After visiting the musee d'Orsay to see the fabulous exhibition of Celine Leguarde's works I came across an original publication. It is the 1902 Bulletin du Photo Club de Paris. Among the many interesting articles on technique, chemistry, new processes, etc. is a long article on lenses.
At the time lenses from photographic equipment suppliers were rather expensive. I imagine that someone realized commonly available glass elements from eye glass opticians could be pressed into service for a much lower cost.
Using cheap lenses seems to have presented a problem of focus. Most of the article contains descriptions of the problem and calculations for the kinds of glass used (crown). The calculations are for focal length, aperture, and assume the color spectrum range of the light sensitive materials (blue into green). Ultimately, what the author gives is a method of accurately calculating how much focus change to apply.
Why all this? Because the point of focus on a ground glass is different than what the light sensitive materials "see." The reason is the ground glass image is full spectrum and the film responds to a very narrow range of visible light. The lenses used had different focus points for different colors.
While I've tried to "clean up" the article so it might be compréhensible to English readers, I've only done this lightly. I find the original language to be very charming as well as providing a view into the French world of the very early 20th century. If this is still accessible to readers, please say so and I will provide a more accurate translation in the sense of current word use, cultural sensitivities, and history.
It goes without saying, but I'll say it anyway, any faults in translation are mine and mine alone.
The Chromatic Blur
Among photography enthusiasts, there are those who value the sharpness of their productions by their weight in gold. The faster and sharper a lens is, the more valuable it is, especially since its price increases very quickly with its speed and sharpness.
Equipped with such a lens, expose short (under-expose), develop hard, and print on citrate: you will have a print that can serve as a mirror, and the model's hairs will be counted on it. If you want to make similar ones in slightly larger dimensions, with a wide aperture to go fast, and a long focus to avoid distortion (1), you will be tempted by these ultra-perfected lenses, but you will first find them to have a definite drawback, not to mention the others: they are very expensive.
I have before me the catalog of a famous foreign company that recommends for multiple uses (including portraiture) very large aperture lenses (1/3.6 to 1/5) covering the entire plate without astigmatism and with extremely fine sharpness. For the 13x18 plate and the 25-centimeter focal length, the lens already costs 535 francs. If you go up to 18x24, it's 865 francs. To make 24x30 (with a 47-centimeter focal length), you have to pay 1250 francs Finally, to achieve a focal length of 61 centimeters, with which the lens will indeed cover 30 × 40, it is necessary to spend 2,500 francs on it: it is a sum and I cannot resist the pleasure of saying that a lens of the same focal length (om, 61) will cost around thirty francs, with the same aperture, in the system of simple lenses that I am going to talk about. It will not have at all the same qualities, except speed, but will have others, which some will find preferable. Let us note that at the distance of 2 meters and with the focal length of o™,61, the enlargement will not be excessive. It will be nearly equal, slightly larger than 1/1 and smaller than the scale of most portrait enlargements which are cut in the color, and it is a size of site that must be achieved. One can approach figure studies that are interesting in themselves, and the merit of the composition lies in the lines of the face and in its modeling. We have just spoken of the photographers who stress “sharpness” in their works. Next to them are the Lewis artists who avoid sharpness of the object and who strive to alter by various artifices the implacable dryness of “sharpness.”
(1) We know that when we bring the lens closer than 2 meters (approximately) to a model who is not in profile, its image is distorted like that given by a garden ball into which we look too closely. According to some authors, the distance to be observed would be even greater, four meters or more.
I will be careful not to speak ill of these artifices because they all have their advantages as well as disadvantages and I will limit myself to simply pointing out the various systems.
[photo by De Pellige]
Pinhole??? The eye of the needle or ahupé has found a learned experimenter and enthusiastic promoters. It beats the reward of cheapness, but the subject must become that of patience because the exposures are counted by minutes, like in Daguerre's time.
Count d'Assche's eyeglass cases had their time of success. They were used a lot in Austria, it seems. In France, I know some skilled artists who own them, but I don't see that they use them often. There too, the relative aperture is very small and exposures are very long (1).
One can carefully focus any object, then slightly alter this exact focus so as to produce a certain degree of blur on the frosted glass, which will be reflected in the photograph. Softness is there, it's true; but it is not distributed uniformly or pleasantly. The curvature of the subjects surfaces and the variable depth of the focal for the various planes mean that the sharpness removed from the main subject most often reappears elsewhere, generally where it would not be desired. This process is used by several artists for lack of anything better, but they are not very satisfied with it.
