MATERIALS OF PHOTOGRAPHY
He who builds a house, manufactures an automobile, works the farm,
or produces a picture, must have certain materials and tools to work
with. In the case of photography the tools consist of the camera and
its accessories, the sensitive films or plates, chemicals and other
supplies.
TYPES OF CAMERAS
_Box Cameras_, when reduced to their simplest form, consist of a light
tight box with a lens at one end and a place for the sensitive film at
the other. Such cameras are usually fitted with a fixed focus single
(meniscus acromatic) lens, one or two ground glass view finders, and
a simple type of shutter. Also, there may be a system of two or three
“stops” for regulating the amount of light admitted by the lens.
A camera of this type is well suited for children and for the beginner.
There is nothing much to get out of order, the camera is ready on the
instant for whatever photographic subject may turn up, and the pictures
obtained compare very favorably with those produced by the most
expensive instruments. However, such cameras have their limitations.
Snapshot exposures are only possible in good light when the sun is
shining on the subject; objects nearer than about 10 feet cannot be
photographed unless an auxiliary lens is used over the regular lens,
and the camera is not so convenient for carrying as a folding type. The
Brownie is one of the best known of box cameras.
_Folding cameras_, such as the Kodaks, are preferred by the great
majority of workers. These range from the very small folding models of
vest pocket size taking pictures 1⅝ × 2½ inches, through the 2¼ × 3¼,
2½ × 4½, 2⅞ × 4⅞, up to the 3¼ × 5½, which is one of the most popular
sizes.
Cameras of this type ordinarily have rapid lenses and many are fitted
with the highest type of anastigmat lenses. Brilliant view finders
are usually furnished as the image is much more clearly seen than in
a ground glass finder. They are ordinarily fitted with shutters which
in addition to time and bulb action may work at automatic speeds from
one second to perhaps ¹/₂₀₀. This makes it possible to get snapshots
of subjects under conditions that would be out of the question with
the box cameras with their slower lenses and shutters. Some of these
cameras are provided with an autographic feature so that data can be
made on the film at the time of the exposure. Some are fitted with
combination backs so that accurate focusing may be done on the ground
glass on the back. Most styles use roll film although some are fitted
to take film packs and cut film.
_Reflecting cameras_ fitted with focal plane shutter, are the type
preferred by many advanced workers. A mirror reflects the image onto
a ground glass, full size and right side up where it is focused and
remains visible in a hood until the very instant of exposure, and thus
it takes all the guesswork out of composition, lighting and focusing.
Some focal plane shutters work as slow as ⅕ second. Such an exposure
is very useful for home portraits indoors where the light is good.
Focal plane shutters also permit of extremely rapid exposures. Cameras
of this type are used when photographing automobile races, athletic
events, etc. Naturally a reflecting camera is somewhat larger and
heavier than an ordinary folding camera.
There are also many scientific, commercial and special purpose
cameras, among which might be mentioned stereoscopic, telephoto,
photomicrographic, enlarging, copying and reducing, lantern slide,
identification, finger print, cirkut, panoramic, studio, view and
motion picture cameras.
Practically all amateur work today is made with small film cameras.
In many of the amateur photographic exhibitions large pictures will
be seen but in most cases the foundation of these pictures was a
small film negative. In selecting a camera it is well worth while to
consider the fact that any camera, even the cheapest will make fair
pictures under favorable conditions. It is only when convenience in
regard to compactness and ease of operation along with the ability to
fully expose and make microscopically sharp pictures under unfavorable
conditions are desired, that the more complicated and expensive models
are of superior advantage.
LENSES[2]
In order properly to understand the purpose and use of the lens it
is necessary to know something about the nature of light. Light
ordinarily travels in a straight line but when a beam falls on a
polished surface such as a looking glass it is reflected back in the
same way that a rubber ball striking the ground rebounds. It is this
_reflected light_ from objects, that reaching our eyes enables us to
see them, and reaching the lens of the camera enables us to photograph
them.
[Footnote 2: For a more exhaustive discussion, see Harting: _Optics for
Photographers_.]
As in the case of sound where low pitched notes are caused by a small
number of vibrations per second and high pitched ones by a great number
so in the case of light the red is produced by long wave lengths and
violet by short ones. Between these are all the other colors from red
to violet—orange, yellow, green, blue. There are wave lengths shorter
than violet called ultra-violet, which are not visible but are very
active in affecting a photographic emulsion. White light is made up by
all colors.
_Refraction_ is another property of light which very directly concerns
our discussion of lenses. When a stick is placed in water at an angle,
it appears to be bent at the surface. The explanation of this is that
in a dense medium, light travels slower than in air. When the beams
strike the surface one part reaches the surface and is slowed up while
the other part is still in the air and traveling faster, thus causing a
change in direction. The same thing taking place in glass explains the
action of light in passing through the lens.
