Nikon F75D - F75 QD = N75 35mm SLR Camera Body Only Instruction Manual page 62

Glossary
—continued
Flexible Program
Flexible Program function temporarily shifts an
automatically selected shutter speed/aperture
combination while maintaining correct exposure.
That is, the desired shutter speed or aperture can
be selected in Auto-Multi Program.
f-number
The f-number represents the aperture value and is
calculated from lens' focal length divided by the
effective aperture opening. The standard numbers
for calibration are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16,
22, 32, etc.
The smallest f-number is called maximum aperture
and the largest f-number is called minimum
aperture. Lenses with large maximum apertures
(smaller f-numbers) are 'fast' lenses that allow
photographers to use faster shutter speeds in dim
light. Lenses with smaller maximum apertures
(larger f-numbers) allow the use of lower shutter
speeds for available light but are also lighter and
smaller than faster lenses. With some zoom lenses,
aperture varies depending on the focal length
setting.
Focal length
The distance from the principal point to the focal
point. In 35mm-format cameras, lenses with a
focal length of approx. 50mm are called normal or
standard lenses. Lenses with a focal length less
than approx. 35mm are called wideangle lenses,
and lenses with a focal length more than approx.
85mm are called telephoto lenses. Lenses which
allow the user to continuously vary the focal length
without changing focus are called zoom lenses.
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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 F75/F75D
operates with the flash sync mode at Normal Sync.
(See "Rear-Curtain 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 F75/F75D 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.
Metering system
See the column on page 32.
Rear-Curtain Sync
Flash fires an instant before the second (rear)
curtain of the focal plane shutter begins to move.
When slow shutter speeds are used, this feature
can create a blur effect from the ambient light, i.e.,
flowing-light patterns following a moving subject
with subject movement frozen at the end of the
light flow. (See "Front-Curtain Sync".)
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 F75/F75D'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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