I will not speak of the flicks produced by the camera during the exposure, nor of the imperceptible swaying that one expects from the model and that a very long exposure encourages, it seems. Here, the blur is even more difficult to control, and surprises in the end results are frequent
During the printing of the positive, a thin transparent sheet of gelatin or celluloid can be placed between the paper and the plate to add “softness” to a print. The amount of blur is adjustable by the number of transparent sheets used and is one of the best “softening” processes available to the artist. One can criticize practitioners for the difficulty of preserving the transparent sheets without scratches and the complication they introduce into the equipment. One can also regret that the blurring is absolutely uniform and blurs the features of the model and the least important accessories in rigorously equal quantities.
It is impossible to end this quick and certainly incomplete review of the blurring process without mentioning the binocular prints that Mr. Boissonnas presented at the 1900 Exhibition. Despite all my esteem for the beautiful productions of this skilled artist, I was not convinced by the brochure that accompanied his works, and I remain convinced that they simply constituted double photographs in a particular way
Readers, especially female readers, who do not feel a special vocation for calculus are asked to take my word for it and skip the small print that follows without reading it. If they are willing to show me the same confidence throughout this article, they will continue to skip the small print and reach the end of the large print more quickly. However boring they may have found the author’s mathematical details, they can say that they suffered little compared to what they avoided.
---------------------------- snip --------------------------
Note: I have excluded from this translation the many pages of calculations and descriptions of how to account for what we would effectively call in English "chemical focus." If anyone is seriously interested I could pass along the original text. It's interesting from a historical perspective, but would be difficult to apply in any meaningful way in the present day.
---------------------------- snip --------------------------
As for lenses, I mainly tried simple lenses and started with simple plano-convex lorgnettes called "periscopic," which can be found at all opticians for an expense of 0.40 francs to 1 franc, depending on the quality, with diameters up to 3 to 4 centimeters, and in all focal lengths from 0.055 to 3.85 (2 inches to 144 inches).
Calculation shows that these plano-convex lenses, arranged with their convexity forward, allow for a fairly large aperture with almost minimal spherical aberration (2). In practice, with exact correction, they produce very good images in the center, for apertures of f/9 and f/8, which allow for quick portraits outdoors or in the studio.
The field of sharpness is very restricted, but at a distance of 2 meters, the model's head is easily contained within it.
By turning the lens over, that is, by directing the flat face toward the light, and by placing a small diaphragm at f/5 in front (at and below), the field of sharpness is significantly increased, but from this point of view, the plano-convex lens always remains inferior to a meniscus (concave convex lens) with its concavity turned forward.
I believe that eyeglass lenses with this meniscus shape are available. They would probably serve well with small apertures, and at f/18 they would allow very brilliant snapshots, because, due to their small thickness and the reduced number of surfaces, absorption and stray reflections of light are reduced to a minimum in simple lenses. But, as I have explained, one would not see any blurring in these photographs taken with very small apertures
Lorgnette lenses are not readily available with diameters greater than 36 millimeters, which, for aperture F/8, corresponds to a focal length of 290 millimeters; if you want longer focal lengths, you must order the lenses, either from an optician, or better, from a factory. I paid 8 francs 40 cents, from a large firm, for a crown-shaped plano-convex lens, which seems perfectly crafted, 12 centimeters in diameter and 70 centimeters in focal length; we see that, even in these large dimensions and prices are affordable when it comes to lenses.
(1) The number of the lorgnette lenses indicates their focal length in inches: one inch 27 millimeters.
Board
The mounts are another matter; they are relatively expensive. A rack-and-pinion portrait lens mount costs 50 francs new, in three inches, and 25 francs used. In four inches, it's double: 50 or more francs. It is difficult to find these large mounts without lenses second-hand. The best thing to do is to do without them. A circular groove is cut into a board in the camera and the lens is fixed to it with points like a glass in a frame. On the front, either a shutter or a cardboard stopper is fitted, which can be ordered from a cardboard maker or made yourself: (fig. 6) a sort of candy box, blackened inside, at the bottom of which there is a hole of the diameter you want for the lens opening
Those who have a so-called portrait lens, of the Peltzval type with three lenses, with diaphragms, can make an excellent anachromatic lens by removing the first two, the front achromatic lens and the diverging flint that follows it.
The last lens of the lens, the one that forms the rear, is a converging crown lens, almost plano-convex with its main convexity directed towards the light. Its focal length is roughly the same as that of the entire lens. To prevent it from bouncing around in the mount, once the flint is removed, it is replaced with a cardboard ring.
