Violet 109
Indigo 47
Blue 48
Green 46
Yellow 27
Orange 27
Red 56
---
360
The sunbeam, the ray of white light, contains powers within it of
which the earlier philosophers had but a faint idea, besides its
accompanying heat. There is a principle associated intimately with
it, which has the power of decomposing and of determining the
decomposition of chemical compounds.
This principle is "Actinism" and is as perfectly distinct in the
nature of its properties from light, as light is from the principle
of heat, with which it is also closely connected.
Actinism may then be considered as the fundamental principle, on
which photography is based, and its power is exerted in forming the
image on the sensitized plate in the Camera, as well as subsequently
in forming or causing the impression on the sensitized paper exposed
to the light beneath the negative.
In this connection we have only to consider the [p087]
application of this Actinism through the medium of a combination of
lenses to form an image on a sensitized plate in the Camera.
It has been shown that when light passes through a prism of glass
its colors are separated; this separation is caused by the unequal
refrangibility of the different colored rays, the violet being the
most and the red the least refrangible of the seven rays.
A ray of light passing through a vacuum progresses in a perfectly
straight line, but all matter, however attenuated it may be, has the
property of refracting or bending the ray of light.
The refractive power of some substances is immense, while that of
others is very trifling. The mode of the refraction depends on the
comparative density or rarity of the respective media. If the medium
which the rays enter be denser, they move through it in a direction
nearer to the perpendicular drawn to its surface; on the contrary,
when light passes out of a denser into a rarer medium, it moves in a
direction farther from the perpendicular. This refraction is greater
or less, that is the rays are more or less bent or turned from their
straight course, as the second medium through which they pass is
more or less dense than the first.
We next study the utilization of this power of refraction in the
manufacture of lenses to overcome the unequal refrangibility of the
colored rays of light.
A lens for use in photography is made of glass as pure and as
colorless as can be procured, and is ground into such a form as to
collect or disperse the rays of light which pass through it. Lenses
are of different shapes, and thence receive different names. [p088]
[Illustration: FIG. 2 E. 1. Double convex. 2. Plano-convex. 3.
Concavo-convex. 4. Double concave. 5. Plano-concave. 6. Meniscus.]
The figures 1 to 6 represent sections of the variously shaped lenses
which are combined for use in photography.
The design in forming lenses is to procure a medium through which
the rays of light from any object may pass and converge to a
corresponding point beyond; the manner in which the rays proceed
through the lens, and then centre in a focal point, will depend on
the form of the lens, its capacity for refraction and the distance
of the object.
The double convex lens may be viewed as a portion cut out of the
side of a sphere. Here, as in all cases of convexity, the focus of
the parallel rays passing through the lens is at the centre of the
sphere. (See fig. 3.)
[Illustration: FIG. 3.]
A plano-convex has only half of the refractive power of the double
convex; the parallel rays, falling on the convex side of the lens,
would converge at the distance of the whole diameter of the sphere.
Thus the focal point at which the rays of light converge is always
regulated by the degree of curvature of the lens. Thus the double
convex lens has the greatest power of converging the rays of light;
the plano-convex has only half the power of the former. Both these
lenses have also the power [p089] of magnifying the image of an
object seen through them in the same proportion.
The double concave and the plano-concave have the power of
dispersing the rays of light and of diminishing the image of an
object seen through them in the same proportion.
[Illustration: FIG. 4.]
The meniscus lens has but a very slightly dispersive power, and the
concavo-convex merely separates the parallel rays to the thickness
of the lens and sends them on parallel as they entered.
All these lenses, having something of the prism in their shape, have
the power to a greater or lesser extent of decomposing the light
that passes through them. This is called chromatic aberration,
because the colored rays do not all converge to the same focus; thus
the image seen through them is surrounded by a fringe or border of
color.
Single lenses lack the power of producing a straight image of a
straight object; the image will have the curve of the lens through
which the light passes to form it; a double convex lens will give a
greater curve than a plano-convex. This is called spherical
aberration.
