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ABOUT SHUTTER SPEEDS
WHAT IS AN ƒ-NUMBER?
The shutter speed used for each exposure is displayed on the data
panel. The following notation is used:
The reciprocal is used for shutter speeds from
1/2000 second to 1/3 second. 125 is 1/125
second.
For shutter speeds of a half a second or longer, a
quote mark is used to denote whole seconds. 1”5
is one and a half seconds and 15” is fifteen
seconds.
An f-number indicates the relative aperture of the lens and lets us
determine how much light it lets through. But why does the number
get bigger when the amount of light decreases? The answer is in
the way it is written, f/4.0, f/8.0. etc. That means the focal length of
the lens (f) divide by 4 or 8. So a 100mm lens at f/4 has a 25mm
effective aperture (100/4) and at f/8, a 12.5mm effective aperture
(100/8). The f-number indicates the size of the aperture as a fraction
of the focal length of the lens.
The f-number series was carefully chosen to
make controlling exposures easy. In the
chart, the aperture and shutter speed
combinations give the exact same exposure.
Notice how the shutter speed changes to
compensate for the change in aperture.
f/2.8 1/2000s
f/4.0 1/1000s
f/5.6 1/500s
f/8.0 1/250s
f/11 1/125s
f/16 1/60s
f/22 1/30s
f/32 1/15s
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