Nikon Camera N90 - Manual (Page 227)

Flash synchronization
The timing of the flash so it fires coincident with the operation of
the camera's shutter. There are two types of synchronization:
Normal Sync which fires the flash at the start of the exposure,
and Rear Sync which fires the flash at the end of the exposure.
f-number
The numbers on the lens aperture ring and on the N90’s LCD
which indicate the relative size of the lens aperture opening.
The f-number series is a geometric progression based on
changes in the size of the lens aperture, as it is opened and
closed. As the scale rises, each number is multiplied by the
factor 1.4. The standard numbers for calibration are 1.0,1.4, 2,
2.8, 4, 5.6, 8, 11, 16, 22, 32, etc., and each change results in a
doubling or halving of the amount of light transmitted by the
lens.
Hyperfocal distance
The closest point a photographer can focus on where the
depth of field includes infinity. When the lens is focused for
hyperfocal distance, the deepest depth of field, covering from
1/2 the hyperfocal distance to infinity, can be obtained at each
f/stop. The longer the focal length, the longer the hyperfocal
distance; the smaller the aperture (the larger the f/number), the
shorter the hyperfocal distance.