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GLOSSARY:

Definitions and short essays discuss the following list of terms in relation to Draper's work.
Although each term in the list links to its place in the glossary, it might be helpful to read once through the entire glossary of optical terms in use during 1839-40.
The links can be used for later reference if necessary.

DAGUERREOTYPE (PLATE SIZES)
PLANO-CONVEX LENS
PLANO-CONCAVE LENS
MENISCUS LENS
BI-CONVEX or DOUBLE CONVEX
LENS ABERRATION
SPHEREICAL ABERATION
CHROMATIC ABERRATION (CHROMATIC DISPERSION)
FLINT GLASS
CROWN GLASS
ACHROMATIC LENS (CORRECTED LENS)
APLANATIC
LUMINOUS RAYS
 VISUAL FOCUS (LUMINOUS FOCUS)
CHEMICAL RAYS
CHEMICAL FOCUS
NON-ACHROMATIC LENS (UNCORRECTED LENS) (CHROMATIC LENS)
MONOCHROMATIC LIGHT
FOCAL LENGTH (FOCUS)
DEPTH OF FIELD
F-STOP NUMBER (f /#)
PETZVAL LENS
DIFFUSED LIGHT
MIRROR-REFLECTED, FILTERED SUNLIGHT
ACCELERATOR CHEMICALS


DAGUERREOTYPE
The first practical photographic process invented by Louis Jacques Mandé Daguerre in 1839. A fine daguerreotype is an extremely beautiful, detailed image formed on the highly polished, silver coated surface of a copper metal plate. The silvered surface, sensitized in fumes of iodine, was placed in a camera for exposure. The plate was developed in mercury vapor.

No negative was involved, and the image produced on the plate appears as both negative and positive when turned in the light. The surface of a daguerreotype is extremely fragile, requiring protection behind mat and glass in folding, book like cases of leather covered wood or plastic. To set off the daguerreotype, most cases also contain linings of dark velvet or silk. Daguerreotypes are one of a kind. Copies required additional exposures.

Aptly termed a "mirror with a memory" by Oliver Wendell Holmes, daguerreotypes were sometimes hand tinted with color to heighten realism. Gazing into the subtle realities of "presence" captured in a great daguerreotype can afford an unforgettable experience. No photographic reproduction equals actually viewing such an image.

Sizes of daguerreotypes were early standardized. A full or whole plate was the largest size ordinarily available. The full plate measured 6-1/2 by 8-1/2 inches, from which smaller sizes were derived. The half plate measured 4-1/4 by 5-1/2 inches. The quarter plate measured 3-1/4 by 4-1/4 inches. The sixth plate measured 2-3/4 by 3-1/4 inches. The ninth plate measured 2 by 2-˝ inches.


PLANO-CONVEX LENS
A lens that is flat on one side and curved (bulged) out on the other. Light rays passing through it converge (bend inward) to a focal point.


PLANO-CONCAVE LENS
A lens that is flat on one side and curved inward on the other. Light rays passing through it radiate (bend outward).


MENISCUS LENS
Suggested by William H. Wollaston in 1812 for use as the lens in the camera obscura. A lens with only one element that is curved outward on one side and curved inward on the other side. Thus, a lens that is convex on one side and concave on the other, or a "concavo-convex" lens. Wollaston suggested that this lens have a stop (aperture) on the concave side facing the distant object. Such a lens would give less aberration and sharper definition than a plano-convex lens. Many simple Wollaston lenses continue to be made today and fitted to box cameras. It is thus probably the most popular photographic lens ever made.
Meniscus Lens[1]


BI-CONVEX or DOUBLE CONVEX LENS
A lens that is curved outward (bulges) on both sides. Usually formed by placing two plano-convex lenses with their flat sides together.


All four types of lenses described above were "uncorrected" and exhibited two forms of
LENS ABERRATION--spherical and chromatic.

SPHERICAL ABERRATION
The "ideal" convex lens would focus all the rays from one point of the object onto one point of the image. This does not happen. From every point of an object, the light rays passing through the extremities of the lens collect at a different point than those which pass through the center of the lens. An infinite number of images are thus formed which causes indistinctness. This effect was often called a "lack of flatness in the field of the camera" or spherical aberration.

