IT has been shown that when light passes through a prism it is not only refracted, but dispersed ; and as lenses are, as it were, but a collection of prisms, we see the same effect produced by them. They will at once be perceived by referring to the diagram (Fig. 21), in which A B are rays of light that, falling upon a bi-convex lens, are refracted and dispersed at the same time. As we have seen (Fig. 9) that the violet rays are bent more out of their course than the red, it is evident that these will meet at a point nearer the lens than the red ones, or indeed than any of the others. If, therefore, we endeavour to obtain an image of the object from which the light rays A B proceed, and place a screen at V, we shall find that we obtain a violet image, surrounded by rings of blue, green, yellow, orange, and red ; and the same thing will happen if the screen be moved back to B, G, Y, 0, or Ronly the circles of surrounding colour will be differently arranged.
To the human eye the brightest part of the solar spectrum is that which coincides with the yellow rays; but the rays which act with most energy upon the silver salts by which the photo. graphic plate is sensitized lie, as we have seen (page 27), towards the other end of the coloured band. Hence, if we wish to obtain a picture by the yellow rays, the plate would be affected most by the blue or violet rays, and we should fail to secure a sharp picture, because at Y the blue and the violet rays would be represented by circles instead of by points. This peculiarity is known as non-coincidence of the visual and chemical rays, and, the lens showing it is said to be nonehromatica defect inherent in all single lenses.
This was the great trouble of the early photographers, the distinction between chemical or actinic and visual achromatism not having been at that time properly recognised. It was for long thought that no remedy existed for this defect. Sir Isaac Newton even was of that opinion. But there is, in truth, a way out of every difficulty, and a remedy for the defect in question was discovered in the fact that, whereas all transparent substances disperse as well as refract light, they do not all disperse and refract it in the same degree. Crown and flint glass, for instance, being of different densities, vary in their qualities in this respect, the former, as a general rule, possessing less dispersing power as compared with refracting power than does flint glass. Correction of the dispersion caused by a crown glass prism therefore can be brought about by the use of a flint glass prism without affecting the diffraction. The same principle is of course applicable in the case of lenses. By placing a concave or negative focus lens of flint glass behind a convex or positive focus lens of crown glass (as in Figs. 22 and 23), the defects due to the breaking up of white light into its component colours is corrected.
In each case the negative focus lens is of flint glass, and its presence is due solely to the necessity of counteracting the dispersion caused by the other lenses. Sometimes three or more lenses may be thus fixed one against another, but the principle remains the same.
It is in this way that the combination known as the achromatic (or “no colour “) lens is arrived at.
Another form of aberration, which has to be taken into account and corrected in a similar way, is known as spherical. It is one which has exercised the ingenuity of lens-makers as much as that of chromatic aberration. It arises from the fact that the rays of light passing through the margin of a lens with a spherical surface meet, or, as we say, come to a focus nearer the lens than those traversing the centre. What is meant will be perceived from the diagram.
The effect of spherical aberration, if present in any marked degree in a lens, is to make it impossible to obtain a sharp image with it. Most lenses exhibit a certain amount of this defect if the glass is employed at its full aperture: that is to say, at its full width. The fault is overcome by the use of ” stops.”
The object of a stop or diaphragm in a lens is to reduce the diameter of its aperture, and so, as it were, to concentrate the rays of light. By so doing it serves a threefold purpose. It gives greater depth of focus by causing the converging pencil of light to fall on the plate at a more acute angle, enhances flatness of field by extending the oblique rays, and increases the defining power of the lens by lessening its spherical aberration. By placing the diaphragm a short distance in front of the glass, the aberration is cured by the stop-page of those conflicting rays which, if allowed to pass, would interfere with the sharpness of the image.
But spherical aberration may also be rectified by making one lens compensate another. Thus when the focus of the marginal rays is nearer than that of the axial rays it is called positive spherical aberration. But concave or negative lenses lengthen marginal rays ; hence by combining suitable negative and positive lenses spherical aberration may be rectified to a considerable extent. When by the choice of glass of appropriate curvatures a lens results that can be used at its full aperture without spherical aberration, it is designated ” aplanatic.”
But aberrations, whether chromatic or spherical, are not the only lenticular defects wherewith the photographer is troubled. There are many others. Indeed, it has been well said that the whole science of photographic lens-making consists in doing the most possible to overcome the inherent defects in lenses,* by arranging such compromises as will effect the best practical results. This necessitates the use of different forms of lenses, because in one class of work a certain fault can be put up with, or ignored, which with another sort of work would be impossible.
For instance, in a lens for portraiture the requirements are quite different to those essential in a lens for architecture or landscape. In the case of likeness-taking, if the lens gives the head with a fair amount of sharpness, the rest of the figure may be allowed to pass with less exactness of definition. An essential point, however, is rapidity of action. In the case of landscape, on the other hand, comparatively equal and sharp definition is required throughout the picture, whereas rapidity is of less moment. As regards architectural work, the great desideratum is that there shall be no distortion.