PINHOLE LENS. The simplest lens which we could use would be a small
hole. If we take a sheet of cardboard and make a hole in it with a pin
and then in a darkened room, hold the cardboard between a sheet of
white paper and an electric lamp we shall see on the paper an image of
the lamp’s filament. The same principle is applied to the making of
pinhole photographs. A very fine smooth hole is made in a piece of thin
brass or tin-foil and this is used in place of the lens. The diagram
(Figure 1) shows how the image is formed by the use of the pinhole.
[Illustration: Fig. 1]
Remarkably good pictures can be made with nothing but a pinhole for a
lens. No object is perfectly sharp but all are equally sharp and the
result may be very fine pictorially. However, the exposure is very
long, taking from a few seconds up to several minutes with the object
in good bright sunlight even when a very fast film is being used. As
the pinhole is made larger so as to admit more light, then the image
will become more and more indistinct until no picture is formed at all.
FORMATION OF AN IMAGE BY A LENS. When a ray of light passes through a
prism it is bent as it enters the glass and again as it leaves it. The
lens works very much the same as two prisms.
[Illustration: Fig. 2]
The lens is used in the front of the camera for the purpose of so
directing the rays of light that they will form an image of the object
being photographed on the sensitive film or plate in the back of the
camera. Figure 3 shows how the image is formed by the lens. It will be
noticed that parallel rays of light are bent so as to go through F (the
principal focus) and that rays passing through the center of the lens
do not change in direction. This determines how far away the image will
be from the lens. If the subject being photographed is 100 feet from
the camera, the image will be formed a fraction of an inch closer to
the lens than will be the image of an object only 10 feet away. This is
shown by dotted lines in the diagram.
[Illustration: Fig. 3]
_Focusing_ is the bringing of the lens to the proper distance from the
film so that the image will be sharp, and as explained this is changed
according to how far away the object is which is being photographed.
This distance from the optical center of the lens to the focal plane
or position of the films when focused on a distant object is termed
_focal length_. When choosing a camera and lens for ordinary usage it
is well to select one having a lens of a focal length at least equal to
the diagonal of the negative. A short focal length lens gives a small
image. For this reason the worker is inclined to approach close to the
subject in order to get a larger image. In such a case any portion of
the subject such as a foot or hand of the person projecting a little
toward the camera, will appear much too large in proportion to the
remainder of the subject. This is often noticeable in amateur pictures.
For instance, the focal length of a lens on a post card size camera
should be from 6½ to 7 inches.
KINDS OF LENSES. The _meniscus lens_ is the simplest type, being made
from a single piece of glass. Such lenses are used only on small fixed
focus cameras. When a beam of light is passed through a prism it will
be noticed that a band of color is formed. This is because the longer
wave lengths such as the red colors are bent less than the shorter ones
such as the violet. A meniscus lens will not focus the various colors
in the same plane and therefore is used only on the very cheapest
cameras.
A _meniscus achromatic_ lens is more fully corrected, being made from
two kinds of glass, flint and crown, and overcomes the difficulty
just mentioned. Such lenses are used on most of the box cameras and
the less expensive folding models. These lenses are made to be used
only with rather small openings and so the user is limited to making a
snapshot exposure in good bright sunlight. A lens such as this is very
fine for landscape work or portraiture but if the subject is a building
or other object containing straight lines, the lines will appear
slightly curved if they come near the edge of the picture. If the stop
opening is in front of the lens the lines will curve outward, but if
behind it then they will curve inward.
Double lenses overcome this defect by combining two single lenses, one
in front and one in back of the stop, so as to get straight lines to
photograph as such, clear to the very edge of the picture. These lenses
can be used at much larger stop openings in proportion to focal length
than the meniscus achromatic or single lenses, and hence the name for
the old type of double lens—“rapid rectilinear.” These older double
lenses still have one defect known as astigmatism or the inability to
photograph both vertical and horizontal lines sharply to the very edge
of the picture.
About 30 years ago, Professor Abbe and Otto Schott working together
at Jena, found how to produce new kinds of optical glass from which
lenses could be made that would give flat field images with the blue
and yellow rays at the same focus. By the use of these new glasses the
opticians have been able to make lenses that give sharp images and a
flat field to the very edge of the picture. These lenses are called
_anastigmat_. Better defining power, however, can only be obtained
by the most careful and skilled work when making the lens, this work
being of a far higher quality than that applied on the older types
of lenses, which accounts for the higher cost of anastigmats. They
can be used with larger stop openings and thus it is possible to make
snapshots on days when the sun is not shining and also to get slow
snapshot exposures near a window if the light is good.
The “_Kodar_” lens is a recent development intermediate in type between
the old “rapid rectilinear” and the anastigmat. It has many of the
practical advantages of the latter though it is an inexpensive type of
lens.