The main object to be considered in the manufacture of a lens for
photographic purposes is to produce one with the least spherical and
chromatic aberration.
Spherical aberration is overcome to a great extent by the use (in
connection with the double convex) of a meniscus lens.
Chromatic aberration is overcome by the use of two glasses of
unequal density in forming one lens; thus [p090] the front lens
of the portrait combination is composed of a double convex of crown
glass and plano-concave or meniscus of flint glass, which two
glasses are sealed together with Canada balsam.
The forms of lenses which are corrected for chromatic and spherical
aberration will be seen in Fig. 5.
[Illustration: FIG. 5.]
These lenses are termed achromatic, and, although each is formed of
two kinds of glass, they are sealed together so as to be practically
one lens.
Every manufacturer of portrait or view lenses, uses the six forms
shown in the diagram (Fig. 2), in some manner peculiar to himself,
but of the six, four will be found in every combination in general
use, varied in radii, construction and dimension, according to the
use for which they are intended.
Formerly the photographer's choice of lenses was restricted to two
combinations, the double combination for portraits and the single
for views. There have of late years been invented a great variety of
lenses, among which and in the order of invention, probably are
Petzval's Orthoscopic, Harrison's Globe, Ross's Doublet, Darlot's
Wide Angle and Rectilinear Hemispherical, Steinheil's Aplanatic,
Voightlander's Euryscope, [p091] and greatest of all, Dallmeyer's
Patent Portrait, Wide Angle and Rapid Rectilinear Lenses.
The combinations of lenses are three, the single, double and triple;
the latter is now no longer in use, or if so, its use is greatly
restricted. The single combinations have greater focal length than
the double, and consequently at the same diameters larger pictures
are obtainable, and they are principally used for landscape or view
work.
The double combinations, so called from having a second pair of
lenses behind the first, which have the effect of shortening the
focus about one-half, whereby the action of the light is
accelerated, and both the spherical and chromatic aberrations more
perfectly corrected, which result in an image more delicate in
definition and more rotund in form, thus peculiarly qualifying them
as portrait lenses.
In the selection of lenses for studio or view work, the intending
purchaser, if desirous and pecuniarily able to avail himself of the
best, will naturally inquire what make of lenses is the most widely
known and used, and it will not take much time to procure a
satisfactory answer to the question.
It has been conceded now for some years, both in Europe and in
America, that the lenses manufactured by J. H. Dallmeyer, of London,
England, are superior to all others, not only for their fitness for
the work for which they are specially constructed, but for their
adaptability to work beyond anything claimed for them by the maker,
and also for a certain undefinable and æsthetic quality inherent in
the negative made by these lenses.
The fact that there is not in the wide world a photographic [p092]
establishment of any note that does not possess one or more of
these lenses is strong evidence of their superiority. In the quality
of the glass used, in the perfection of finish and adjustment, in
softness, crispness and depth, in rapidity, illumination and every
quality that recommends a lens, the Dallmeyer lenses are unrivalled.
The portrait combination now in general use, was first constructed
from calculations made by Professor Petzval, of Vienna. Its optical
components are, a front crown lens of unequal convex curves to which
is cemented a double flint lens of unequal concave curves. The back
combination is a crown lens of unequal convex curves and a
concavo-convex flint lens at a little space from it. (See Fig. 6.)
[Illustration: FIG. 6.]
For more than a quarter of a century this form of lens had been used
without material change in its construction, until Mr. Thos. Ross,
by a modification of the curves, succeeded in flattening the field
and increased its rapidity by shortening the focus, but left it with
the peculiar shallowness of focal depth, especially in the larger
sizes, which has been the torment of photographers to this time.