Depending on whether the main convexity of this lens is directed forward or backward, it can provide all the services I have described with regard to spectacle lenses. With the convexity in front, the largest diaphragm (1/4.5), and exact correction, we obtain an extreme blur, with a very curious effect
The resources of chromatic blur are not limited to single lenses, and it seems that all double or triple lenses could be established without looking for achromatism and by reserving the variables that the optician has at his disposal to correct other aberrations: sphericity, curvature of the focal surface, astigmatism. It also seems that notable simplifications could thus be made to the lenses, with corresponding price reductions: the floor is given to the manufacturers.
In the meantime, while they provide us with an abundance of excellent anachromatic lenses, of great perfection and extremely cheap, we can, from now on, produce chromatic blur with good uncorrected double lenses, because these lenses have existed on the market for a very long time, disdained by artists who, ignorantly, pass by happiness. Image of life! A well-known company, which I can cite without being accused of advertising (because I indicate enough ways to do without it), the Steinheil company has been selling so-called Periscope lenses since 1865, composed of two simple crown menisci The company's brochure states that in small diameters, they allow apertures from F/12 to F/15 with a fixed correction, and that the sharp image embraces 60° with large diaphragms and 90° with small ones. These are the conditions of a good semi-wide angle rectilinear lens, the one that manufacturers readily call universal. If they are achieved with a fixed correction in small focal lengths, it is likely that they are also found in large ones, with a variable... and exact correction.
The prospectus I mentioned also states that there are commercially available, under the name Bistigmat, two-lens lenses that are nothing more than imitations of the periscope. I don't know these imitations, but they may be excellent. Many lenses are nothing more than more or less successful copies of famous types. The more complicated the lens, the more difficult it was to establish its corrections, and the more likely the copy is not to achieve them. A lens as simple as the periscope must, on the contrary, be imitated with deplorable ease for the scientist who took the trouble to calculate it the first time
These imitations are also found in most photography bazaars. At the beginning of their lens catalog, you generally encounter a type that is humbly titled imitation rectilinear or rectilinear fashion, and which is intermediate in price between the achromatic lens, which is the simple lens formed of two glued glasses, and the least expensive and least pretentious rectilinear lens, which is composed of two simple lenses placed symmetrically. Well, greet the imitation rectilinear lens, because it is an anachromatic that doesn't know it! It is composed of two symmetrical crown menisci, and its largest aperture is f/8 to f/10.
In this state, it will take you quick portraits with a pleasant oftness of lines. From f/15 to f/18 you will have good snapshots in a sharp angle of 60° and stopped down between, and it will provide a respectable wide angle covering an angle of 90° if its mount allows it.
I have before me two catalogs A and B, and here are the prices of the rectilinear imitations (the focal lengths are not indicated).
To cover
9X12
13X18
18X24
Catalog A Catalog B
11 francs 13 francs
22 francs
10 15 20
For those who want to try these lenses, I remind you how to correct them. F being the focal length in millimeters, the correction in millimeters is approximately F and exactly: FX 0.0176
It is worth doing this multiplication exactly once and writing the round number of millimeters on the mount. Then write a correction curve on the camera's tail, as I indicated on page 94, or correct in your head by measuring the magnification (p. 85).
I also remind you, and this applies to all anachromatic lenses, that the larger the aperture of the diaphragm, the greater the blur, even with exact correction and rigorous focusing
Below f/9, this focusing presents no difficulty, and the frosted glass does not reveal that the achromatism correction is lacking. Above f/9, the black lines on a white background begin to be bordered by an iridescent fringe that becomes more pronounced as the aperture increases, and, for very large apertures, the focusing appears uncertain. It is best to focus on slightly large printed characters, black on white, and to stare at the glass when these characters appear dark violet-blue, almost black, without a red or yellow fringe.
As for the services that anachromatic lenses, single or double, can provide, they fall into various categories.
These lenses are excellent at short and medium focal lengths (0.5 to 0.30). At a large aperture, they provide quick portraits, softened by their precious blur; stopped down, the lenses...
Double lenses replace, for all possible uses, lenses four or five times more expensive. Finally, the landscaper who needs various focal points to meet the needs of the viewpoint, while covering his plate completely, finds in a set of anachromatic menisci an ideal kit, of minimal bulk and price.
In the long focal lengths, 0.30 to 0.60 and above, anachromatic lenses have inestimable merits. The aridity and exasperating detail of the corrected lens are particularly odious here; the softness, the blending of the anachromatic, especially pleasant. Whether it is a question of figure, genre, or still life, the difference is extraordinary. And what's more, the anachromatic with the longest focal point costs almost nothing, while the corrected lenses reach fantastic prices as soon as the covered plate begins to grow seriously.