Spherical aberration can be reduced by two methods:
1--A stop or diaphragm can be placed at some distance in front of the lens which transmits only light rays that pass through the diaphragm's opening and thus excludes the light rays which would otherwise pass through the extremities of the lens. A stop always blocks a great deal of light from passing through the lens. Giroux's first daguerreian camera used a stop and therefore admitted too little light to take portraits.

2--A convex lens focuses light rays passing through its center to a point more distant from the lens than the focus of the rays passing through its extremities. A concave lens produces the opposite effect. A convex and a concave lens can thus be combined to cancel out the worst effects of spherical aberration in the convex lens alone.


CHROMATIC ABERRATION (CHROMATIC DISPERSION)
Lenses have another defect beside spherical aberration. Light rays of different colors (wavelengths) undergo different refraction as they pass through a lens. A lens bends red rays the least and bends violet rays the most. An infinite number of images of different gradations of color fill the space of the focal distance. A lens that is stopped down with a diaphragm to correct spherical aberration has its focal length increased which increases the area of chromatic aberration (the space through which the differently colored images are dispersed). This problem can be corrected somewhat by using two different types of glass in the convex and concave elements of a lens. FLINT GLASS which had a low refractive index (bent light rays the least) was utilized to make the concave element and CROWN GLASS which had a high refractive index was utilized to make the convex element.

Such a lens made of convex and concave elements carefully calculated to minimize spherical aberration and using two different type of glass, flint and crown, carefully calculated to minimize chromatic aberration was called an
ACHROMATIC LENS or CORRECTED LENS.

Achromatic lenses thus corrected chromatic dispersion by using a combination of crown glass (convex) and flint glass (concave) that focused light of all wavelengths upon the single focus of best visual effect.
Achromatic lens[2]

The lens used in the first daguerreian cameras manufactured by Alphonse Giroux of Paris were "16-inch achromatic telescope objectives of plano-convex exterior form, about 3 1/4 inches in diameter, mounted back-to-front with plane side towards the subject, and with a 15/16 inch diameter stop situated about 3 inches in front of the lens. Used in this way, the lens has a considerable amount of undercorrected spherical aberration, and even at the working aperture of f /17 the image is quite hazy, and the field decidedly inward curving."[3]


APLANATIC LENS
An "aplanatic" lens was an achromatic lens that had a reversed sequence of crown (convex) and flint (concave) glass lenses. The name was coined by Sir John Herschel to indicate freedom from spherical aberration but it was probably not much more effective.
Aplanatic lens[4]


Light in the blue-violet wavelength range of the spectrum affected daguerreotype plates singly-sensitized with iodide more and sooner than light of other colors (wavelengths). The light rays toward the violet end of the spectrum which had this greater intensity of photographic effect were referred to as the CHEMICAL RAYS of light. The spot at which the lens focused blue-violet light was known as the point of CHEMICAL FOCUS. Light rays much closer to the red end of the spectrum exerted the greatest action on the retina of the human eye, and such rays were referred to as the LUMINOUS RAYS of light.. The point at which the lens focused them on the daguerreian plate became known as the VISUAL FOCUS or LUMINOUS FOCUS.

Most early operators, unaware of these principles, logically focused lenses by sight at the visual focus. This effectively missed the point of clearest focus for the daguerreotype which was at the chemical focus. Even when achromatic lenses were used, poor images usually resulted because although such "corrected" lens were supposed to focus the light rays of all colors (wavelengths) to about the same point so that the visual focus and the chemical focus were the same, a certain amount of chromatic dispersion/aberration always remained. This was because aberration could be minimized at only one area of the spectrum at a time. In the early days of daguerreian photography all available achromatic lenses were designed as telescope objectives. In achromatic lenses produced for the telescope, most attention was paid to the accurate focus of the rays at the red end of the spectrum. This enhanced the visual effect but left considerable aberration of the rays at the violet end of the spectrum. Since the violet rays exerted the greatest action on the daguerreotype plate, a telescope lens produced less sharp photographic than optical images. This effect was only corrected after opticians made achromatic lenses to accurately correct the focus of the rays at the chemical focus (violet end of the spectrum) and thereby sacrificed the sharpness at the visual focus. In both types of achromatic lens however, the chemical and visual focus were still separated and thus the photographer's knowledge and skill were critical in making an effective picture.[5]


Draper clearly understood the difference between the visual and chemical focus in an uncorrected lens, but perhaps to avoid complications inherent in the nonuniform manufacture of achromatic lenses, he still preferred to employ simple plano-convex flint glass lenses used in combination. All such lenses were interchangeably known as NON-ACHROMATIC LENS, UNCORRECTED LENS, or CHROMATIC LENS. Draper clearly admitted that "The risk of failure by employing an uncorrected lens, is greater than the risk by a good achromatic, or a reflector".[6] Nevertheless, his optical experience in Virginia made him quite skilled in the manipulation of light with uncorrected lenses and he used them successfully to produce daguerreotype portraits.

