nary unorthochromatic plate quite capable of giving good results, and for this work would recommend a fast plate which will enable us to photograph even non-actinic buildings instantaneously on crowded thoroughfares. Naturally the latitude in exposure is greatly reduced, but one soon accustoms himself to the speed of his plate, so that exposure soon becomes of secondary importance. Another advantage of the fast plate is that it permits of the use of a very small diaphragm—an item of considerable importance in architectural work, where a

great amount of detail and clearness is always required.

For a higher class of work where color and tone effects are desired, the orthochromatic plate is advised. The principal benefit derived from its use besides the true rendering of color luminosities, is a more brilliant negative with better detail and a clearer rendering of distance. For monuments and similar views, where the natural scenery or a sky silhouetted with clouds is intended to embellish the picture, an orthochromatic plate is indispensable.

F

OR all-round photographic work a complete color correction is generally regarded as impracticable. It necessitates the use of a deep color screen which prolongs the exposure considerably, and of red-sensitive plates which can only be handled in complete darkness or in a light that is very little removed therefrom. The consequence is that non-orthochromatic plates are still used for the many purposes for which they are quite suitable, though a great deal of amateur photography is done with orthochromatic emulsions. When we remember that practically all the roll film made is orthochromatic, and that orthochromatic plates of various brands enjoy a great popularity, there can be little doubt that, as far as the amateur is concerned, more exposures are made on color-sensitive plates and films than on those which do not possess that quality.

If such plates (and in what follows the word plates is used as including films) are exposed without a light filter of some kind, very little is gained. It would require an expert to pick out of a number of negatives those which had been taken. on ordinary and those on orthochromatic; and even the expert might be puzzled or wrong. To understand this, it is necessary to consider the fault for which orthochromatism is a remedy.

The light to which our eyes are most sensitive is yellow and yellowish green; they are less sensitive to bright red and to blue, and much less so to deep red and to violet. Therefore, a patch of the most intense violet-blue that could be got, seen on a background of equally intense yellow, would form a contrast of a powerful character, the violet appearing dark on a bright ground.

As every photographer knows, this is not at all parallel with the result on an ordinary plate. To the yellow and yellowish green it is almost blind, to blue it is very sensitive, to violet also it is sensitive, and there are other rays, which the human eye cannot perceive at all, which affect the plate powerfully. These rays are called the "ultra-violet." To red light, also, the plate is insensitive, so that light of this color can be used to illuminate a dark-room. The expression "illuminate a dark-room" is not quite the bull it seems. A "dark-room" must be dark to the sensitive materials that are to be manipulated in it; provided this is so, it cannot be too light to the eyes.

Orthochromatic plates are coated with an emulsion which, by means of certain dyes, has been made more sensitive to green, yellow, and sometimes to red, than before. But although this is the case, these dyes are not able to give to the

emulsion the same sensitiveness as the eyes possess. It is still very much more sensitive to blue as compared with yellow; in fact, the difference is still so great that the advantage of the dyes is almost entirely hidden, as has been already mentioned. It is to remedy this state of things that a light filter or color screen is used.

The fact is sometimes overlooked that the photographer has a very useful color screen in the lens itself. Glass is comparatively opaque to the ultra-violet rays just referred to, so that they do not play so large a part in photography as they would do, for example, if our lenses were made of quartz, which is much more transparent to them. The atmosphere also absorbs a great deal of the ultraviolet light which reaches it from the sun. In order to make the photograph show colored objects in the same strength of light and shade as the eye sees them, a plate must be used which is sensitive to light of all the colors to which the eye is sensitive; and we must also interpose in the path of the light a light filter which will absorb ultra-violet, violet, and blue rays to such an extent that the excess of sensitiveness to such light which is possessed by the orthochromatic plate is counterbalanced.

It might be supposed at first that all that need be done would be to adjust the light filter to the plate so that their combined action was to get exactly and always the same effect as is seen by the

But while this is perfectly poseyes. sible, and indeed work with such a combination is done daily, it is open to the objection that the plates can only be used in darkness, or almost darkness; while the light filter prolongs the exposure necessary to such an extent as to make hand camera work, except under the most favorable conditions, very difficult. So that, in ordinary everyday photography, it is usual to resort to a compromise.

Orthochromatic, or isochromatic, plates have their sensitiveness to green and yellow increased very much, but are still only slightly sensitive to red. With such materials, therefore, we can use a fairly bright red light in the dark-room, provided it is the right sort of red-that

is to say, a deep ruby and not an orange. Plates that are sensitive to red as well are called "panchromatic." We see, then, that the light filter used with orthochromatic plates is not one which makes the rendering absolutely accurate, but one which gives a perceptible improvement without causing too great an increase in the exposure.

