Photometer 2 Instruction Manual DLGelectronics.com
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).