Uncorrected lenses made of flint glass passed more light than crown glass (and thus more light than achromatic lenses made with crown glass) because the flint glass had a low refractive index and "the least dispersive lenses intercept the greatest number of chemical rays."[7]

Passing more light made a lens "faster" and thus better suited for portraits. Draper perfected his technique ever further by limiting the exposure of his plate to just exactly the time it took for the blue-violet (chemical) rays of light to affect the daguerreotype plate. He stated: "if the plate be withdrawn at a certain period, when the rays that have maximum energy have just completed their action, those that are more dispersed but of slower effect, will not have had time to leave any stain. We work, in fact, with temporary MONOCHROMATIC LIGHT."[8]

Draper's work with chromatic lenses of pure flint glass apparently held a significant advantage, in the hands of a skilled operator, over achromatic lenses made of a combination of flint and crown glass. This advantage would have lasted until at least the spring of 1840, when the French began to produce large achromatic lenses that passed enough light to negate the advantage of flint over crown glass. It was in the spring that Draper himself switched to the use of French achromatics for portraits.


FOCAL LENGTH (FOCUS)
Focal length of a lens is defined as the distance from the optical center of the lens to the plane of focus (film plane) when the lens is focused on an object located at the point of infinity.
--Shortening the focal length increased the amount of light falling on the plate.
--Shortening the focal length allowed greater depth of field (see below). Or as Draper stated: "the camera must be shortened to obtain a sharp picture".
The focal length can be shortened by adding a second convex lens element. Convex lenses cause parallel light rays to converge. Adding a second convex lens caused the converged rays from the first convex lens to converge more. Shortening the focal length in this way also had the effect of limiting the operator to a smaller image (usually ninth-size instead of a whole plate)


DEPTH OF FIELD
is the area of sharpness in a picture--the extent of the scene, from near to far, that will be sharp in the picture. Three variables effect the depth of field.
--The shorter the focal length of a lens, the greater the depth of field.
--The smaller the aperture of the lens, the greater the depth of field.
--The greater the distance to the subject, the greater the depth of field.
Bringing a camera up close to attempt a portrait radically lessened the depth of field. To accomplish a close-up portrait of the human face and not the whole human figure, Draper could not move further back to increase his depth of field. He also could not use a lens of small aperture because it would not pass enough light. Required to use a large aperture lens up close to his subject, his only recourse was to find a way to decrease the focal length in order to get adequate depth of field. He accomplished this by adding a second lens (thus making his lens into a bi-convex). A bi-convex lens shortened the focal length and increased the amount of light on the plate (as explained above).

The term "depth of field" or "depth of focus" was not in general use until about 1860. In 1839-40 the term did not exist. Early daguerreians had to express the concept using different words. William Henry Goode stated: "objects very distant from that to which the focus was adjusted, are not accurately delineated".[9]


F-STOP NUMBER (f /#)
The F stop and its relation toDraper's use of lenses is most clearly explained in a letter to the writer from Irving Pobboravsky:

The f /# engraved on lenses nowadays combines both lens diameter and focal length into a single number indicative of lens speed. The f /# is simply the ratio of focal length over diameter. So when Draper says he used a lens with 14 inch "focus" (focal length) which is four inches in diameter he is talking about an f /3.5 lens -- (14 / 4) = 3.5. Such a lens admits almost 13 times more light than the lens Daguerre specified for the Giroux camera. This is a very fast lens.

His Sept. 1840, Pil. Mag. Article also mentions a double convex objective with a united focus of 8 inches and whose diameter is 4 inches narrowed to 3-1/2 inches. 8 / 3.5 gives an f /2.3 lens, even faster than the f /3.5.

To calculate the relative speed of two lenses, for example the f /3.5 and f /2.3 you square each f/# and take the ratio: (3.5 x 3.5) / (2.3 x 2.3) = 2.3
This means that the f /2.3 lens transmits 2.3 times more light than the f /3.5.

