the Royal Photographic Society in 1881 drew up some rules, which were revised in 1891 and again in 1901. The former standard unit f /4, and the numerical notation used with it, have been abolished in favour of the unit f /1 established at the International Congress in Paris 1900. Intensity ratio is defined as dependent upon the effective aperture of a lens, and not upon the diameter of the diaphragm in relation to the focal length of a lens. The effective aperture of the lens is determined as follows. The lens must be focused for parallel rays. An opaque screen is then placed in the principal focal plane, and a pinhole is made in the centre of the plate (in the axis of the lens); an illuminant is placed immediately behind the pinhole itself, when the diameter of the beam emerging from the front surface of the lens may be measured. (It will be found that except in the case of the diaphragm being placed in front of the lens, the diameter of the diaphragm itself is seldom that of the effective aperture.) Every diaphragm is to be marked with its true intensity ratio as above defined, but the present intensity ratios are retained in their order of sequence: f /1, f /1·4, f /2, f /2·8, f /4, f /5·6, f /8, f /11·3, f /16, f /22·6, f /32, f /45·2, f /64, &c., each diaphragm requiring double the exposure require by the preceding one. In other cases apertures are to be made in uniformity with the scale, with the exception of the highest intensity, e.g. a lens of f /6·3 would be marked for f /6·3, f /8, &c. The corresponding numbers are known as f numbers, but are only applicable or a lens focused for distance. Other systems of notation are in use, but the above is generally adopted (see Fabre, T.E.P. Suppl. C. 38). Special diaphragms are in use for process work with ruled screens (see N.S. Amstutz, Handbook of Photoengraving, 1907). Standards for the screws of photographic lens-flange fittings, and for the screws fitted to cameras for attachment to time stand or for fixing movable parts, have also been laid down (Ph. Journ. 1901, 25, p. 322).
Instantaneous Shutters.
The general use of rapid dry plates and hand cameras has rendered it necessary to have some mechanical means of regulating exposures in small fractions of a second, especially for objects in rapid motion, and this instantaneous shutter has become an essential part of modern photographic equipment in many forms and patterns, but practically three types are preferred—the between-lens shutter, the roller-blind shutters, used before or behind the lens, and the focal plane shutter, in front of and close to the plate and forming part of the back of the camera. The usual limit of rapidity of the two former is nominally about 1100 of a second, and for ordinary purposes higher speeds are seldom required, while with the latter speeds of 11000 to 12000, of a second may be attained.
Two important factors in the use of lens shutters are the rapidity or speed, measured by the total duration of exposure from opening to closing, and the efficiency, measured by the ratio of the time during winch the shutter is fully open and the time occupied in opening and closing. Both factors are more or less variable, either with differences of construction, of diaphragm opening or of position of the shutter with regard to the plate and lens. In any case the efficient exposure is always less than the actual, and may be considerably so.
The rapidity required of a shutter in photographing moving objects is regulated by the minimum time necessary to produce a well-exposed image upon the plate, with a loss of definition, or blurring, by displacement not exceeding 1100, or preferably 1200 to 1250 of an inch, if enlargement is extended. This will depend on the state of the light and the illumination of the object, the relative intensity of the lens as measured by its effective aperture and focal length, the sensitiveness of the plate, and the amount of effective light passing through the shutter during the exposure. The amount of displacement to be guarded against depends upon the rate of movement of the object, the direction in which it is moving with reference to the axis of the lens, its distance from the camera, and the focal length of the lens. It will be proportionately less as the distance of the object increases, and as the rate of its motion and the focal length of the lens for a given distance decrease, and vice versa It will be greatest when the object is moving at right angles to the axis of the lens, and least when the motion is directly towards it; but in that case there will be some increase in the apparent size of the object as it approaches the camera. For example: An object moving 1 m. an hour advances 17·6 in per second. With a lens of 5-in. focus this would represent a displacement on the ground glass, for an object 50 ft. away, amounting to ·146 in. per second, and it would require exposures between 115 and, 137 of a second to give maximum or minimum displacements of the image between 1100 and 1250 of an inch. An object at the same distance moving ten times as fast would require 1-10 of the above exposures. If, however, the distance be increased, the possible exposure may also be increased in the same proportion, so that the object moving 10 m. an hour at 500 ft. distance would only require the original exposures of 115 to, 127 of a second. On the other hand, the limits of exposure for an object moving 1 m. an hour within 10 ft of the lens would be between 175 and 1185 of a second. This is entirely independent of the sensitiveness of the plate, and only represents the maximum duration of exposure permissible in order to reduce the blurring of the image between certain limits. The sensitiveness of the plate, and the intensity and amount of light acting upon it through the lens and shutter, must be adjusted so as to produce the desired photographic effect within that time. With a lens of 8 in. focal length the displacement would have increased in the first instance to ·23 in. per second, and the maximum exposure permissible would be from 123 to 157 of a second. This shows that there is an advantage in using short-focus lenses for very rapid exposures. In practice, most work of this kind is done upon quarter-plates (414×314 in.) with lenses of 412 to 5 in. focus. As the displacement will be greatest for an object moving at a right angle across the axis of the lens, an exposure sufficient for this case will be sufficient for any other. Sir William Abney has discussed this question practically in his Instantaneous Photography, and it is treated mathematically by W. B. Coventry in his Technics of the Hand Camera, in which will be found formulae and tables for ascertaining the distances and limiting exposures for moving objects, allowing for a blur of 1100 of an inch. In foreign treatises the limit is usually calculated for a displacement of 110 of a millimetre, or about 1250 of an inch.
