Pentax Optio Digital Camera 750Z - Manual (Page 176)

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Reflected light exposure meters, in general, are not used to obtain a
single correct exposure. They are normally used to make light readings
of various parts of the subject, but the lighting around the subject can
greatly affect the camera film (or CCD on a digital camera) due to the
color and material of the subject, even under similar lighting conditions.
As the metering range can be specified fairly precisely with spot
exposure metering such as that provided on this camera, to capture the
desired picture with a camera, you must meter the brightness at various
points on the subject and then decide which value based on what
standard to use in the camera settings. The following describes the
common methods for using the exposure value metered with a reflected
light exposure meter as a reference for making the appropriate
photographic decisions.
This method is used to obtain the most suitable exposure value by
averaging the brightest area (highlight) of the subject with the darkest
area (shadows).
To use the Optio 750Z as an exposure metering with this method, first
meter the highlight of the subject and then meter the shadows. As
described in Step 6 on p.173, the first and second metered values (and
those thereafter) are saved and then appear on the EV scale. The
average of those values can be easily derived by using the four-way
controller (45) to move the exposure value to the point exactly between
the first and second values. This is the average of the two values.
Because this method obtains the exposure of the intermediate level
between the brightest and darkest areas of the subject, it is effective for
capturing a well-rounded picture with balanced lighting. However, when
the subject has a significant difference between the highlight and the
shadows, it is recommended that you take at least three exposure
readings, calculate the average and determine, yourself, which area of
the composition represents the intermediate level. To obtain a fairly
appropriate exposure value with a single metering, try placing a gray
card with a reflectance of 18% perpendicular to the camera’s optical axis
when taking readings.
Using the Reflected Light Exposure Meter
Averaging Reading Method