Mr. Dallmeyer was the first to improve upon the [p093] original
portrait combination, and in his new Patent Portrait Lens he has
most ingeniously obtained a diffusion of focus at the will of the
operator. By a quarter or half turn of the cell of the back
combination the focus is diffused, giving an agreeable softness in
place of the shallow plane of excessive and wiry definition so
familiar to the photographer.
[Illustration: FIG. 7.]
The diagram, Fig. 7, shows a section of Mr. Dallmeyer's New Portrait
Lens. There are two actinic combinations, of which the front
resembles the Petzval lens; the back combination differs as regards
the ratio of radii of the lenses used, the crown being a deep
meniscus and the flint a deep concavo-convex, with their adjacent
surfaces dissimilar; their positions also are reversed, the
concavo-convex of flint occupies the external position, instead of
as in the Petzval, and this lens being mounted in a cell capable of
being unscrewed supplies the means of regulating the spherical
aberration of the system at will. The lower portion of the diagram
[p094] exhibits a plan of the mount of the back flint glass lens;
this cell admits of being unscrewed, one or more parts of
revolutions of the screw indicated by an index and divisions; with
this back lens screwed home this combination has all the good
qualities of the old form of portrait lens, but with a flatter field
and wider illumination.
There are three distinct classes of portrait lenses. The first are
lenses of large diameter and aperture compared with their short
focal length. In this class the greatest rapidity is obtained at the
sacrifice of flatness of field. Of this class are the B and C lenses
of Dallmeyer.
The second class are lenses of equal diameter and aperture with
those just described; but with about double their focal length, and
therefore less rapid, but with more field and wider illumination; of
this class are the A series.
In the third class are long focus lenses, which, at three inches
diameter have fifteen inches focal length; with the result of a
larger and flatter field; they are, however, necessarily slow, but
well calculated for out-door views, groups and copying. Of this
class are the D series, which, since the introduction of the rapid
dry plate, have become available for ordinary portrait work in the
studio.
[Illustration: Fig. 8.]
Fig. 8 represents the Dallmeyer Wide Angle Rectilinear Lens. It
consists of two cemented combinations, [p095] each composed of a
deep meniscus crown and a deep concavo-convex flint glass lens;
between the two, dividing the space in the proportion of their
respective diameters, is placed a revolving diaphragm, the largest
aperture of which is f/15; the position of the stop being nearer the
back combination avoids the central spot or flare.
This lens embraces an angle of 90 to 100 degrees; it is quite free
from distortion, and particularly adaptable for taking views in
confined situations, such as interiors, views in narrow streets, &c.
Being a double combination its work is more finished and round than
that by the single lens.
[Illustration: FIG. 9.]
[Illustration: FIG. 10.]
Probably the two greatest rivals to the Dallmeyer Wide Angle Lens
are the Actinic Doublet of Ross (Fig. 9), and the Steinheil
Aplanatic (Fig. 10). The Ross Doublet consists of a crossed crown
lens, cemented to its correcting flint lens, which is a crossed
concave, the whole forming a deep meniscus the focus of which is
equal to the back combination or about double that of the equivalent
focus of the complete instrument, the posterior meniscus combination
consists of a meniscus crown lens cemented to a concavo-convex flint
lens. The two combinations are mounted rigidly with a rotating disk
of [p096] diaphragms or stops placed midway between the lenses.
The Steinheil consists of a front and back corrected combination of
precisely the same shape, mounted rigidly and having a rotating disk
of apertures placed midway between the lenses.
These are three distinct types of view lenses of wide angle, and a
comparison of the diagrams will show the wide difference in the
construction of the two latter from the Dallmeyer; and while they
stand unrivaled for the perfection and beauty of their work, which
is about equal, the Dallmeyer obtains a great advantage over the
others in the thinness of its lenses, which enables it to work with
greater rapidity, and indeed it has been used with great success
with the drop-shutter for photographing moving objects and other
similar out-door work.
[Illustration: FIG. 11.]