Distortion is of several kinds; some of these, however, may sometimes be attributed to a defect in the lens, which in reality arises from inexperience on the part of the operator, as when he plants his camera too near the object to be photographed, and in the case of a portrait has the hands or feet projected forward and represented of disproportionate magnitude. One of the most common forms of distortion justly chargeable to the lens is that known as ” curvilinear distortion,” the characteristic of which is that all the straight lines, except the axial ones, are shown curving either inwards or outwards. The defect arises from the circumstance that the margin of a lens refracts to a greater degree than its centre, the effect being to ” condense ” the outer rays into a smaller space, and so to make them converge. The fault becomes more pronounced by the useas in the case of a single landscape objectiveof the diaphragm in front. If the objective be turned round, so as to allow the light to traverse the lens before it reaches the stop, the nature of the distortion is altered. The rays then, in place of being condensed, are expanded, with the result that the marginal lines are bent outwards.
These two forms of distortion are known respectively as “outward” and “inward,” or ” barrel ” and ” pin-cushion.” Their nature will be readily understood from Figs. 25 and 26. The defect is here greatly exaggerated of course. Indeed, as a matter of fact, few lenses distort to any very appreciable extent, and then only when the angle of view is a large one, and long straight lines occur at the edges of the plate. Still in the early days of photography the evil was much felt and a cure earnestly sought after. One way in which it was attempted to overcome the difficulty was by making the sensitized plates curved instead of flat. When obtained, however, they were not easy of manipulation. This caused them to be little used, and they in consequence soon dropped out of existence.
Several makers of lenses came very near the discovery of the remedy, although they appear to have missed it becauseas is so often the caseof its very simplicity. Gradually, however, it began to dawn upon opticians that if a stop in front of a lens gave barrel-shaped distortion, and a stop placed behind the lens caused distortion, of the “pin-cushion” character, a diaphragm placed betwixt two lenses would have the effect of correcting both errors.
Andrew Ross appears to have been the first to put a lens on the market that came approximately near to a solution of the difficulty of curvilinear distortion in a portrait-objective of two piano-convex achromatic lenses divided by a diaphragm. His son, Thomas Ross, went a step further by substituting meniscus glasses for the piano-convex ones, not however until others had laboured in the same field with more or less success. Finally, in 1857, came the “orthoscopic lens, the joint production of Petzval and Voigtlander. The late Mr. Traill-Taylor thus gives the history of the orthoscopic :
“In 1840 Professor von Ettinghausen, having returned from a visit to Paris when the daguerrotype process was engaging the attention of the scientific world, remarked to Petzval (a mathematician of Vienna) that Daguerre, with whom he had been in direct intercourse, made use of a lens having a small diaphragm, by which a great loss of light ensued, and inquired if he (Petzval) could not devise a better form of lens. Acting upon this hint Petzval instituted researches, and the year following (1841) gave to Voigtlanderat that time an optician enjoying a high reputation the formulm for two objectives, both of them working without a diaphragm. One had a large aperture and a short focus, and gave great concentration of light over a large area; the other had a longer focus, and was capable of covering a large field. The former was the now well-known and universally used portrait lens, the other being the orthoscopic, which was allowed to lie perdu for several years afterwards.”
The orthoscopic (” correct seeing “) was considered a great advance on the lenses of those days, when the single achromatic was the only lens in use for landscapes, and gave an increase of aperture with improved flatness of field. But it was a mistake to suppose that it was entirely without distortion, although such as it had was of the opposite form to what had theretofore been common.
The orthoscopic consists of a plano-convex, or nearly flat achromatic meniscus, together with a back lens of bi-concave crown glass, joined to a flint-glass meniscus.
The portrait objective of Petzval, above referred to, is a combination consisting of two achromatic lenses placed some distance apart. The front one is a plano-convex, or else of so slightly meniscus shape as to appear almost plane. It is composed of a crown-glass bi-convex joined to a plano-convex flint lens. The back lens is a bi-convex composed of a double-convex crown and a concave-convex of flint glass. These lenses are usually mounted so as not to touch one another.
Before quitting this subject of distortion it is necessary to mention another very important aberration, one which figures very largely in all recent photographic literature, that, namely, known as astigmatism (a word derived from the Greek, and meaning not coming to a point). ” If,” says Zentmayer, “we focus a well-defined round object, situated in the axis of a lens of wide aperture, on a screen, we shall find the image round; even if we move the screen in and out of the focus the image will only become less sharp ; but if we turn the lens sideways, so as to get the image of the same object formed by pencils oblique to the axis, then we shall observe that it is no longer possible to form a sharp image of the object, and by moving the screen in and out of the focus the image appears elongated, horizontally or vertically.”
Astigmatism is therefore a serious fault for a lens to possess, if present in any marked degree. It is usual to find it in lenses corrected for great flatness of field, as it is called, although the defect is seldom wholly absent from any lens. One of the great aims of modern lens-makers, therefore, has been to produce non-astigmatic combinations. Into the formation of the lenses of which these are composed, we find entering one of those new discoveries which have played so considerable a part in the evolution and development of photography. I refer to the new kind of glass known as Jena glass.
