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10
Field of View
Determining the field of view is important if you want to get an idea of the size of the object you
are observing. To calculate the actual field of view, divide the apparent field of the eyepiece
(supplied by the eyepiece manufacturer) by the magnification. In equation format, the formula
looks like this:
Apparent Field of Eyepiece
True Field (in degrees) =
Magnification
Using the example we started with in the previous section, we can determine the field of view
using the same 25mm eyepiece. The 25mm eyepiece has an apparent field of view of 50°. Divide
the 50° by the magnification, which is 20 power. This yields an actual field of 2.5°, or two and a
half degrees. To convert this to feet at one thousand yards, multiply the actual field of 2.5° by
52.5. This gives us a field width of 131 feet at one thousand yards.
PHOTOGRAPHY HINTS
Metering
Celestron spotting scopes have fixed apertures and, as a result, fixed f/ratios. To properly expose
your subjects photographically, you need to set your shutter speed accordingly. Most 35mm SLR
cameras offer through-the-lens metering which lets you know if your picture is under or
overexposed. Adjustments for proper exposures are made by changing the shutter speed. Consult
your camera manual for specific information on metering and changing shutter speeds.
Reducing Vibration
Releasing the shutter manually can cause
vibrations, producing blurred photos. To reduce
vibration when tripping the shutter, use a cable
release. A cable release keeps your hands clear of
the camera and lens, thus eliminating the possibility
of introducing vibration. Mechanical shutter
releases can be used, though air-type releases are
best.
Blurry pictures can also result from shutter speeds
that are too slow. To prevent this, use films that produce shutter speeds greater than 1/250 of a
second when hand-holding the lens. If the lens is mounted on a tripod, the exposure length is
virtually unlimited.
Another way to reduce vibration is with the Vibration Suppression Pads (#93503). These pads
rest between the ground and tripod feet. They reduce the vibration amplitude and vibration time.