There are several types of lenses which are used for special purposes.
Two of the most common are wide angle lenses and telephoto lenses.
_Wide angle lenses_ have a very short focal length for the size of the
film or plate covered and are especially useful for photographing in
confined positions, such as in a room where it is desired to include
the greater portion of it. They are also rather necessary when trying
to photograph buildings on the opposite side of a rather narrow street.
The perspective is rather poor when working up close, as nearby objects
appear too large in proportion to those more distant. The room looks
much larger than it really is. Wide angle lenses must always be used
with a smaller stop opening than those of ordinary focal length. For
example, the Bausch & Lomb Medium Wide Angle Protar Lens Series IV has
a maximum speed of f.12.5. The extreme wide angle Protar lens made by
the same firm has its largest stop opening at f.18. The Goerz Hypergon
lens has only a maximum opening of f.22, but this lens embraces an
extremely wide angle of 135 degrees. The lens on the average hand
camera includes an angle of perhaps 42 to 49 degrees.
_Telephoto lenses_ on the other hand are most useful for photographing
objects that are so far away as to appear too small with a normal lens.
Not only are they useful for photographing animals, birds, and other
wild life but are useful for taking pictures of persons or buildings
that are not usually accessible. As the light reflected from the
subject must cover a larger proportion of sensitive film, it naturally
follows that the exposure would be longer than when no telephoto
lens was used. A steady tripod must be used as the least vibration
will spoil the picture. A longer bellows draw is generally necessary
although some of the new fixed magnification telephoto lenses do not
require a long bellows extension for the equivalent focal length.
_Soft focus lenses_, an example of which is the Wollensak Verito f.4
lens, produce an image which is very pleasing for certain types of
pictorial and portrait work.
STOPS AND SHUTTERS
When using a camera, it is very necessary to know something about
_stops_ or diaphragm openings.
A very instructive experiment for the beginner, if he has a camera
with a ground glass focusing screen, is to set it up in front of some
well illuminated object and focus it on the ground glass screen by
moving the lens nearer or further from the ground glass. A coat or
other dark cloth should be thrown over the head of the operator and
back of the camera to exclude the light. When the shutter is set so as
to remain open and the largest stop opening is used it will be noticed
that there is a brilliant image of the subject on the ground glass but
that it is upside down. If the picture being examined is a landscape
or other subject containing objects at different distances, it will
also be noticed that those at a certain distance away will be sharply
defined whereas others nearer and further away are somewhat diffused or
blurred. In other words, they are out of focus.
If the size of the stop opening in front of the lens is decreased, it
will be noticed that the image of objects nearer and farther than the
one focused on becomes sharper, and that the image is not as bright as
before. Thus we see that the two principal functions of the stop are to
increase or decrease the depth of focus and to regulate the amount of
light admitted to the sensitive film. This last function makes it an
important factor in exposure. In the case of portraits it is usually
better to have only the subject sharp for if the background were also
sharp it would detract from the interest of the subject. For landscapes
it is more necessary to have the entire picture sharp.
The amount of light reaching a given portion of the sensitive film
depends not only on the size of the stop opening but on the distance of
the lens from the film. With longer focal length lenses the distance
is greater and therefore the light spreads over a larger area. Thus it
is less concentrated, and so the exposure needs to be longer with the
same actual size stop opening. The diaphragm or stop opening is always
referred to in terms of its proportion to the focal length. In one
system the stop is expressed simply as a fraction of the focal length.
Thus, f.8 means that the diameter of the aperture or opening is ⅛ of
the focal length. For example, if the lens opening or stop is ½ inch
in diameter and the focal length is four inches, then the stop is f.8.
Again, if the focal length is 8 inches and the stop opening is one
inch the ratio would be the same, and the stop would be marked f.8.
In the latter case four times as much light would have been admitted,
yet it would have had to cover four times as much space thus giving
equivalent illumination to the film. The comparative exposure of two
stops varies with the square of their f. values, the larger numbered
one being the slower and requiring the longer exposure. In the U.
S. (Universal System), the stops are so marked that each succeeding
smaller one requires twice the exposure of the next larger.
The following table gives a comparison of the two systems:
F. f.4 f.5.6 f.6.3 f.8 f.11 f.16 f.22 f.32 f.45
U.S. 1 2 (2½) 4 8 16 32 64 128
Most of the rapid rectilinear lenses have U. S. numbers. Anastigmat
lenses usually have f. markings. The single lenses are more often
marked with three or four numbers, 1, 2, 3 and 4, of which each
succeeding larger numbered stop usually admits about half as much light
as the next smaller and therefore requires approximately twice the
exposure.
The _shutter_ is a mechanical device for controlling the length of the
exposure. One of the simplest forms is shown in the following diagram:
[Illustration: Fig. 4
Fig. 4 shows the position of slide B when the smallest stop is in front
of the lens.]
