Sunday, May 25, 2008



           In three out of four studios that I have visited in the course of the last year or so, I found the dark slides of the camera in general use out of register with the focusing screen. The present use of rapid plates enables the photographer to use diaphragms to an extent that disguises considerably the want of fine definition, resulting from the plate not being truly in focus. In the days of wet collodion, when, in order to secure a portrait free from the unsharpness, due to movement of the sitter, it was the custom to use the lens at full aperture or nearly so, photographers had to be, and were, careful to see that their dark-slides were truly in register with the focusing screen.
           Although the want of sharpness in the negative, due to the slides not being in true register is less conspicuous when using the smaller apertures to which the use of rapid plates has accustomed us, it is still very desirable that the coincidence of position of the plate and the focusing screen should be as complete as possible, if only for the power obtained to get properly exposed results with the shortest exposures, particularly in the case of portraits of children. A photographer, the proprietor of a high-class establishment, doing a large business, writes me that since the correction of his slides his operator generally takes portraits of children with the open lens with excellent results, and with fewer failures and continent economy of time and plates.
           The plan which I employ to ascertain the truth, or want of it, of the registry of the slides, is to take a strip of wood about 3/5 or ½ an inch in thickness, rather longer than the width of the slide, and to drive a screw through it. The strip is laid across the front of the frame of the focusing screen, and the screw advanced until the point just touches the screen itself. A plate is now put in the dark-slide, the back is buttoned down, and the shutter is drawn. The strip is then laid across the front of the slide, and if it is in true register, the screw point will just touch the plate. It is desirable to repeat this trial with the plate placed both vertically and horizontally, and with each carrier that may be in use with the slide. A waste negative ix better than a plain glass for the purpose, as a paint scratch on the will indicate very closely coincidence of the two surfaces.

           The reason for having the wood a little longer than the width of the slide is (as I have found in one or two cases) that the top or bottom or one of the sides may be higher or lower than the opposite and the extra length of the wood allows the screw to be tried on different paris of the plate. The (crew is inserted at the place where it will come over the middle of the screen when one of the ends of the wood is just at the outer edge of the frame. This arrangement allows full use to l made of the extra length of wood for the testing the truth of adjustment away from the centre of the screen. The extent of deviation from coincidence is ascertained by placing slips of card of different thicknesses between the point of the screw and the focusing screen, or the plate as the case may be, until piece is found of a thickness that just fills the space.
           If it is the focusing screen that is found to be nearer to the front than the plate, a cardboard matt is rut of the same size outside as the ground glass; about a quarter of an inch wide all round except at the corners, where it is rounded inside for strength. A photographer generally has a stock of old mounts of various thicknesses, but if there is not one of just the thickness required, one or more thicknesses of cartridge paper may be pasted on to a thinner card, and when dry used for the purpose. The card matt is dropped into the frame, and the glass replaced and the slip-beading pinned in gain.
           If it ia the plate that is too near the front, slips of card are glued on all round to the rabbet of the dark-slide. It may be that some of the carriers may require adjustment independent of what has been done to the dark-slides. In this case they may be trued by gluing slips of card along the edges or the front, or, if the error is in the other direction, by reducing the thickness of the wood in the same place.
Of course, a neater job may be made by sending the slide and frame of the focusing screen to a camera maker or to an intelligent cabinet maker, if (which is not often the case) the camera can be put out of use for the time, but the home cure method described has answered perfectly well.


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