Nikon Nikon N75 instruction manual Guide number, Flash-to-subject distance, 122

Glossary-continued

Flexible Program

Flexible Program function temporarily shifts an automatically selected shutter speedlaperture 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.

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'sposition-at the exact moment of exposure.

Lock-OnTM 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 N751N7500 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 N751N7500 is 12139 (ISO 100, m/ft .). Guide numbers are quoted in either meters or feet. Guide numbers are used to calculate the flstop for correct exposure as follows:

guide number

flstop = -------

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 =-=----,-,-----

flstop

Useful for determining the maximum flash-to- subject distance for flash photography.

122