13o. Desirable Characteristics of a Shutter. These are somewhat numerous, and are rarely found combined in a single instrument.
A shutter should act without appreciable lag the moment that it is released ; it should work without appreciable vibration so as not to impair the sharpness of the image,' and without rebounding at the moment of closing (which would give a parasite image of the brightest parts of the subject). In cases where the lens is used for portraiture it is very advantageous that the shutter should work silently ; it ought not to limit the useful field even when the lens is considerably out of centre, nor limit the effec tive aperture of the lens. It ought to work equally well when used in different positions ; it should be light, compact, and strong its efficiency should be as great as possible. When used on an instrument in which the sensitive surface remains otherwise uncovered (such as magazine cameras, or studio cameras with repeating backs) it should not open during setting. Lastly, it ought to give a reasonable range of exposures which are very nearly con stant and correctly stated.
This last condition is certainly one of those the non-fulfilment of which leads to most serious mistakes. No shutter gives constant exposures over a period of years, for even when not being used it is impossible to prevent the steel of the springs from undergoing changes, i but over a period of several days the exposure time should be the same at the same setting, and especially should this be the case when the shutter is used several times in succession, a characteristic, however, which does not always hold. Failure to fulfil this condition is particularly frequent in shutters in which the regulation of the exposure is done by a friction brake, like a shoe-brake of carriages, the effect of which is very variable with the temperature and the hygrometric state of the air. On the other hand, regulation by means of an air-brake gives sufficiently constant results, provided the com pression cylinder is not worn or clogged with dust.
The numerical indication of the times of exposure corresponding to different settings of the shutter is very often far from the truth. It may be generally said that for times less than 115 second the times indicated are appreciably less than the real times. This, however, may be arranged purposely to allow for the fact that amateurs, even with exposure times greater than they think they are using, still take a large proportion of under-exposed negatives. In other cases there is no simple relation between the times given and the actual ones. With certain cheap shutters, times indicated as Ii5oth and I/loath second both correspond to the same actual times of about 1/25th second, whilst sometimes on the same shutter the exposure marked as the shortest is longer than one of the others indicated as relatively long. In the case of certain rare types of shutter where the times of operation are fairly correctly indicated (some latitude is obviously necessary in the adjustment in construction) for the shortest and longest exposures, the indications are often inexact in the neighbourhood of 1/25th second, since the variations of exposure may be relatively large for a very small variation in the adjustment.
Several good makers prefer to replace the graduation in actual exposure times by a series of numbered marks, at the same time indicating approximately the exposure corresponding to each of these numbers.
To cover usefully a given range of times" with the smallest number of different exposures, it is best to have a series of exposures in geometrical progression (preferably each one double the preceding one).
Exposures greater than a second are not generally made automatically, largely because for focussing it is necessary to keep the shutter open for some time. By moving an index on the outside of the shutter the coupling of the mechanism is changed, and the continuous working of the shutter becomes discontinuous ; a first operation of the release opens the shutter, which remains open until the release is again pressed.