Monday, May 26, 2008

Practicus In The Studio: Studio Exposures.

            Correct exposure is one of the most important of the factors in making perfect photographs, yet the majority of portra approach it in the most casual manner, and apparently trust to a sort of sixth sense to tell them how long to keep the shutter open, or as one Mid to me, "It is like taking a dire into water; when I press the bulb, I do not know when I am coming to the surface.” With long practice it is possible to work successfully in this sab-conscious way, but most people will find it desirable to have some definite idea of the number of seconds necessary to give the exact quality of negative which is aimed at. I want you to take particular notice of those last words. There can be no fixed standard of exposure or density in portrait work, or we should all arrive at one monotonous style, without that touch of individualism which now distinguishes our best photographers from one another. Twenty or thirty years ago there was an established ideal of a clear, sparkling negative ranging from clear glass to opacity, and a high-class operator who did not conform to it had little chance of employment. Many negatives which would be appreciated to-day were then thrown aside as failures, because they were too soft or too hard to print in the limited range of media then available, but now we are more free to choose our methods, and can produce negatives to satisfy our own artistic instincts. Therefore it is necessary if we are to be consistently good in our work we must not trust to "flukes" for our successes, but to study the conditions under which our particular class of negatives can be obtained.
            On asking one of our best known outdoor photographers bow he secured such uniformly perfect negatives. I was told that they were obtained by "exposing to suit the developer.” This was in the pre-Watkins days, when no attempt had been made to systematize development and most people believed that the clever worker owed his success to modifying the developer according to the appearance of the imago, often beginning with plain pyro solution, and working up the negative by adding alkali and bromide drop by drop. This idea is now exploded, proving that my friend was a true prophet when he asserted that the prime factor in producing the negative was correct exposure Hurter and Driffield, to whom photographers owe so much, have taught us that the amount of silver affected by light when a plate is exposed is in definite proportion to the length of exposure given, bat this assumes that all the so affected is reduced by the developer, or, in other words, the plate is “developed right out," which is rarely the case in studio work, most portraitists finding that such a procedure produces too much contrast. This fact has been recognized by both plate and developer makers who prescribe different times of development for portrait, landscape and copy negative, the former always being much shorter than the latter two.
            To establish a correct method of exposure we must make a few experiments, working with a standard developer, and a fixed time of development, which may be obtained by the factorial system, the only variation being made in the exposure. It is convenient and economical, besides assuring uniformity of rapidity in the emulsion, to make several exposures on one plate, and this can easily be done in most studio cameras by fixing a small mask in the camera back and marking the slide so as to show when the plate is in position. The easiest size is to work three upon a half-plate, cutting a mask with opening two inches by four and fixing this in the existing carte or cabinet mask. If the slide has notches for single exposures, and also for repeating two C.D.V. on half-plate, the centre notch may be used, but new marks s little farther from the centre must be made for the two end exposures. For my own use I have made a repeating back which allows of four exposures, each three inches by two clear, from the rebate upon a half-plate, and this I find handy for many other purposes.
            The exposures, which must, of course, be upon the same subject, may be varied in any proportion which the operator desires. Usually double at each step will be found as good as any for portrait work, as our negative will then show us the effect of one, two, four, and eight seconds' exposure. The result will be rather surprising to those who try it for the first time; for, supposing that the one-second exposure gives a thin but printable negative, it will be found that the eight-seconds section, although thick and slow to print, will also yield a passable result. That, however, is not my point, which is that the operator should now select the exposure which gives him the quality of imam- he wants, or if none quite pleases him should give an exposure between the two which he judges to be nearest correct. So far so good. Now all depends upon correctly estimating the value of the light, and this can better be done with an exposure meter than by the exercise of personal judgment. If we use an ordinary Bee meter and note the time taken to match the tint at the. time of making our exposures we shall be able to establish a ratio between meter time and exposure for any light or lighting. For example, if we find that our selected exposure is four seconds, and that it took right minutes to get the tint, we haw the proportion of half a second for each meter minute. Naturally I do not propose that anyone should make meter tests while sitter waits, but an occasional test between whiles can easily be managed. The plate speed and lens aperture must be unchanged, or due allowance must be made, or this system will be worse than useless.
            It is often found that when strong effects of light and shade are being tried for that the negatives turn out hard and chalky and do not at all represent the model as seen by the artist. There are two causes of this, both closely connected, under-exposure and over-development, the latter being due to an attempted to force out shadow detail. Now, if development had been done by time without regard to the appearance of the image, we should have retained the detail in the high lights, but the shadow detail would still have been wanting. Longer exposure would remedy this without giving flatness, unless-unite an unreasonable time were given. This class of subject affords an excellent field for the progressive series of exposures already recommended; or if it be thought that the effect cannot be judged from so small s plate two full-sized exposures may be made, one receiving three times as long as the other, both being developed for the same time in the same dish.
            It is important when making experiments in exposure to keep not only to one make of plate, but to the same grade. Emulsions vary in character, and two grades which are, perhaps, marked 200 and 240 H and D, cannot be relied upon to give the same quality of image, even if the difference in speed be accurately allowed for; much more is this the case if two makes of plate he mixed up. For the same reason one developer should be adhered to, and for printing quality land adaptability to various subjects and lightings there is nothing to beat the old-established pyro-sods. Remember that a negative is only a means to an end, and that "pretty" negatives do not always give the best of prints. Although not strictly within my subject, I feed that at the present time of year it is not amiss to on that pyro is less affected in its action by variations of temperature than most other developing agents. I have only recently found the slow action of another developer mistaken for under exposure, with the result that the exposures were increased and flatness resulted.
            A point which mart not be missed is the effect of the distance between lens and sitter upon exposure. This is always allowed for in copying, but is often overlooked in portraiture. Most operators know that a large head requires more exposure than a full length, other things being equal, but perhaps could not toll you why. There are two reasons, one being the increase in the focal length of the lens a the sitter approaches the camera, and the other the flattening of the lighting by the greater amount of atmosphere which intervenes as the sitter is placed further from the camera. Let us consider the former case, assuming that a head measures 9 inches in height and we are making a 3 in. image of it: this adds one-third to the camera extension, supposing we are using an 18 in. lens working at F/6 for infinity; one-third added to the focal length gives us 24 ins - in other words, we are working at F/8, which requires practically double the exposure. When taking a full-length cabinet the reduction would be l/12th, which would only add an inch and a half to the original focal length, and this we could safely ignore so far as exposure is concerned. In the second case the increase in exposure is only apparent, not real. If there is a certain amount of fog over the shadows it covers the bare glass, but there is no more detail in the shadows than there would be if the atmosphere were perfectly clear. In London, where the atmosphere is as thick in winter as it is in most places, many photographers use a lens of shorter focal length than they would otherwise, in order to avoid this flattening.
            In conclusion, let me impress upon the notice that correct exposure is the key to satisfactory results. Leaving colour effects out of the question, any arrangement of light and shade can be correctly reproduced if the proper exposure be given. We can flatten the scale by over-exposure, we can sharpen it by under-exposure, so that if we hit the happy mean we shall get upon our negative what we saw when looking at the sitter. Surely such a consummation is worth taking pains to attain, instead of following the usual "hit or miss" way.



