Sequence viewing > Photographic Exposure - Resource - ©
Lloyd Godman
Light
Meters
Taking
more accurate exposure readings is done with a
light meter.
All meters work on this basis of converting light
into electric current, which can then be measured
on a scale and translated into appropriate shutter
speeds and f numbers relative to the film ASA.
While some meters work on this basis of converting
light into electric current like a solar panel,
others have a battery which provides power to
a Cds or Cadmium sulphide cell which are photo-resistance;
meaning that its electrical resistance changes
in proportion to the light it receives.
Selenium
Cell Meters
The
hand held meter uses a cell behind a window
in some convenient part of the casing.
This cell is normally pointed toward the
subject. Meters of this kind are electrically
self contained needing no battery as the
selenium cell generates the current (solar
cell). Their disadvantage is that sensitivity
is directly related to the size of the
cell and can take up a large area of the
meter surface. Cell response is comparatively
weak so that readings in dim light are
difficult and unreliable. |
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With
the simplest type of these meters a red
needle moves indicating the intensity
of the light. As the dial on the meter
is moved the circle of the green needle
is moved until it lines up with the red
needle. When the dial is moved to line
the two up, the readings are read off
on the scale. So we could have 1/60 second
@ f2.8 or 1/8 second @ f8. Both would
give us the same exposure. Reading around
the dial, there are a whole range of combinations
we could chose from.
Of course we have to set this to the correct
ASA or speed of the film. |
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Historic Light meters
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