The question we have to determine, therefore, is how far to carry the compromise, and it is evident that this will depend on the nature of the work to be done. The writer cannot refrain from putting on record his opinion that, for every photographer (except the mere beginner) where the prolongation of the exposure is not a serious matter, and other circumstances do not offer difficulties, the best result is to be obtained with a panchromatic plate and a color screen deep enough and of the right tint to make the correction complete: the compromise of the orthochromatic plate and not very deep light filter is only fully justifiable when there is a necessity for it. No one who has learned to use panchromatics for landscape work, for example, is ever likely to go back to other material.

Self-screen Plates

In the "self-screen" plate we have an orthochromatic emulsion which has been dyed so as to act to some extent as its own light filter. The consequence is, it is impossible to make such plates as rapid as the simpler orthochromatic plates, since any light filter to act at all must cut some of the light and, if it does that, must necessitate a longer exposure. If the light filter is in the emulsion, this is equivalent to making that emulsion slower. Self-screen plates are therefore not among the fastest, but the dyeing is not carried very far. Probably they are about half as fast as they would be did they not contain the screen dye, so that they are still quite fast enough for all ordinary purposes. They can be used in the hand camera, and generally for work for which the very utmost possible degree of sensitiveness is not an essential.

But a light filter which does not prolong the exposure more than twice, although better than nothing, is still very

far from giving as full a correction as we are often glad to have; and so it comes about that separate light filters, which can be put on the lens for use as required, are largely employed by photographers. They are not of any practical value with non-orthochromatic plates, as they increase the exposure to an inconvenient extent; but they can be used with advantage either with the ordinary kinds of orthochromatic plates or with self-screen plates.

It is customary to describe these light filters as "three times," "five times," and 'five times," and so on, according to the effect which they are supposed to have upon the exposure. We write "supposed to have" because the classification is only a very rough one; it may serve to indicate to which groups a filter belongs, but is not by any means a reliable guide for exposure.

When a light filter has been bought, one of the very first things which should be done is to find out what increased exposure it entails in ordinary circumstances; that is to say, with daylight during the greater part of the day. Toward evening when the light is more yellow, and by artificial light when it is very much more yellow, the exposure increase necessitated by the filter is very much less; so much so, as far as artificial light is concerned, that it is not usual to employ a filter with it at all; the light is yellow enough, that is to say, it is comparatively poor enough in blue or violet, not to require it.

Although the orthochromatic plates of different makers are not quite alike in their color sensitiveness, they resemble each other nearly enough for the same color screen to be used with them indiscriminately. If the photographer uses self-screen plates he will find that a color screen, which is, say, a five times screen with orthochromatic plates that do not embody any light filter, does not call for quite so great an increase as that. The difference, however, is a very slight one, and the actual increase should be found by trial.

It has been the writer's frequent experience that the increased exposure required is generally under-estimated by the maker or seller of the screen. Seven or eight times is the increase generally

entailed by a "five-times" screen, if full advantage is to be taken of it. It is a curious fact that if the negative is in the least under-exposed the screen does not seem to exercise its full effect.

The Most Useful Color Screens

For work with self-screen plates a five or even a ten times filter is the most useful, as giving the most complete correction. For general work, of course, the plates will be used without any other screen than that which has been incorporated with them, but when this is not sufficient, then, within reason, the deeper the screen that is used with them the better.

With those orthochromatic plates which are not provided with any screen in the emulsion itself, and with roll film, it will be found that a two- or three-times screen is often very helpful. It will enable the cloud forms to be got on a landscape negative with full printing value, and at the same time it does not make the exposure so long as to put hand camera work quite out of the question. It should be supplemented by one which calls for twice or three times as much increase, for work where correction is more important than keeping the exposure short.

Using Two Screens at Once

It has been suggested that when a deep screen is needed, one weak one may be superposed on another, or a weak and a strong one used together. There is generally nothing to be gained by this, since the strong filter cuts out what the weak one cuts out and something besides. That this is so can be seen by holding one partly over the other: the two screens together, if their color is the same, and they only differ in depth, will not appear appreciably denser than the deeper screen by itself.

So far from a "six times" and a "three times" screen requiring nine times the exposure when the two are used together, the writer found by actual trial that the two screens combined did not call for an exposure appreciably longer than was needed by the "six times" screen used by

itself. And if the exposure necessary is not longer, it is evident that the screen cannot be serving any useful purpose. On the other hand, the use of two separate filters, with the thick glasses of which they are composed, is likely to affect the definition prejudicially.

When we come to the use of panchromatic plates, the screens used are almost sure to belong to the "K" series made by Eastman. These are made in three depths, known respectively as K1, K2, and K3, and, although they are primarily designed for use with the panchromatic plates, they are quite suitable for orthochromatic plates of any kind. The K1 screen, used with an orthochromatic plate, may be regarded as about a 'three times" screen, and the K2 a "ten times." With the panchromatic the K1 increases the exposure something less than twice, the K2 about four times, and the K3 about six times; but the actual figures of the increase required by each batch of emulsion are given in the box with the plates, so that accurate work can be done at once without any need for experimenting.

