Vivitar 800mm Mirror, Инструкция по эксплуатации

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NOTE: In order to compensate focusing when normal positioning may be altered due to extreme
temperature change, the focusing ring has been designed to allow rotation beyond the fixed
marks of the distance scale. To avoid mistakes, always focus while looking through the viewfinder.
NOTE:
No adjustments in focusing are necessary when taking infrared photography as required
with normal lenses.
4. Light and Contrast Control
Since the reflex lens is not equipped with a diaphragm system, the use of filters plays an important factor in
the control of light and contrast.
5. Filters
Normal (IA Skylight): This filter should be used when no other filters are necessary.
ND 2X & ND 4X (Neutral Density): These filters control the passage of light since no diaphragm
is provided in the lens. Exposure factor is 4X with a ND4X, so that the light intake through the
lens is decreased as if a similar lens were stopped-down by 2 stops. Note that ND filters reduce
light passage only, while the depth of field is not changed.