An efficient shutter should fulfil the following conditions: It should be light and compact, simple in construction and action, strongly made, and not liable to get out of order; capable of being set without admitting light into the camera; easily released with a slight pressure of the finger, if a pneumatic release is not fitted, and free from any tendency to shake the camera on release. It should open and close quickly, allowing the largest possible proportion of the exposure to be made with the full aperture, and it must not cut off any of the effective light passing through the lens, but should distribute it evenly all over the plate: though in landscape work it is an advantage to give the foreground more exposure than the sky. It should be adjustable for variable instantaneous and for prolonged or “time” exposures. With a good shutter there is less risk of shaking the camera in short “time” exposures, from 14 to 1 second, than there is in taking off a cap. Shutters working between the lenses must permit of the use of diaphragms in the lenses, and of alterations of speed while set. Above all, a shutter must be constant in its action, giving short and variable exposures always correctly or relatively so, an important condition which cannot always be fulfilled, and the exposures marked on the indicator should lie capable of being repeated with tolerable certainty. Shutters should also be adaptable gr use with different lenses. Three methods of varying the speed of a shutter are in use: (1) by altering the length of the slot; (2) by the retarding action of a pneumatic brake; (3) by varying the tension of a spring. The latter is considered by W. B. Coventry as far the best. They are usually released by the pressure of the finger on the end of a lever holding the moving part in a state of tension; or better, by J. Cadett’s system of pneumatic pressure, applied by means of a compressible rubber bulb and tube, which may drive a piston acting on the lever holding the shutter, or inflate a collapsible bulb at the other end of the tube and thus exert the necessary pressure on the lever. With W. Watson’s “Antinous” release a flexible wire acts directly on the piston or trigger release of a cylinder shutter. It is also adapted for roller-blind, focal plane, flap, and various forms of between-lens shutters. It is durable, effective and convenient (see fig. 3). In many cases both methods can be used as desired, the mechanical release being preferable on account of its convenience and freedom from liability to shake the camera.
The following are the principal types of instantaneous shutter: (1) Flap, (2) drop, (3) combined drop and flap, (4) rotary, (5) roller blind, (6) focal plane, (7) moving blade central, (8) iris. They can be applied in four different positions: (a) in front of the lens; (b) centrally, near the diaphragm; (c) behind the lens (d) immediately in front of the sensitive plate. They all, however, come under two main classes: Lateral, including those in which the exposure commences and ends at the circumference of the lens aperture; and Central, those in which the exposure begins and ends at the centre of the aperture. Some of them are “lateral” in their single form and “central” when double. The form and position of the effective aperture of a shutter, relatively to the lens and plate, have a strong influence, either favourable or unfavourable, on the amount of effective light passing through the lens, and its even distribution over the plate. This is especially the case during the incomplete phases of opening and closing the aperture. It seems to be agreed that the best position for lens shutters of the lateral type is behind the objective, and for those of the central type, between the component lenses. In this latter position the whole of the plate is illuminated during the full period of exposure, with a gradually increasing intensity, until the full opening is reached, and then the illumination gradually falls off until the shutter is closed. The most effective shutter is one in which the first and third phases of incomplete illumination, during the opening and closing, are the shortest compared with the second phase of full opening.
With the focal plane shutters, however, different portions of the plate are exposed in succession, the lens working at its full aperture and efficiency throughout the exposure.
To secure successful results in using instantaneous shutters, the operator should make himself acquainted with the working of his shutter and its efficiency in various circumstances of exposure with the lenses, plates and developer he proposes to use; ascertaining the actual value of the various exposures marked on the indicator, and,
