DLG PHOTOMETER 2 Manual de instruções - Página 4

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Photometer 2 Instruction Manual

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INTRODUCTION TO FOCAL PLANE EXPOSURE METERING

A photographic exposure meter measures lighting and indicates camera settings to give

a nominally optimum exposure of the film. Several methods of metering are widely used.

Reflected Light Metering

Wide angle reflected light metering gives an overall reading of the luminance of the

scene and will indicate camera settings such as to render the overall scene average

luminance at a mid-grey level. This is the most common technique and gives good

results with "normal" scenes. However, in an unbalanced scene, with a dominant dark or

bright feature, the exposure may not favour the feature of interest. As an example,

imagine a scene with the mouth of a cave in an otherwise brightly lit daylight scene.

Using camera settings indicated by a wide angle reflected light meter will give an

exposure optimised for the average across the scene including the dark cave interior

and the brightly lit daylight outside. It is likely that the detail within the shadow of the

cave will not be satisfactorily rendered, with the negative underexposed in this area and

failing to show any detail within the cave shadows. Similar situations can arise when

photographing dark objects against the sky, or in a snow scene, or when trying to

photograph a brightly lit subject against a dark background.

Incident Light Metering

Incident-light metering partially addresses this issue by indicating an exposure setting

based on the light falling on the subject of interest in the scene. In our cave example, an

incident light reading could be taken within the cave in an attempt to optimise the

exposure for the shadowed cave interior. However, the incident light reading obviously

does not take into account the reflectivity of the subject.

Large-Format Focal Plan Metering

With large format photography, the possibility exists to measure the actual illumination at

specific points in the image, and to optimise the exposure for features of interest in the

image. Further, it is possible to measure the illumination at various points in the image

where it is desired to get good rendering of shadow and/or highlight detail, and to

optimise the exposure to achieve the best compromise. Furthermore, by determining the

total range of luminance across the shadows and highlights of the image, the film

processing may be optimised to give the best results.

The focal plan metering system lends itself to use of the Zone System, devised by Ansel

Adams and Fred Archer. An excellent description of this system is given in Adams' book

The Negative (1981).