CHAPTER II.

LENSES.

[Sidenote: Optics.]

We do not intend to incorporate in this chapter elementary optics, as the subject is well known to most educated men, but in case any reader should know nothing of light and optics, we recommend him to get [Sidenote: Ganot’s Physics.]Ganot’s Physics, and thoroughly master at least the paragraphs of Book VII., on “Light,” that we enumerate below.[10] This may seem a little formidable, but our reader will find that with a very simple knowledge of mathematics he can easily understand all the sections marked, and it is our opinion that light and chemistry should be studied directly from systematic text-books that treat of those subjects. In the Appendix we shall refer to some additional books which we consider advisable for the student to read, but for the present we strongly recommend him to thoroughly master the parts of Ganot that we have cited, and to avoid all other desultory reading until he has done so.

Footnote 10:

Namely, paragraphs 499, 500, 501, 502, 503, 504, 506, 508—the Laws of the Intensity of Light, 509—Photometers, Rumford’s and Bunsen’s, 510, 511—first proof only, 512, 513, 514, 518, 519, 524, 525, 528, 533, 536, 537, 538, 539, 540, 542, 543, 544, 551, 552, 554, 555, 556, 558, 564, 565, 566, 567, 568, 569, 570, 571, 572, 573, 574, 575, 576, 579, 580, 581, 582, 583, 584, 602, 604, 612, 615, 616, 617, 618, 619, 620, 621, 625, 626, 627, 628, 629, 631, 632, 634, 635, 636, 637, 639, 640, 641, 645, 646, 650, 652, 655, 656, 659, 661, and 664.

Far too much time has been given, and far too much importance has been hitherto attached, to the subject of optics in connection with photography. Much time and expense would have been saved had the pioneers of photography had good art educations as well as the elementary knowledge of optics and chemistry which many of them possessed, for without art training the practice of photography came to be looked upon purely as a science, and the ideal work of the photographer was to produce an unnatural, inartistic and often unscientific, picture. It is, indeed, a satire on photography, and a blot which can never be entirely removed, that at the very time the so-called scientific photographers were worrying opticians to death, and vying with each other in producing the greatest untruths, they were all the while shouting in the market-place that their object was to produce truthful works. At length, when the most doubly patented distorting lenses were made to meet their demands, they, with imperturbable self-confidence, presented a sharp, untrue photograph, insisting upon its truth. “A truer picture,” said they, “than drawing;” “truer than the eye sees,” some said. In short their picture was absolutely perfect. When a lens giving a brilliant picture, with all the detail and shadows sharp, and the planes all equally sharp, was at last produced, the scientists were _in excelsis_. But, alas! they proved themselves as unscientific as they were inartistic! Had they but taken up their simplest form of lens and used it as a magnifying-glass, they would have seen immediately that all was not right, and instead of clamouring for the artistic falsities of “depth of focus,” “wide-angle views,” “sparkle,” and the other hydra-heads of vulgarity, they might have set to and made the lens which was required. It was but a simple thing that was required.

[Sidenote: Dallmeyer’s long-focus landscape lens.]

The question then arises—What is the best lens for artistic purposes? That lens is _Dallmeyer’s new long-focus rectilinear landscape lens_. This summer (1888) we used one of these lenses and were delighted with it.

[Sidenote: Why this lens is the best.]

Why is this the best lens for our purpose? is the question that naturally arises. It is the best because being what is called a long-focus lens, it cannot be so ignorantly employed as can lenses of shorter focus, there is no appreciable marginal distortion, and with open aperture the outlines of the image are softly and roundly rendered, and in addition the relative values seem to us to be more truly rendered by it.

[Sidenote: Best focal length to use.]

This lens then being, as we think, the best for artistic work, the next question that arises is what focal length of lens must we use to get the best results. The student will be told _ad nauseam_ that if he places his eye at the distance of the focal length of the lens from the photograph he is inspecting, all will be well. Such, however, is _not always the case_. He may prove it for himself by taking a lens of short focus and photographing any suitable object placed too near to him, and he may then place his eye at the distance of the focal length, and if he be an artist, he will immediately detect that the drawing is false, and the distance is dwarfed and pushed together as compared with foreground objects, whilst in a true drawing the proportions must be true between the foreground objects and distant objects. This misuse of the lens is what leads to the production of so many photographs false in drawing, and it is evident that since many of these falsely drawn photographs have been and are a basis for many scientific purposes, the deductions based upon them will have to be reconsidered.

