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Film Plane Indicator
This mark is engraved on the top of the camera beside the film rewind knob, just to the left of the pentaprism, to indicate the exact position of the film plane. It is not used in general photography, but it is helpful in
Distance Scale
The distances on the scale are calibrated from the film plane to indicate the focused distance from film plane to subject.
The scale is not generally used except when confirming depth of field, performing guide number calculations in flash photography, or using infrared film.
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Infrared Index Mark
Since infrared light rays have longer wavelengths which focus on a plane slightly behind that of ordinary visible light rays, it is necessary to slightly adjust the focus of the lens when using black and white infrared film. The infrared index mark engraved on the lens barrel is used for this purpose. After focusing the same as usual, note the tiny red dot engraved on the lens barrel just to the right of the distance index, and turn the focusing ring to align the focused distance with this red dot. For instance, if the focus is at 5m on the distance scale, turn the focusing ring to align the 5m mark with the red dot. After that focusing correction, you can release the shutter.
When using infrared black and white film, visible light rays must be kept out by means of a deep red filter (Rl) over the lens. When using infrared color film, there is no need to