This form is known as a rotary shutter and may give snapshot exposures
of approximately ¹/₂₅ of a second. An instantaneous exposure is made
by pushing lever “C” across in one direction only. Ordinarily the
largest of the three stop openings (controlled by slide “B”) is used
for snapshots in good light, the middle one for snapshot exposures of
very distant objects and for time exposures of interiors. The small one
is more often used for time exposures outdoors. To make a time exposure
slide “A” is pulled up and then when lever “C” is pushed across in one
direction the shutter will open and remain open until lever “C” is
pushed across in the opposite direction. Figure 5 illustrates one of
the more comprehensive types of shutters. The size of the stop opening
is controlled by moving “B” to the proper f. value.
[Illustration: Fig. 5]
FILMS AND PLATES
Photography begins with the chemical action of light on compounds
of silver, particularly those of chlorine and bromine. This action
is familiar to anyone who has added a solution of silver nitrate to
a sodium chloride solution and watched the resulting precipitate of
silver chloride darken after a short exposure to sunlight. Briefly,
films consist of a base or support of either glass or transparent
cellulose on which is coated an emulsion which holds in suspension
some sensitive silver salts. Silver bromide is the principal sensitive
material in films and plates. This, together with the gelatin
which is added to keep it from settling, makes up the emulsion. The
different degrees of sensitiveness are obtained by varying the amount
and duration of heat to which the emulsions are subjected during
manufacture. Other substances may be added which also affect its
sensitiveness.
The silver bromide or other compound in the emulsion is so sensitive
to light that an exposure in the camera for but a very small fraction
of a second under good lighting conditions, is sufficient to produce
a developable image. The emulsion is composed of innumerable grains,
perhaps from 10 to 30 billion per square inch, lying several deep.
The number acted on during exposure depends on the intensity of the
light and the duration of its access to the film. The light action is
not visible after exposure but every grain that was at all affected
during exposure can be developed. This is explained later in Chapters
III and IV. It naturally follows that if the sensitive film is exposed
to light at any time other than when the picture is being made, for
even the smallest fraction of a second before being developed, it will
be completely fogged as all of the grains of silver bromide will be
affected and when developed will be completely dark.
In addition to speed another characteristic of modern films and plates
is _latitude_. This means that the exposure can be varied considerably
without appreciably affecting the final picture. Thus, if the least
correct exposure was ¹/₁₀₀ sec. this could perhaps be increased to
¹/₂₅, or four times as much, without affecting the final result except
that the negative would be denser and require a longer time in printing.
Films or plates with very slow emulsions are more often used for
copying and other work where it is desired to have considerable
contrast and where speed is no object. The most rapid emulsions
are necessary where extremely short exposures under ideal lighting
conditions must be given, as for athletic events, and for slow snapshot
exposures in poor light.
Ordinary plates and films are sensitive to the shorter wave lengths of
light such as the ultra-violet, the violet and the blue, but little if
any to the yellow and red. For this reason a blue flower or similar
subjects will photograph so as to appear almost white, whereas a yellow
or red one will appear nearly dark as the light reflected will not
affect the film to any extent.
To counteract this some plates and most films are made somewhat _color
sensitive_ or orthochromatic by the incorporation in the emulsion of
certain dyes which makes it less sensitive to blue light and more to
the yellow-green rays.
For the exact rendering of reds and other colors, plates and films are
sometimes still more fully corrected and these are termed panchromatic.
While they are sensitive to all colors, yet they are somewhat more
sensitive to the blues than the reds, and therefore a filter is often
used.
The _filter_, which may be a piece of colored gelatin between two
pieces of glass, is placed over the lens when photographing certain
colored objects, and its function is to absorb or hold back the blue
rays or certain other ones as desired. This will give time for the
yellows and reds to affect the film and thus a denser deposit of silver
will be obtained which will result in that part of the print being
lighter. As the filter holds back some of the most actinic light, it
is evident that the exposure will have to be longer than when no
filter is used. The length of the exposure will depend upon the type
of sensitive emulsion being used and the kind of filter. A variety of
filters can be obtained which will absorb different colors and thus
it is possible to have any given color photograph lighter or darker
if desired. For full correction so that all colors will photograph
to appear relatively as dark as they should, it has been found that
Eastman Commercial Panchromatic Film and a Wratten K-3 filter will
produce the desired result.[3]
[Footnote 3: Discussed more fully in _Color Films, Plates and Filters_.
Eastman Kodak Company.]
PRINTING PAPERS
Printing papers are sensitive in the same way as films or plates except
that the emulsion is slower, and is coated on a paper base instead of
glass or cellulose. The more rapid enlarging papers have a slow bromide
emulsion while ordinary contact developing printing papers are usually
silver chloride. The various contrasts and surfaces will be discussed
in the chapter on printing.