Sunny f16 Rule
This
handy exposure rule should be committed to memory by all outdoor photographers. The rule
states that you will always get a "correct" exposure of a sunlit object
if you set your aperture to f16 and your shutter speed to the inverse of
the film speed, or any equivalent combinations of this exposure. For example,
if you are shooting ISO 100 film, shooting at f16 at 1/100 second, or any
equivalent setting, will give a "correct" exposure on sunlit subjects.
Now for a few caveats. The subject
must be primarily front-lit by direct sunlight and it must not be be within two
hours of sunrise or sunset. Also, I include the words
"correct" in quotation marks to highlight the fact that what is correct
for the camera might not be correct for what you want. There are times
when you may want to purposefully over or under expose a scene. A half
stop under exposure can yield more saturated colors. A half stop over exposure
may make colors look more pastel. But the rule provides a reliable starting
point to work from.
Also, when shooting slide film and
the subject is bright white like snow or white sand, you should
underexpose a half stop or so to avoid washing out the details in these
bright areas. Likewise, if the subject is dark in color, such as a black bear,
you should over expose a half stop or so to avoid losing the fur details.
These adjustments are required to take into account that these subjects
would otherwise be outside the exposure latitude of the slide film. These
adjustments are not necessary with print film because print film has wider
exposure latitude.
Note that the above paragraph may
seem counterintuitive if you are used to using your camera meter and exposure
compensation. But the Sunny f16 Rule does not involve your camera's
meter and is a totally different situation.
Note that you don't always have to
set your aperture at f16 to follow this Rule. Any equivalent exposure will
work, such as f11 at 1/250 second or f8 at 1/500 second in the example
above with ISO 100 film.
For side lit subjects, you may want
to overexpose an additional half-stop exposure from the Sunny f16 Rule and for
backlit subjects, you should overexpose an additional full stop. These
adjustments help you capture some of the detail in the shadows.
For a complete discussion on meter-based exposure, see my exposure articles starting with
Exposure
101.