           In the early days of orthochromatic photography the dye generally used for the preparation of light filters was picric acid, this having the advantage of simplicity and cheapness and of great efficiency, picric acid absorbing the ultra-violet almost completely, and having a wry sharp cut in the spectrum. The disadvantage of picric acid, however, is that it is unstable to light, filters made with it soon turning brown. For this reason the early gelatine filters were made chiefly with tartrazine, which is very stable and gives permanent niters. Tartrazine, however, has the disadvantage that its absorption in the ultra-violet is unsatisfactory, and even moderately deep tertrazine filters transmit appreciable amounts of ultra-violet, this detracting very much from their efficiency. For this reason filter yellow, introduced by Hoechst in 1907, rapidly displaced tartrazine as the best dye for filter-making, and has held that position ever since.
           Filter yellow is extremely stable, absorbs the ultra-violet strongly with the exception of a transmission band at 300μμ, which, since it is absorbed by glass, is of little importance, and hat a satisfactorily sharp out for the preparation of orthochromatic filters. A disadvantage of filter yellow which has always been recognized, however, is the fact that its absorption curve was less sharp than that of picric acid, and for many purposes, especially the preparation of very light filters, a dye possessing the stability and ultra-violet absorption of filter K, but of greater sharpness of cut, would be desirable.
           When the need for light-filters of high efficiency for aerial photography arose the need for such a dye became pressing, and we undertook a search for such a material. After a great number of trials it was found that suitable absorption and stability were possessed by the phenyl-glucosazones.
           When certain sugars, such as glucose, are warmed with a solution of phenyl-hydrazine in dilute acetic acid, yellow precipitates are produced possessing definite crystalline structures, by which the sugars may be characterized. These yellow substances are known as osazones, those formed with phenyl-hydrazine being termed phonylosazones. On measurement of the absorption spectrum of glucose-phenylosazone it was found that the absorption curve was very sharp and extended far into the ultra-violet, and since the material is well known to be stable, it appeared that a dye prepared from it would possess the properties required for the preparation of light yellow filters.
           Glucose-phenylosazone is insoluble in water, so that to obtain a dye it is necessary to have a salt-forming group present in the molecule, and to produce a dye suitable for use with gelatine it was desirable that this group should be an acid one. To obtain such a derivative of glucose-phenylosazone which will form salts with metals it is merely necessary to substitute for phenyl-hydrazine a derivative containing an acid group and condense glucose with it in the same manner. Several such derivatives were tried, and the most satisfactory result was obtained with glucose-phenyl-osazone-para-carboxylic acid. This was prepared in the following way:-
           Para-nitrotoluene was oxidized to give para-nitrobenzoic acid. This was then reduced to para-aminobenzoic acid, which was diazotized, and gave para-hydrazino-benzoic acid or phenyl-hydrazrne-para-carboxylie acid. The glucosazone of this acid is a yellow crystalline compound insoluble in water and almost insoluble in alcohol. It forms a sodium salt which is extremely soluble in water, but which can be precipitated from concentrated solutions by the addition of alcohol, and this sodium salt of glucose-phenyl-osazone-para-carboxylic acid has been adopted by us for the preparation of light-filters under the name of "Eastman Yellow."


In Fig. 1 are shown the absorption spectra of tartrazine, filter yellow, and Eastman yellow, from which it will be seen that the Eastman yellow has a sharper cut than filter yellow, and almost as strong an absorption in the ultra-violet.
           Light-filters prepared from it retain these characteristics, and these light-filters have been prepared and specified under the names of EK 1 and EK 2 light-filters. A special filter for aerial photography has been adopted by the American forces under the name of Aero No. 1.


Fig. 2 shows the absorption curves of these light-filters.
           As regards stability, it was found on test that the new dye was not quite so stable to light as filter yellow, but was superior to all other yellow dyes tried, and its stability is amply sufficient for the preparation of light-filters, since it requires weeks of exposure to direct sunlight to produce a change even in the lighter-coloured filters.

C. E. K. MEES.

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.