Tuesday, June 3, 2008

Some Notes On Print-Meters

           Of the many forms of print-meters, or actinometers, the type dependent upon miniature negatives of graduated density is preferred by some, as no matching of tint is required, and for occasional work with only a few frames exposed it certainly is convenient. On the other hand, the most accurate of all probably is the "single-tint" tint-matching type, almost essential when many frames have to be kept going and taken in and out during the day's work, but it requires constant inspection, and if one tint is over-done accidentally only a rough estimate is possible to compensate for the over-printing. Single-tint meters, such as Johnson's, supplied by the Autotype Company, are provided with a roll of P.O.P., which under varying atmospheric conditions does not always make a good match: with the surrounding tint, the yellow glass above necessarily being of insufficient depth to remove colour contrast. Greater accuracy in reading is secured by not attempting to effect a colour match, but to work in the following way: If the nearest edge of the rectangular aperture is viewed obliquely from a fair distance with the eyes partially closed, it will initially stand out lighter than the darkening silver paper beyond it, and at a certain stage will merge into the tint and be lost, which point is taken as "one tint." Tests have shown that this method largely eliminates the personal equation, one printer, practically speaking, registering the same number of tints as another, whereas in the case of colour matching by gazing directly downwards on the meter wide differences in the estimation of what constitutes a tint have been found. A variant of "Johnson's" is the circular meter with disc refills: it is cheaper, but for professional use the former is the better. In some cases celluloid is used to protect the tint, but it is an indifferent substitute.
           To insure accuracy with thin or medium negatives, the meter should register three or four tints during the printing, accuracy being of more importance when printing platinotype or palladiotype by meter than with the carbon process, which has greater latitude in exposure, or, rather, errors in exposure are more readily corrected in development. But a quick-printing single-tint meter is a decided nuisance when dense negatives are being dealt with, and in such a case a fixed-out lantern plate dyed yellow with bound-on cover-glass can be placed over the meter and will be found very useful to slow down its indications. As a matter of curiosity, it may be mentioned that the extreme variation in the rate of contact printing met with in one trade printing concern ranged from three minutes to twenty hours, the negatives - being exposed to the same
Some Notes On Print-Meters Fig.1

mercury light, and each negative affording good prints. The perpetrator of the twenty-hour gem had "faithfully promised his customer a dozen prints at the end of the week," as is usual in such cases.
           Other forms of print-meters based on tint-matching are illustrated by "Sawyer's" and the "Akuret," the sensitive paper being exposed under translucent tints of different densities. They possess an advantage over the single-tint type, as no movement of the paper is required whilst the negatives are being printed, but their scope is not so wide. The familiar Wynne's meter, dependent upon numbers successively printing out, is a favourite with many, though others experience a difficulty in deciding whether any particular number hat, or has not, appeared Finally, the type first alluded to is the device of Mr. H .J Burton, on the lines of which the making of an efficient home-made article is to be described.
           Not a few have attempted to make print-meters of the graduated miniature negative order, by copying a photograph in the camera, or by exposing by contact a dry-plate behind a positive, in either case successively on different parts of the plate, with an increased time of exposure for each small negative, but this method is very uncertain, and the results are usually far from satisfactory for fairly obvious reasons. It is also obvious, when mentioned, that to attempt to design a meter capable of indicating exposures from thin negatives up to those of extreme density, either means an undesirable multiplication of the tin; guide negatives, or an equally undesirable abruptness in the translation from one to another. For unnaturally dense negatives, one or more pieces of ground-glass placed over the meter will meet the case.
           Fig. 1 represents a quarter-plate negative of nine prints stuck on white cardboard, which are reduced by copying in the camera to a little less than 2*(1/2) inches high over all, in position shown on the plate. The prints should be of the same subject, preferably a portrait rather large on the plate, and whilst it is an advantage that the set be uniformly lit, even illumination is not essential. Over-exposure is to be avoided as bright, but fast printing negatives with almost clear glass shadows are the moat suitable. If the ground prints through it is blocked out.
           Assuming the figure to represent the glass side of the plate, and the negatives to read from left to right in the usual way, the graduation in printing rate is effected by covering No. 2 with one layer of celluloid, about the thickness of that used for cut films, No. 3 with two layers, and so on. Fig. 2 shows the first sheet of celluloid applied with extremities extending beyond the miniature negatives,

Some Notes On Print-Meters Fig.2

but clear of the edges of the glass; on the right about 1*(1/3)th inch, on the other three sides more space is available. The next sheet has space 2 cut away, and is stuck down at each end with a touch of celluloid varnish, the procedure being continued mutatis mutandis until No. 9 is covered with eight thicknesses.
           When the first test of the meter is made everything may appear right, the lower numbers being nicely graded, and on printing further the faint images of the highest numbers apparently the same. But here exists an unsuspected trap, for on printing, say, No. 5, to the "pretty" stage, No. 6 may now be found to be almost indistinguishable in depth, and therefore require further ho. ding back, together with the numbers following it.
           Accordingly, the only safe plan is to test each number right through the series at its "pretty" stage against the next less printed one. Some pieces of stripped thin roll- film may be found of service when only a very slight holding back is demanded. The celluloid covering is then edged all round with cardboard slips stuck down with seccotine, and a thin cover-glass bound on. If the lower numbers print too quickly for the negatives in use, the cover-glass may be of ground-glass. A three-quarter-view quarter-plate printing-frame holds the finished article; white wood frames are sold sufficiently deep to take it. A packet of 3*(1/2) x 2*(1/2) ordinary gelatine P.O. P. provides the meter paper, and will keep for years if stored in an airtight tin with some dry calcium chloride. Self-toning papers are not so good for the purpose. When the number indicating the correct exposure is found by trial, naturally only a narrow strip of paper need be utilized afterwards.
           In the model constructed, which has worked well, for identical depths of printing the last number requires about six times the exposures of the first, a range sufficient to satisfy most requirements. This ratio, of course, only holds good for the particular thickness of celluloid employed. Comparative values might be given each negative if a single-tint meter is at hand, or improvised, by registering the number of tints necessary to bring each negative in turn to the pretty stage.

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