This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Focal Plane Shutter. The focal plane shutter is one which is placed directly in front of the sensitive plate, and is composed of an opaque cloth curtain or blind, each end of which is mounted on a spring-actuated spool, or roller. The simplest form of the focal plane shutter has one slit cut in its center, which can be made narrow or wide, according to the desires of the operator, while another form has a series of fixed openings or slits of different sizes. The curtain is wound on one spool and when the spring is released it instantly winds on the other, by which operation the slit or opening is drawn across the surface of the sensitive plate, allowing the light passing through the lens to act upon the sensitive plate. (See Ills. 51a to 51d)
Illustration No. 50 "B. & L." Volute Shutter. See Paragraph No. 693.
Illustration No. 51 Photographs made with Multi-speed Shutter on a 3A Kodak See Paragraph No. 705.
696. The amount of speed, or amount of exposure given the sensitive plate depends entirely upon the tension or power of the spring, and upon the width of the opening or slit in the curtain. The narrower the slit and the greater the tension of the spring, the more rapid is the exposure. The focal plane shutter forms part of the camera body, and opens or uncovers the plate or film, having nothing whatever to do with the lens. Therefore, focal plane shutters may also be termed plane shutters.
697. All of the modern reflex cameras are fitted with focal plane shutters, these being built in the cameras. It is possible, however, to purchase focal plane shutters and fit them to the rear of most view or hand cameras. The shutter consists principally of the rectangular wooden frame, having the spring-actuated rollers at top and bottom.
698. The focal plane shutter, working as it does directly in front of the sensitive plate instead of between the lens (thus being considerable distance from the plate), admits fully two times as much light, speed for speed, as any other kind of shutter, with the exception of the Multi-speed Shutter (described later). Consequently, if with the fast between-the-lens shutter full exposure is obtained with say 1-50 of a second, then with the focal plane shutter, same opening and same lens, the plate will get a similar exposure at 1-100 of a second.
699. When employing a shutter between the lens the entire aperture must be open for some parts of the time of exposure, and the longer it is open, in relation to the time of opening and closing, the better. The focal plane shutter need not expose the whole area of the plate at the same instant, but the opening in the curtain of the shutter may be narrowed down to a mere slit, and if this slit is passed across the plate at a moderate speed the duration of the exposure will be very short.