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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 (in degrees)
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 52°. Divide the 52° by
the magnification, which is 24 power. This yields an actual field of 2.2°. To convert this to feet at one
thousand yards, multiply the actual field of 2.2° by 52.5. This gives us a field width of 116 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.