Until of recent years flint and crown glass were the substances exclusively employed for the fabrication of objectives. The first named is a silicate of potassium and of lead; the second is a silicate of potassium and of chalk. Flint glass strongly disperses and refracts the rays of light, while crown glass possesses but a feeble power to disperse and refract. Although united they compensate each other’s defects to some extent, that is, they render it possible to vary the refractive power of lenses, yet they do not allow of an alteration of the dispersive power in a like manner.
In 1842 a French glass manufacturer named Grimaud had the idea of substituting the borates for the silicates in the making of glass. There was no demand for the new thing however, and the use of boric acid was abandoned almost as soon as indicated. For one thing it was recognised that there was a lack of stability about the new material. However, what in France proved to be a non-marketable substance, met with better success abroad. Turned to advantage for optical glasses by Herr Schott of Jena, it ere long became known far and wide as Jena glass.
With it Dr. Abbe soon found that it was possible to make lenses without the presence of secondary spots. Zeiss, the optician, undertook to construct objectives for the microscope on the lines indicated by Abbe. The result was such as to delight microscopists. It need hardly be said that this first success led to others. From the microscope to the photographic objective was but a step, and it was quickly taken, Voigtlander, Steinheil, Harnack, Darlot, and Berthiot, all turning their attention to the new glass. For the objectives thus constituted was claimed a great amplitude and clearness of field, together with considerable diminution in the length of exposure.
Nor was this all. For Dr. Rudolph, taking his stand on the one hand upon a general principle established as regards uranophotography, in 1885, by Dr. Hugo Schroeder, and on the other, adopting a plan proposed by Dr. Abbe, he was able to give to the optician Zeiss the elements for the construction of an objective composed of two simple meniscus lenses of crown glass, between which was placed a lens of smaller diameter and of triple correction. This central portion of the objective was little more than a plate of glass with parallel surfaces, and had but little share in the lenticular action of the combination. Nevertheless it played a most important part in the whole, admirably correcting all the chromatic and spherical aberrations of the objective.
This combination was given the name of Apochromatic Triplet with central correction. The correcting lens being composed of the baryte flint glass of Jena, the achromatism is corrected for three colours in place of two, and the secondary image totally disappears. It is further claimed for the combination that it gives an image entirely free from distortion as well as from the central flare spot. Moreover, the field of view is flat, and the astigmatism still possible from the oblique rays does not exceed that obtained with good aplanatic lenses of corresponding aperture.
So much it seemed necessary to say about the Jena glass and the lenses made from it. It need hardly be added that while the new material is still on its trial, so to speak, it already bids fair to supplant the time honoured flint glass of English manufacture, as well as to revolutionise the whole art of lens-making.
The above particulars, and the triplet of J. H. Dallmeyer, will give an idea of the way in which combinations came to be built up, in order to secure achromatism and non-distortion. The latter was at one time very highly thought of by photographers, and still holds a distinguished position amongst objectives.
These examples will serve to illustrate the principles of photographic lens construction. It should, however, be noted that one disadvantage attending these combinations is that the more the surfaces are multiplied the greater is the loss of light. Some makers have sought to overcome the difficulty by cementing the separate glasses together, and to a certain extent they have succeeded. The disadvantage, however, attendant on this loss of light has become of much less importance since the introduction of quick plates than it was in the old portrait days.
The subject of lenses is such a large one that one might extend these remarks to almost any length. Sufficient has been said, however, to give the reader an insight into the depth of scientific research which has been necessary on the optical side to bring photography to its present standpoint. One would like to add something about the different makers of lenses; but space will not permit.
It need hardly be said that a person who takes up photography seriously can hardly get along with a simple objective. Although the beginner may do so, and, indeed, will find it advisable to confine his attention to one lenssay a rapid rectilinear, or a good single lensuntil he has thoroughly mastered it. For general worklandscape, groups, and portraiturean achromatic, with an aperture of f/10 or f/11 (i. e., — or 1/11 of the focus) is a good lens to select. For landscape or sea-views only a wide angle lens is the best These are single objectives.
In doublets, as a combination of two is called, one of the best for general purposes, for rapid or instantaneous views, for copying, enlarging, etc., is the rapid rectilinear of, say f/8. Where an objective for photographing interiors and for architectural work in confined situations is required, a wide-angle rectilinear of double the above aperture might be chosen.
One of the most useful wide-angle lenses is that of Dallmeyer, of which an illustration is given. Although Dallmeyer made these lenses for the most part as here shown, that is, with a front lens of larger diameter and longer focus than that of the back lens, other makers generally give the two combinations the same diameter.
One advantage of possessing a wide-angle, double rapid rectilinear, or portrait lens is that the half combination may be turned to advantage as a single landscape lens of a narrower angle and of about twice the focal length of the whole. A useful doublet specially for portraits is one of f/3 or f/4. Of course if a half combination be used in this way the length of exposure has to be considerably increased. For some years past manufacturers have been turning their attention to the making of lenses in series, so that a photographer might go out with a complete battery, so to speak, suitable for every kind of work, and capable of use in the one moment. By careful selection, a man may thus have, neatly secured in a pocket-case, every focus, either singly or in combination, for which his camera is adopted. Such sets of lenses, if carefully worked out, are extremely useful and cannot be too highly recommended.