The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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lens out to its equivalent focus, and to replace the ground-glass or focussing-screen by a piece of cardboard or thin metal plate, in the centre of which, and central with the axis ol the lens, is punctured a minute pinhole. The camera should now be taken into a dark-room, and a lighted candle placed close to
Fig. 44.
the pinhole in the cardboard. On examining the front lens, a circle of light will be faintly visible, or may be rendered still plainer by breathing on the lens or dusting it with a little French chalk or plain powder. A pair of dividers may be used for taking the diameter, and this may be at once read off on applying the same to a foot rule, and the focal length divided by this will give at once the true aperture of the stop.