826-1.htmlFinishing Up Exposure and How It Works - A Beginner's Guide
http://www.uglyhedgehog.com/t-485845-1.htmlFor further reference:
"A Memorial Volume Containing an Account of the Photographic Researches of Ferdinand Hurter & Vero C. Driffield : being a reprint of their published papers, together with a history of their early work and a bibliography of later work on the same subject" - Authored by the Royal Photographic Society of Great Britain
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"The Negative" - Ansel Adams
Although these references discuss film sensitivity and processing, there is some correlation to digital. "The Negative" discusses the foundation of exposure.
--Bob
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