While the possession of a wide angle lens is indispensable in the
varied selection of out-door subjects, yet for many purposes it has
been found they are unsuitable; and to meet this want Mr. Dallmeyer
has constructed a modification of the wide angle, which he calls
Rapid rectilinear. Its construction is shown in Fig. 11. The lenses
of the front and back combination have the same general form as
those of the wide angle, but they are of smaller diameter, being
constructed for angles of 60 to 70 degrees only. It is four times as
rapid, and is in fact an aplanatic and symmetrical lens, and may be
regarded [p097] as the most perfect lens extant. As it admits of
the use of a larger aperture it is well adapted for interiors, where
there is space for its use, and for almost every purpose of out-door
photography, requiring short exposure and no greater angle.
The use of this lens has been greatly extended since the
introduction of the rapid dry plate, which admits of its employment
as a portrait lens in the studio, and in fact it has become a very
popular instrument for portrait work, especially for the larger and
life sizes, so that there is a strong probability that before many
years the Rapid rectilinear and the D series of lenses of Dallmeyer
will be the most useful and the most profitable lenses for portrait
work in the studio that have ever been constructed, unless the near
future shall give us something not now thought possible in optics.
Dallmeyer lenses are sold only by E. & H. T. Anthony & Co., who are
the agents in this country, and as these lenses are quite expensive,
and are indeed beyond the means of many, the Messrs. Anthony keep
other and cheaper lenses, which are good of their kind, both for
portrait and for out-door work, among which are the E A lenses for
portraits, and the Platyscope and other lenses for views, etc.
To those desiring advice on the choice of a lens, this much may be
said. If it is intended to procure a Dallmeyer, it is only necessary
to know the limit in size of the work to be done. If in portraiture,
you select a lens that will cover the size of plate you intend to
use. If in landscape, or architectural work, or copying, consult the
catalogue and order the lens that meets the requirements of the
case. There is said to [p098] be no difference in these lenses;
that is, all lenses of the same series and size are exactly alike,
the glass is of the same density, ground to the same curve, and
polished to the same degree of fineness, so that each one is as good
and no better than any of the others of the same kind and size. It
is the same with the view lenses; one may order with confidence, and
not be disappointed by receiving an inferior lens, while expecting
the best in the world.
It is not so, however, with any other make of lenses; at least I
have never heard such a claim made on the part of any other maker,
hence for the selection of a lens by any other maker, some thought
and experiment are justifiable and even necessary. Under such
circumstances, the following method of testing a lens will be found
useful.
When of several lenses of the same size it is desired to select the
best, attach one to a camera box and focus it on some long object
placed parallel with the axis of the lens; adjust the focus so that
the part of the object nearest the lens shall be in focus, but near
that point where it would begin to lose sharp definition. Mark the
position of the carriage on the rail, then turn the focus forward so
that the same point will be as near the other extremity of sharp
definition, then mark the position of the carriage on the rail and
note the distance between the two marks, which will indicate the
depth of sharp focus. Now stretch a line across the room and focus
the instrument on the centre of the line, and note to what distance
on each side of the centre sharp definition extends; this will
indicate the flatness of the field. Try all the lenses in the same
manner, and secure the one that has the greatest [p099] depth of
focus and the greatest extent of sharp definition on the horizontal
line.
Expensive lenses should be treated with much carefulness. They
should be kept free from dust and dampness, and should never be
cleaned or rubbed with cloth or the handkerchief; nothing but fine
chamois should be used to clean a lens, the fine polish of which
(one important factor in its usefulness and value) is so easily
injured by abrading its surface with anything of a fibrous or gritty
nature.
The lens should always be covered when the day's work is done and
while the studio is being cleaned up, to exclude dust, etc., from
the glasses.
Examine the lens every morning to see if the glasses are at all
dimmed by damp or dust, and if so, use the chamois. You must not
expect good work from lenses whose surfaces are in any way dimmed.
Study your lens with care, and learn all its good and weak points,
and so enable yourself to take every advantage of instrument, light
and pose in making a sitting.