Practice question · Numerical answer
Sunny 16 at ISO 100 gives f/16 at 1/125. Loading ISO 200 film instead, what shutter denominator does the rule give at the same aperture?
Hints
- Doubling the film speed buys exactly one stop.
- One stop faster than 1/125.
Show the answer
250
Why
250. Doubling the sensitivity means half the light is needed, so the shutter halves the time: 1/125 becomes 1/250. The rule is usually stated as one over the film speed, which gives 1/200, the nearest marked speed is 1/250, and a third of a stop is well inside negative film's tolerance.
Practise Exposure and Metering
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