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.
Efficiency. When using this word in connection with an instantaneous shutter, it is understood to mean the relation between the light passed and the total time from the shutter beginning to open and ceasing to close. Every part of a lens acts as a lens, and as soon as a shutter on a lens has begun to open the uncovered part of the lens projects the image on the plate. The brightness increases until the lens is entirely open, and then decreases until the shutter on closing cuts off all light, but any movement in the object will be recorded whether the lens is partly or fully open, so that the time from the first opening to the final closing of the shutter is determined by the rate of movement of the object being photographed, if it is desired to secure a perfectly sharp image. Should the time of a certain exposure be 1-100 of a second, the loss of light the plate sustains on account of the opening and closing amounts to 50 per cent. Thus the efficiency of the average between-the-lens shutter is low.
701. The efficiency of the focal plane shutter is double that of a lens shutter, because the image is formed on the blind of the shutter, the lens being fully open, and the image is just waiting, as it were, to pass through the slit and impress itself on the plate. So far as light action on the plate goes, the focal plane shutter set at 1-100 of a second would give double the exposure of a lens shutter set at the same speed. This is of immense advantage when working at high speeds, or in a dull light on moving objects.
Accuracy. Many of the cheaper lens shutters are notoriously inaccurate; the speed marked 1-25 of a second may actually be 1-10 of a second, while that marked 1-50 of a second may be 1-60. With a focal plane shutter the spring may be left alone and the width of the slit changed to produce variations of exposure. Thus, if at a certain spring tension, with a slit 2 inches wide, you secure an exposure of 1-50 of a second, it is obvious that with the same tension and a one inch slit the length of exposure will be exactly half.
Safety. The focal plane shutter affords a protection to the plate, in that the blind is close to the sensitive plate and any trace of stray light entering the camera, no matter whether through the diaphragm or through pinholes in the bellows, or cracks in the camera box, is kept from producing fog. The blind or curtain should be frequently examined for pin-holes, and if a plate is to be kept in readiness for an exposure for any length of time, it is well to place a cap on the lens.
Distortion. This disadvantage theoretically exists with the focal plane shutter, any moving object being distorted to a certain extent. In the majority of cases, however, this distortion is not perceptible to the naked eye or to the casual observer. Distortion is greatest when using a narrow slit and slow movement of the blind; i, e., weak tension of the spring. In practice, therefore, the wiser course is to use a fairly strong tension and to keep the slit as wide as necessary to give the calculated exposure. From an examination of many focal plane exposures you will observe that the distortion is only apparent in cases where an object containing straight lines was moving rapidly. For instance, the funnel of an express locomotive.