PHOTOGRAPHY-16

 PHOTOGRAPHY-16

GUIDE NUMBER [GN]

The GN is a safe tool for estimating the right level of exposure for a given film speed [ASA], SUBJECT-FLAH DISTANCE for every flash gun. Exposure being a product of intensity of light falling on a sensitive recording device, and the duration of light received on such a surface. While the duration of exposure is determined by shutter speed, that of intensity is governed by the volume of light let in through the Aperture [Diaphragm]. For flash light, aperture gets priority , for, the shutter speed is insignificant as the ‘stay’ of light on an object is far too short for even the highest of shutter speeds . It is only important that the ‘throw’ oif light happens when the shutter is ‘opened’- the act named exposure. This ‘synchrony’ is ensured by all modern camera designs and the photographer has no burden of ensuring synchrony.

Working of GN

For a Film rated at ASA 400, IF aperture f8 is suggested for an object at a distance of 8 ft, the GN= 64 . APERTURE f8 X distance in ft 8  =8X8= 64 –the GN. For an object at a distance of 12 ft the Aperture can be estimated by dividing the GN by the distance involved 12 ft= 64 by 12= 5.333 on the diaphragm ; since 5.333 cannot be set, select the nearest value of the scale f 5.6. Mark f is a convention to indicate aperture value . The most common  Aperture series on lenses run thus: f 16, 11, 8, 5.6, 4, 3.5, 2.8 or 2.7, or 1.8 or 1.7 and so on. All these f values must be understood with a common numerator 1, meaning 16= 1/ 16, 8= 1/ 8 and so on. So, the smaller the f number the greater the opening [ more light let in ] and larger the f number the smaller the opening [restricted opening for light entry] . Either influence exposure by regulating the light volume.

VALIDITY OF  GN

GN values hold good when the flash unit/ bulb are fresh and well kept. With wear and tear, the light out put declines and the estimation from given GN  may fail to work to expectation.

 

When in doubt, one can assess the current GN by logical derivation making use of bracketing technique. Bracketing refers to the act making 3 pictures of the same object held at the same distance, but employing 3 different Aperture values in the proportion of  +,  normal, - [ f 5.6, f 8, f 11] where 8 is mid value for exposure, 5.6 is + value and 11 is minus value]. Of them one would make proper image densities for details. Using that frame and its f value multiply by the distance to get to know the correct GN VALUE . Say if the distance was 7 feet , and the best frame was by f 5.6, the GN = 5.6 x 7 =39.2 0r 39 to be stringent. When in need or doubt we can detect the workable GN for different film ASAs  relevant for work. This way GN  gives a tool for locating the best  f setting for the current status of the flash for the distance for a defined ASA It would stay valid for 8 months to a year subject to the quantum of use and upkeep of equipment.

Always GN is a variable for film speed [ASA / IS / DIN] as international units in specifying film speed and its behaviour asa ‘brand’ property.  That way, photography using artificial light opens venues for delectable trials in lighting to enhance the ‘moulding effect’ of human faces through some imaginative lighting, in place of harsh frontal lighting.

More to follow.                     Prof. K.Raman