Nikon F55 - F55 35mm SLR Camera Instruction Manual page 55

Glossary
—continued
Focus Tracking
Enables the camera to analyse the speed of a moving subject according to the focus
data detected, and to obtain correct focus by anticipating the subject's position—at the
exact moment of exposure.
Lock-On™ Autofocus keeps focus firmly on a main subject during Focus Tracking even if
some other object momentarily blocks it in the viewfinder.
Front-Curtain Sync
The flash fires an instant after the front curtain of a focal plane shutter has completed its
travel across the film plane. This is the way the F55/F55D operates with the flash sync
mode at Normal Sync.
Guide number
The guide number indicates the power of a flash in relation to ISO film speed. The guide
number of the built-in Speedlight of the F55/F55D is 12 (ISO 100, m). Guide numbers are
quoted in either metres or feet. Guide numbers are used to calculate the f/stop for
correct exposure as follows:
guide number
f/stop = ———————————
flash-to-subject distance
Using a selected aperture, we can calculate the required flash-to-subject distance with
the formula:
guide number
flash-to-subject distance = ———————
f/stop
Useful for determining the maximum flash-to-subject distance for flash photography.
ISO film speed
The international standard for representing film sensitivity. The higher the number, the
greater the sensitivity, and vice versa. A film speed of ISO 200 is twice as sensitive as
ISO 100, and half that of ISO 400 film.
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Slow Sync
A flash technique for using the flash at a slow shutter speed. Flash shooting in dim light
or at night at a fast shutter speed often results in a flash-illuminated subject against a
dark background. Using a slower shutter speed with the flash brings out the background
details in the picture.
The F55/F55D's Slow Sync mode extends the automatically controlled shutter speed
range down to 30 sec. (in Auto-Multi Program, Aperture-Priority Auto) or 1 sec. (in
Night Portrait mode).
Vignetting
Progressively diminished illumination on the film from the centre to the corners. There
are two kinds of vignetting—natural vignetting caused by the lens, and vignetting that is
caused by improper use of accessories such as a lens hood or filter.
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