[Sidenote: Experiment for finding a rough rule for the use of lenses.]

The next question is, what proportion, as a rule, should the focal length of the lens bear to the base of the picture to give approximately true perspective delineation? This proportion should be as two to one, that is, the focal length of the lens should be as a rough working rule twice as long as the base of the picture. We arrived at the result by making a series of drawings on the ground glass of the camera, and comparing them with a perspective drawing made upon a glass plate. Opticians have arrived at the same conclusion, for we find this is the rough rule stated by Mr. Dallmeyer in his “Choice Lenses.”

[Sidenote: Comments.]

The falsity of the statement that photographs are always true—a statement that has been in vogue from the earliest photographic days—is then apparent. [Sidenote: False drawing producing false tonality.]It will now be obvious why some lenses make ponds of puddles, and otherwise falsify the landscape. This fact would have long ago been noticed had artists always seen the landscape from which the photograph had been taken. Another thing which a wide-angle lens, if wrongly used, does, is, in the case of a picture with clouds, to draw down and crowd together the clouds, and define them more sharply than the eye sees them, so that when the negative is printed they appear too strong in value, and the whole picture is thrown out of tone, and is therefore false and inartistic, even if the lens be correctly used; this fault is generally present in pictures taken with these lenses.

[Sidenote: Lenses recommended.]

It will be seen from our remarks, therefore, that the only lens we recommend for artistic work is Dallmeyer’s new rectilinear landscape lens. At least two of these should be obtained of different focal lengths, one of which is advertised to cover a plate a size larger than that used by the photographer, and the second to cover the same sized plate that he uses. In addition a rapid rectilinear lens as advertised to cover a plate of the same size as his camera, will be found very useful for quicker work. [Sidenote: Lenses for special purposes.] For special purposes, for example in photographing beetles, or fish, or flowers for scientific manuals, the finest lenses procurable must be used, and sharpness, brilliancy, &c., are vital qualities in such cases, for the work desired is diagrammatic and not artistic, but in these cases also the greatest care must be taken to use the lenses properly, so that the drawing is correctly rendered. Ignorant critics and enthusiastic partisans alike have claimed for photography, as its chief merit, “truthfulness.” As has been shown, a photograph may be very false indeed.

[Sidenote: Composite photography.]

Another chimera is that of “composite photography,” to which we shall again refer. When Mr. Galton tells us he uses an ordinary portrait lens for his work, and gives no other details, that is quite sufficient, in our opinion, to seriously impair the value of his “composites,” even were there no other considerations.

[Sidenote: Portraits taken with rapid rectilinear lens.]

The only really artistic series of photographic portraits we have ever seen, namely, those by Mrs. Cameron, were taken with the next best lens to that advocated, namely, a rapid rectilinear lens, but even they would have been improved by the use of the new lens. We have besides seen here and there really artistic portraits by others (but these were the result of chance, as no second picture was ever produced by the same worker), and they were taken by a rapid rectilinear lens. Mrs. Cameron, though not an artist, had knowledge enough to see that the portrait lenses of the day were undesirable for her work. And here it may be remarked that a great ignorance of optics is as harmful as wasting too much time upon its study. One industrial portrait photographer, who has very occasionally succeeded in producing an artistic picture, prides himself, we are told, on not knowing what lens he uses. Such a man can never be an artist, for he cannot know whether his work be true or false. To appreciate falseness in drawing requires considerable training. An average judge of photography might discover gross distortion of limbs, due to violent perspective; but how many would notice the false drawing in a face which is taken with a portrait lens?

[Sidenote: Diaphragms.]

