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Getting the exposure right can have a significant effect on how easy it is
to classify your image in HemiView, and on the accuracy of your
calculations. Your aim is to have areas of sky at the top end of the
brightness range, and areas of canopy at the lower end. You can check
this on your image by looking at the brightness graph (p 80).
If the image is overexposed, the sky areas will have a tendency to spread
in the final image. Underexposure is less of a problem, but will reduce the
amount of visual detail of the canopy. The camera’s automatic exposure
will tend to adjust the exposure too much for open and closed canopies.
We suggest two approaches to dealing with this.
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·
With the camera in Programmed Auto exposure mode, note the
exposure values under a section of canopy with about 50% sky
visible. Use the command dial to select the highest f-number
consistent with a shutter speed faster than 1/60 second. This is to
minimise camera shake, but use the smallest aperture possible to
give the greatest depth of field in focus.
·
Set the camera to manual exposure, and set the shutter speed and
aperture to these values.
·
Use this exposure setting for all your hemiphotos, as long as lighting
conditions remain the same.
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·
Set the camera to Programmed Auto mode, and set Auto Bracketing
to 3,
±
1.0 (p 116).
·
Take 3 consecutive pictures at each hemiphoto site. These will have
exposure value adjustments of 0, +1.0, -1.0 You will have to reset the
self-timer and focus for each shot.
·
Examine the images, and use the one that gives the least spreading
of sky areas, and enough detail of canopy areas, for analysis by
HemiView.
·
In relatively open canopies, you may need to further overexpose, by
setting the exposure compensation to +1.0 or +2.0 (p 109). In very
dense canopies, you may need to set exposure compensation to -1.0
or -2.0.