[photograph by C. Puyo]
Now, long focal points, by allowing perspective, provide figures, genre, and still lifes, perspectives that the eye much prefers to those of short focal points.
Our eye is a lens that moves and reasons, so the impression it feels of a subject is not that of the rigorously geometric and flat perspective of which the iris would be the point of view. The eye only sees clearly at a very small angle, about 6º. This clear brush, he moves it successively and rapidly over all the parts of the scene that he wishes to embrace, and the perceived image is always normal to the visual ray; consequently, the whole is a perspective traced on a sphere and not on a plane, and it is the instantaneous memory of these successive perspectives that constitutes vision.
But that is not all: the eye knows, by experience, the real size and direction of the lines that perspective distorts on the edges of the painting, and in the instantaneous memory that it keeps of them it restores them in part in their real sizes and directions
In short, when the eye is placed too close to a scene to be able to truly encompass it in a single glance, it composes, by unconscious reflection, an instantaneous memory little different from the impression it would have had if it had been placed far enough away to truly encompass it all at once. So that by stepping back, thanks to a long focal length, the artist achieves more or less the perspective that the eye believes it sees, even up close.
This distant perspective is very satisfactory, whereas if the distance from the camera to the subject falls below a certain limit that can be set approximately at 2 meters, whatever the focal length, the photographic image reveals distortions that the eye does not notice
If it's a full-frontal portrait, the nose is too big and the ears too small, like the image given by a garden ball or a spoon. If the lens has captured a genre scene or a still life, the objects or subjects in the foreground appear enormous, and those in the background seem tiny. In the front, a Brodbignac chair, and in the background, furniture from Lilliput!
With the long-focus lens, there's nothing like this to fear. But we've seen that this lens is unbearable if it doesn't produce blur! So...
And now let the reader not imagine that I believe I have discovered simple objectives. On the contrary, I think that they must have already been used to create confusion in this country and in others, and several times since the late Daguerre. I am ashamed of not knowing it, to say so myself, and I console myself by thinking, with an eminent philosopher (1), that ideas are always found in common; it the only thing personal is the way of exposing and illuminating them."
11) Ad. Coste. Revue de Sociologie
I would like to point out, however, that the process described above differs notably from that of the spectacles recommended by Count d'Assche and by Messrs. Watzeck and Loæhr, of Vienna. Anachromatic lenses work at a large aperture, even when they are formed from a single plano-convex lens. These are true lenses which clearly cover a certain surface, and in this clear field there remains no blur other than the chromatic blur. Spectacles, on the contrary, are slow, because they are always strongly diaphragmed (small diametere, small f/stop?) at least, and often. For such apertures, we have seen that the chromatic blur becomes imperceptible. The remaining blur can only come from an inaccurate correction of the focus or from spherical aberration and astigmatism, defects from which biconvex lenses are affected to a high degree
The question has another aspect to which the author of these lines is drawn by a secret tenderness, and which he asks permission to explain in two words. Stand firm and be kind enough not to laugh.
Photography provides artists with a powerful means of expression: nowhere is this better known than in this house.
To sciences of every kind, it offers processes of investigation and recording that nothing can replace, and it even accumulates materials for history. Just ask Presidents on their travels!
But photography has yet another role, certainly broader: to provide a healthy and captivating distraction to a multitude of good people who are growing larger every day.
Healthy, because it exercises and develops attention, care, and taste; captivating, because it is a commonplace observation that the demon of photography quickly reigns supreme over the hearts it has bitten
Such a man, whom an evening club absorbed in a disturbing way, has become the most orderly husband since his tête-à-têtes with the red lantern began. Farewell, queen of spades! Away with the queen of hearts! Mothers, wives, you will never know what you owe to gelatin-bromide!
The anachromatic lens can extend this beneficent empire by putting optical resources within reach of the humblest purses that the achromatic lens achieves through a difficult construction and for an excessive price. One economy easily leads to another, and certainly photographic equipment involves other simplifications. The use of negative papers instead of plates has been the subject of recent studies that appear full of promise.
May these studies succeed, and many others with them, so that photography continually expands its domain and becomes more popular every day. May the anachromatic lens guide it towards the crowd, and may it compete not only with the Cercle, but also with the Cabaret! This is the grace I wish for it.
[photograph by DE PULLIGNY]
No comments:
Post a Comment