However, this higher speed translates into very poor image quality because these lenses are uncorrected for lens aberrations, and because of shallow depth of field. . . . For example, if an f /2.3 lens of 8 inch focal length is focused on a person's eyes you may find his nose is out of focus. Depending upon the artfulness of the photographer the resulting image could range from impressionism to downright awful. You would of course be capable of getting a correct exposure but you would likely not be happy with the image quality. However, it might be interesting to see the image quality of a daguerreotype portrait made using a pair of biconvex lenses such as Draper described. It might be better than expected"[10]
It is interesting to note that Draper states clearly in his letter of July 1840 to Sir John Herschel that he took his famous portrait of Dorothy Catherine Draper with exactly the pair of 4 inch biconvex lenses of 8 inch focal length (f /2.3) mentioned by Pobborsvsky above. The visual quality of that image has held up well in competition with any extant portrait with possible origin in the spring or summer of 1840.

It might also be interesting to observe the quality of an image taken by a lens that Morse describes in his 20 November 1840 letter to E. N. Horsford. Morse stated that he had prepared a lens (using Draper's principles and instruction) which allowed him to take indoor portraits in 5 seconds. The lens contained five elements as follows: "2 large achromatic 4 inches diameter, a plano convex, a double convex each of 4 inches, and a large double convex 6 inches in diameter"[11] Such a complex lens system may well have approached the optical quality of the sophisticated Petzal lens.


PETZVAL LENS
A lens invented by Josef Max Petzval of Vienna which became available to portrait photographers after 1843. It effectively minimized chromatic and spherical aberration. It became standard for most cameras and was exported all over the world.

Like Draper's lens system, Petzval's increased the amount of light falling on the plate and shortened the focal length by employing two converging lenses instead of one. The two lens elements that Petzval used were a carefully calculated achromatic cemented pair of lenses backed by an achromatic air-spaced pair of lenses.
[12]


DIFFUSED LIGHT
This was a term used by Draper and early operators to denote Indoor light.
The "diffused daylight" basically meant sunlight shining through a window or other controlled opening. This light was then MIRROR-REFLECTED into the subject's face after being FILTERED through some medium that reduced the glare. The medium Draper used to take much of the glare out of sunlight was blue glass. Glass of this color only passed sunlight in the blue, indigo, & violet wavelength range of the spectrum.


ACCELERATOR CHEMICALS
Draper (and Morse through the fall of 1840) probably never used bromide or other accelerator chemicals because as he stated: "in 1840 . . . many excellent daguerreotypes were made, by the use of the ordinary spectacle lens. But at that time our material sensitive to light was iodide of silver only. Iodide of silver is affected very little by other rays than the indigo. The image of the sun from an uncorrected lens on iodide of silver is extremely sharp. When . . . prepared with only an iodide, it is not essential that the camera lens be carefully achromatic. The bromide of silver is, however, decomposed in a larger part of the spectrum, and with it, it is important to work with corrected lenses.[13]


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1.Rudolf Kingslake, "Early Landscape Lenses", Image (March 1955), 21.
2.Ibid.
3.1.Rudolf Kingslake, "First Daguerreotype Lenses", Image (May, 1953), 29.
4.Kingslake, "Early Landscape Lenses", 21-22.
5. Josef Max Petzval, "On the Camera Obscura", London, Edinbrugh and Dublin Philosophical Magazine and Journal of Science (January 1859), 5-14 (hereafter cited as Philosophical Magazine).
6.John William Draper, "Remarks on the Daguerreotype," American Repertory 1 (July 1840): 403.
7. John T. Towson, "On the proper Focus for the Daguerreotype," Philosophical Magazine 15 (November 1839): 381-85.
8.John William Draper, "On the Process of Daguerreotype, and its Application to Taking Portraits from the Life," Philosophical Magazine, 3rd ser., 17 (September 1840): 220.
9.William Henry Goode, "The Daguerreotype and its Applications," American Journal of Science 40 (1841): 137-38.
10. Irving Pobborsvsky to writer, 26 September 1993.
11.Samuel F. B. Morse to E. N. Horsford, 20 November 1840, Eben Norton Horsford Papers, Rensselaer Polytechnic Institute, Troy, New York.
12.William Welling, Photography in America: The Formative Years, 1839-1889 (New York: Thomas Y. Crowell Company, 1978), 39.
13.Towson, "Focus," 381-85.