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 Shutters. The "focal plane" means the position that the plate should occupy for the picture upon it to be sharp; hence a focal plane shutter does not actually work in the focal plane, or it would be in contact with the plate it should expose - but it ought to be very near it. Illustration No. 51b shows the Graflex Focal Plane Shutter, while in Illustration No. 51c its relative position when fitted to ordinary hand or view cameras is clearly demonstrated. As explained in paragraph 695, there are two general types of curtains employed in focal plane shutters. One of these types is shown in Illustration No. 51d; the other is only one-fifth as long, the slot or opening being adjustable, instead of a definite width as is the case in the Graflex shutter illustrated. The focal plane shutter is a very important part of the reflecting type of cameras, the principle of which instrument is shown in Illustration No. 51a. The lens always remains open, the image being cast on a mirror, which, in turn, reflects it onto a horizontally placed ground-glass in the top of the camera. When the exposure is to be made the mirror is thrown upward, by pressure on a lever, and at the instant the mirror is out of range of the lens the focal plane shutter is released, making the exposure on the sensitive plate.
Wollensak Shutters. The shutters manufactured by the Wollensak Optical company, illustrated on page 316b, are of a most practical and durable construction. The Autex Shutter, shown in Illustration No. 51e, is a type of the later models which are now superseding the automatic and regular shutters previously on the market. The Optimo Shutter (see Illustration No. 51 f) has a range of automatically con-troled exposures, ranging from one second to 1/300 of a second, the latter being sufficiently fast to cover any emergency. The high speed efficiency is made possible by the star-shaped aperture formed by the points of five leaves which. in turn, revolve in making the exposure. The Studio Shutter (see Illustration No. 51g) is made especially for the professional photographer, and its Iris diaphragm principle of construction is also one of special merit, as the shutter thus acts as a diaphragm.
Illustration No. 51a. Sectional View Showing Graflex Principle. See Paragraph No. 718..
Ill. No. 51b. Graflex Focal. Plane Shutter. See Par. No. 718..
Illustration No. 51c. Showing Relative Position of Focal Plane Shutter. See Paragraph 718..
Ill. No. 51d. Curtain of Graflex Focal Plane Shutter. See Par. No. 718..
Illustration No. 51e. Wollensak Autex Shutter. See Paragraph No. 719..
Illustration No. 51f. Wollensak Optimo Shutter. See Paragraph No. 719..
Illustration No. 51g. The Studio Shutter. See Paragraph No. 719..