Supplied with his lenses, the student will find “stops,” or diaphragms. The name, “stop,” suggests its use. By making the light pass through a contracted hole, the weak marginal rays are cut off, and the image is therefore made sharper all over, spherical aberration is reduced, and the depth of focus is increased. But though diaphragms are used to correct an error, yet the ignorant use of them is as great a source of error. One of the causes of sharply defined and false heavy shadows in the much-vaunted “sharp photographs” is due to focussing sharply, and “stopping down,” that is, to using a small diaphragm. This is the invariable practice of most photographers.

[Sidenote: Modified diaphragms.]

Some ingenious workers have suggested modifications in the construction of diaphragms, with a view to improving the picture; one of these being a paper diaphragm, made translucent with castor oil; but we have not found any advantage in these novelties. It is, however, a legitimate field for experiment, and translucent diaphragms might be tried in indoor work and bright out-door effects.

[Sidenote: Intensity of lens.]

The student will often see in photographic papers that a lens works at F/8, or F/32, or some other number. This simply expresses the ratio between the working aperture and the equivalent focus of the lens, and is obtained by dividing the equivalent focus by the working aperture. F/8 then means the aperture is one-eighth of the focal length of the lens referred to. The rapidity of lenses are compared in this way by squaring the denominators of the fractions thus obtained; when the results will give the ratios of rapidity. [Sidenote: “Depth of focus.”]By “depth of focus” is roughly meant the sharp rendering of the different planes of a landscape, or any object with more than one plane in one plane. Needless to say, this quality, greatly sought for in lenses by photographers, is a thing to be carefully avoided in artistic work, as we shall show later on.

[Sidenote: Flare spot.]

By a flare spot is meant a circular spot on the focussing screen, which receives more light than the surrounding field; it is said to be caused by the diaphragms being wrongly placed. The same effect is produced when the sun shines into the lens, the light being then reflected from the brass tubing of the lens, and it is for that reason that the lens must be carefully shaded during exposure, when the sun is directly in front of the camera.

[Sidenote: Angle of view.]

The angle of view included by a lens is an important consideration, and we shall refer to this later on; here we shall only show how this angle may be determined when the student wishes to do so. The angle depends on two factors, the length of the base line of the picture, and the focal length of the lens. This is practically determined by ruling a horizontal line the actual length of the base line of the picture, and drawing from the centre of this line a perpendicular equal in length to the focal length of the lens. Completing the triangle, we have in the angle contained by the two sides of the triangle the required angle, which can be measured by an angle measurer. Experience shows that if the base of the picture is greater than or equal to the focal length of the lens, the angle included will vary between 53° and 90°; but if the base is less than the focal length, these angles will vary between 44° and 19°, or less. It will be seen, therefore, that the long-focus lenses give more suitable angles of view for pictorial purposes.

[Sidenote: Hints on lenses.]

Delicate optical instruments, like lenses, must, it is needless to say, be carefully protected.

A good lens should be free from scratches, striations, dull patches, due to imperfect polishing, and veins; but air bubbles do not affect its value, for it must be remembered that the shape of the hole through which the light passes does not affect the image, save only by cutting off some of the light. Thus, if a wafer be stuck to the centre of the lens, the image will be found unimpaired. Dust and dirt, however, though they do not seriously impair the definition of the image, yet cut off much light, as will occur to any one when he thinks of the difference between the light of a room, when the windows are dirty, and when they are perfectly clean. Lenses should not be left in bright sunlight, for this causes a change that slows them, the dark also injures them in certain cases, for, as all microscopists know well, darkness causes a change in Canada balsam, with which lenses are cemented together.

Mr. Dallmeyer insists that lenses should be kept dry and free from sudden changes of temperature, otherwise they may tarnish or sweat, as it is called. Any one who has been troubled with this sweating will never forget it. Our experience is that the best way to keep lenses is in small leather, velvet-lined cases. We generally keep with them a piece of soft chamois leather, or an old silk handkerchief. No compound of any kind should be used to clean lenses, if anything appears to be going wrong with them, they should at once be sent to the maker.

[Sidenote: View-meter.]

A valuable little tool is a view-meter. The handiest and compactest we have seen is that supplied in telescopic form.