With these plates the K3 gives a rendering which is approximately correct, and therefore should be used whenever circumstances permit. The K2 is intermediate, and is suggested as being suitable for portraiture, where, to avoid the necessity for retouching, the color correction must be as complete as possible, though at the same time long exposures cannot usually be given. The booklet Real Orthochromatism, published by the Eastman Kodak Company, will be found to be a good supplement to this article, as it goes into the subject in a more ad

vanced manner than can be done here, and contains much that is of interest on orthochromatic work, whether the reader is a user of the Wratten productions or

not.

Over-correction Highly Improbable

It is sometimes said that the use of color screen is carried to the extent of over-correcting the rendering; but, except with experimental home-made screens, this is hardly likely. None of the commercial screens, unless deliberately made for that purpose, will over-correct; in fact, with the exception of the K3 and one or two special kinds, none of them correct fully. Photographers are so accustomed to an incorrect rendering of colors that they are apt to regard any approach to truth as an exaggeration, while the disappearance of color contrasts in a correct rendering may easily suggest that the orthochromatism has been overdone. It may be overdone from the point of view of the impression required, but it may none the less be a perfectly correct monochrome rendering of the subject.

Inasmuch as orthochromatic work on the whole must be better than nonorthochromatic, by eliminating a defect inherent in the ordinary emulsion, one cannot but wish to increase the number of photographers who follow it. The hope of doing a little toward this end has been the cause of this article being written, and if the writer has at times appeared to be teaching his photographic grandmothers to suck eggs, he asks these experts to forgive his presumption for the sake of the cause which he is advocating. -GEORGE BASSETT, in Photography.

these again are interrelated. The field of view is determined by the focal length on a given size of plate.

This plate size we will consider fixed. The first thing will then be to choose a focal length to include a given angle of view. If architecture and interiors will be the chief aim, a wide-angle lens is wanted-i. e., a short-focus lens, three inches on a quarter-plate, and fourand-a-half inches on a half-plate. For ordinary work a five-and-a-half-inch lens gives most pleasing results on a quarterplate, and an eight-inch on a half-plate. For portraiture a longer focus is desirable seven or eleven inches respectively. It will be seen from the table that the angle of view diminishes as the focal length increases, and the smaller the angle of view the flatter the picture. Hence the reason for the above recommendations.

TABLE I.-Angle of VIEW

For

When considering the aperture to choose, one must be governed by the extent of one's pocket a good deal, for the price of a lens increases as its rapidity, caeteris paribus. For architectural work large aperture is not a necessity, though it is a convenience for focussing. Indeed, large aperture is inconsistent with large angle; f8 is generally fast enough, though lenses are made as rapid as ƒ6.5. ordinary work f6.5 is rapid enough for snaphsots in bright weather, though if short exposures are to be made all the year round, f4.5 is a desideratum. Indeed, press workers will do well to take an ƒ3.5 lens, so as to be prepared for all conditions of weather. For the studio, also, a lens cannot be too fast. The only doubt that an amateur can have is, then, whether he should use an f6.5 or f4.5. Is the increased rapidity worth the extra expense?

We propose then to consider the advantages and disadvantages of an ƒ4.5 lens. First of all, it has less depth of

focus than the f6.5, though by stopping down this can always be remedied. It must not be forgotten, however, that diffusion of the background is sometimes an advantage. Another drawback is the necessity for extra care in focussing the more rapid lens, a small movement of the focussing screen rapidly blurring out the image. Again, as I said above, aperture and field of view are opposed to one another; i. e., the lens of larger aperture will not have the same covering power as the slower lens. The designer of the lens had, above all, to consider the question of aperture, and choose the forms of his lenses accordingly. Moreover, the large aperture demands very careful correction of aberrations, and this cannot be obtained over a very large field. It should not then be expected that the f4.5 lens will give good definition beyond the corners of the plate; per contra, the f6.5 lens should, and therefore allow for a reasonable rise of front. If, therefore, the rising front is much used, the ƒ6.5 lens had better be chosen.

Again, with regard to illumination. Here the f4.5 scores. If one looks through a lens and turns it sideways, it will be seen that the aperture appears smaller and smaller, till it finally vanishes. This means that the illumination on the plate falls off as one proceeds from the center to the edges. Really the lens only works at its full aperture in the middle of the plate. The greater the circle of illumination of the lens, the more light there will be getting to the corners of the plate. Hence, the lens should cover at least one size larger plate than it is used for, i. e., a quarterplate lens should illuminate a half-plate,

and so on.