A negative of which the exposure has been " correct " should show, even in the image of the deepest shadows, a silver deposit greater (however slightly) than the chemical fog ap pearing in the parts of the plate or film which have been covered by the rebates or turn buttons of the dark-slide. 2 This comparison is best made by laying the dry negative, emulsion side down, on a sheet of white paper in a well lighted place. If the negative is very dense the examination is best made by transmitted light in front of a sheet of ground glass. Care should be taken not to choose for the comparison a protected portion adjoining a region of very great density, since the chemical fog may be accompanied by irradiation or halation.
Any great difference between the density of the image of the shadows and that of parts covered by the rebates indicates that exposure has been longer than necessary. Unless the possible maximum of correct exposures has been exceeded, no harm will have been done other than an increase of the time needed for printing.
In an over-exposed negative, the contrasts in the high-lights are distinctly reduced, but owing to the considerable density of these parts of the image, it is not possible to make sure of this except by inspection of prints correctly printed on the paper best suited to the negative in question.
377. Beginners, and also some experienced workers, somewhat readily confuse the notions of density and of contrast. Just as it is possible at the sea-shore, i.e. at a very low altitude, to find a very steep path, and a fiat or only very gently rising road in a high mountain region, so too it is possible to have a very thin negative (density almost nil in the shadows) which is nevertheless very contrasty (very great differ ence between the extreme densities). On the other hand, a very dense negative (very great density in the image of the deepest shadows) may be very weak (small difference between the density of the high-lights and that of the shadows).
The usual photographic terminology is rather confusing as regards the designation of the characters of the negative, one and the same term being sometimes applied to very different characters. The following definitions will be adhered to in this work, though their arbitrary character is fully realized.
It is essential to realize that the gradation of the image is determined solely by the difference between the extreme densities. The density of the shadows determines only the time of the exposure necessary to obtain a print. For
instance, it is possible to superpose on a weak negative a piece of neutral grey smoked glass or another negative which has been uniformly fogged. The time necessary to get a print may be ten times as long, but once the equivalent printing times have been ascertained, the results will be identical with or without this supple mentary uniform density. Indeed, two identical negatives, one left uncovered and the other covered with a neutral grey coating allowing one-tenth of the light to pass, will appear identical if the first is illuminated by a lamp of Io candle-power and the second by one of loo candle-power at the same distance.
378. A negative is not an end in but only a means to an end, viz, the photographic print. A negative must therefore not be judged by itself, but only according to the prints which it can give by proper printing on a sensitive paper of suitable character.
To say of a negative that it has been correctly developed relatively to a given method of posi tive printing is the same as saying that its development was stopped at the moment when the difference between its extreme densities was that suited to the selected method of printing.
A print made from a negative which has been insufficiently developed cannot have both whites and blacks at the same time. The general appearance of the print is grey; its contrasts are insufficient.
On the other hand, a print from an over developed negative easily gives whites and blacks, but some of the details visible in the negative are merged in the whites or in the blacks. The image shows a complete scale of greys, but with exaggerated contrasts.
The rock on which the beginner usually comes to grief is his ignorance of the characters that a negative should possess in order to yield good prints by the usual printing processes. The rational apprenticeship of the photographer should, in our opinion, begin by printing from negatives of suitable quality. For instance, two negatives may be used, one suitable for print-out paper or rapid bromide paper of weak contrast, and the other for a paper of very great contrasts, which negatives must, of course, be supplied with advice as to their correct use. Having acquired experience in printing, the beginner will be in a position to judge what results to expect from his negatives, whereas for lack of a guide, it is usually impossible for him to distinguish between faults in the negative and errors in printing.