* f16 Rule: The correct exposure for an average subject in bright sunlight is 1 over the ISO number of the film, with the camera's lens opening set at f16.
This rule allows you to estimate the exposures you'll need with a film based on its ISO rating. For an ISO 50 film, this is 1/50th second at f16. For an ISO 400 film, it's 1/400th second at f16. If your camera's shutter doesn't have exactly the speed given by the rule, just choose the next higher speed. For ISO 50, you can use 1/60 second. For ISO 400 film, use 1/500 second.
The 50 ASA [slow] films are the correct choice when:
* the finished enlargement must have minimum 'graininess', or
* the finished enlargement must have maximum detail, or
* the light will be strong enough to allow an acceptable combination of shutter speeds and lens openings.
The ISO 400 [fast] films are the correct choice when:
* graininess is not very important or perhaps even desired, or
* the amount of light available will be low, or
* shutter speeds must be high in order to stop motion.
On a perfect, clear, sunny day, f/16 is to be used. Days that aren’t clear and sunny can still use the Sunny 16 rule with some minor changes. Extremely bright days with distinct shadows use f/22. Hazy sun and soft shadows use f/11, cloudly days with barely visible shadows use f/8, and overcast days with no shadows use f/5.6. The same shutter speed rule applies. In our example of using ISO film speed 100, the shutter speed will always be 1/125, regardless of the aperture used.
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