Choice, speed, on DXing

Press, 22 October 1986, Page 35

Choice, speed, on DXing

Film speeds confuse some people. Although the DXing of cameras and films now usually ensures that film speed matches camera setting, there is still the problem of choice

of speed and also occasionally a glitch in the automation.

Film speed is the speed at which the emulsion reacts to light, and so relates to shutter speed

and aperture. The present standard designation of film speed is the International Standards Association (ISO) scale. If an ISO speed is stated in full it has two numbers separated by a bar, for example 100/21deg., where 100 refers to the old American Standards Association (ASA), and 21 deg. to the old German standard DIN (Deutsche Industrie Norm). In New Zealand the DIN figure is usually left out, making this film speed simply 100 ISO. This trend is emphasised by the manufacturers’ common practice of including the ISO figure in the film’s name. The ASA and DIN standards were originally based on different film characteristics, but the effect, though not the numbering, was much the same. On the DIN system,

each doubling of speed added three to the DIN rating; on ASA, doubling the speed doubled the ASA number. As the latter system is both more convenient and more widely accepted, it will be used from here on through in the ISO version.

The film speed series followed by most film types is ISO 25, 50, 100, 200, 400, 800, 1600, 3200, though few if any include the whole range. The interval between neighbouring speeds corresponds to a “stop” on the camera setting, so that if, for example, the required aperture in existing light is f 4 for 100 ISO film, it will be a stop smaller at f 5.6 for 200 ISO if the shutter-speed is the same. Similarly, if the f-number is kept constant a shutterspeed of 1/250th second on 200 ISO film cor-

responds to 1/500th second on 400 ISO. Some are worried if film speeds do not exactly correspond to camera markings. This matters little with amateur colournegative (print) films, as these now have considerable “latitude” — the degree of wrong exposure which will still give an acceptable print. Professional films of all types, and all monochrome and some colour transparency films, have less latitude and should 'be exposed with care; but control of developing means exposure may purposely be varied for special effects. Professional films are so labelled, and cost a little more than their amateur counterparts. The uses and characteristics of various speeds of film will be dealt with in a future article.