is the distance between it and the point to which it converges
parallel rays. The rays of light are parallel when they issue from
an object at an infinite distance. For ordinary practical purposes,
any object, that is not nearer than a thousand focal lengths of the
lens may be regarded as at an infinite distance, that is the image
of an object so far off, and the image of the sun or stars (which
are situated at the nearest approach to an infinitely great distance
that we know of) would if separately focussed give an inappreciably
small difference of position of the focussing screen. But no
photographic lens is very thin. The measurement from the back
surface of the lens to the screen, when focussed on a distant
object, is called the "back focus," but this is of no use whatever
except as to the determining of the camera length necessary. The
"equivalent focal length" is the focal length (or focal distance) of
a thin lens that would give the same effect, so far as focal length
is concerned, as the lens in question. When the simple expression
"focal length" is used, it always refers to the equivalent focal
length. The single word "focus" is sometimes used erroneously
instead of "focal length."
The focal length of all lenses (except to a very small extent, with
single or so-called "landscape" lenses) is proportional to the
linear dimension of the image that it gives under similar
conditions. For example, a lens of 6 inches focal length will give
just the same amount of subject on a quarter plate that a lens of 12
inches focal length will give on a whole plate, because the linear
measurement of the whole plate is exactly double that of the quarter
plate. The easiest way to compare the focal lengths of two lenses,
is to focus both on a fairly distant object or view, and to measure
in the image the distance between two fixed points in both cases.
The proportion between these measurements is the proportion between
the focal lengths of the lenses. By this method the focal length of
any lens can easily be determined if one has a lens of known focal
length.
If a lens is first focussed on a distant object, and the focussing
screen is then moved back until the image of any object is of the
same size as the object, the distance travelled by the focussing
screen is exactly the focal length of the lens. It is however
exceedingly difficult to get at the same time an image of an exactly
predetermined size, and to secure the very best definition, so that
it is more convenient to get the image as near as it happens to come
to the size of the object and then to allow for the difference, as
then nothing interferes with the operation of focussing. The best
near object to use is an accurately divided scale, and the details
wanted in addition to those mentioned above are the comparative
lengths of the image and the object. To get these, two fine marks
are made on the focussing screen, and the distance between these is
the length of the image. The scale is focussed with critical
exactness and so that it falls over these marks, then the amount of
the scale represented between the marks can be measured, and the
divisions counted for the length of the object. The distance over
which the focussing screen was moved between the two focussings is
to be multiplied by the length of the object and divided by the
length of the image, and the result is the focal length of the lens.
_Aperture._--The "aperture" of a lens is the diameter of the
cylinder of light that it can receive and transmit. If the diaphragm
is in front of the lens, the hole in the diaphragm is the aperture,
but if the diaphragm is behind a part of the lens, so that the
incident light passes through a lens first, the hole in the
diaphragm is not the "aperture," the "aperture" is larger because
the lens condenses the light before it gets to the diaphragm. The
aperture of any lens can be measured by focussing a distant object,
then replacing the focussing screen by a sheet of cardboard with a
pinhole in the middle of it. In a dark-room a light must be placed
behind the pinhole, and a bit of ground glass held in front of the
lens. A disc of light will be seen on the ground glass and the
diameter of this is the diameter of the aperture, or simply, the
"aperture," with the diaphragm employed.
_Rapidity._--The rapidity of a lens depends almost wholly on its
focal length and aperture. The thickness of the glass makes a little
difference, and at every surface in contact with air there is loss
by reflection, but these and analogous matters are of comparatively
little importance, and as they are uncertain and cannot be
determined it is customary to refer rapidity to the focal length and
aperture only. The aperture found, that is, the diameter of the
effective incident cylinder of parallel rays, should be divided into
the focal length, and the diaphragm corresponding to the aperture
should then be marked with a fractional expression indicating the
proportion of aperture to focal length. Thus if the aperture is one
eighth the focal length, it is marked _f_/8, if a sixteenth _f_/16,
and so on. All lenses with the same aperture as so marked may be
regarded as of equal rapidity whatever their focal lengths may be.
Now the more rapid a lens is the shorter the exposure that it is
necessary to give for any subject, and the exposure required is
proportional to the square of the figure in the expressions as given
above. Namely 8 and 16 squared give 64 and 256 which are as one to
four, the proportional exposures required. Or we may say that 8 to
16 are as 1 to 2 and square these and get 1 to 4 the proportional
exposures.
[Illustration: Fig. 6.]
The best way to mark stops is, for example, _f_/8 and _f_/16, as
these expressions are universally understood, but some persons think
that the relative rapidities or intensities are better, others
prefer to express the relative exposure necessary, and every system
of numbering on these plans has a unit which is merely empirical,
not one of them adopting the only true or scientific unit of _f_/1.
Zeiss has recently changed his unit from _f_/100 to _f_/50.
Dallmeyer marks some of his lenses now with the practical
expression. The following table may be of service to those who
happen to have lenses with their diaphragms marked on any of these
empirical systems.
-------------------------------------------------------------------
| Royal | | | | |
| Photographic | | Paris | Zeiss | Zeiss |
_f_/ | Society. | Dallmeyer. | Congress.| (old). | (new). |
-------------------------------------------------------------------
3.16 | | 1 | 1/10 | | |
3.2 | | | | 1024 | 256 |
4 | 1 | | | | |
5 | | 2.5 | 1/4 | | |
4.5 | | | | 512 | 128 |
5.66 | 2 | | | | |
6.3 | | 4 | 4/10 | 256 | 64 |
7.07 | | 5 | 1/2 | | |
8 | 4 | | | | |
8.66 | | 7.5 | 3/4 | | |
9 | | | | 128 | 32 |
10 | | 10 | 1 | | |
11.3 | 8 | | | | |
12.25 | | 15 | 1.5 | | |
12.5 | | | | 64 | 16 |
14.14 | | 20 | 2 | | |
15.81 | | 25 | 2.5 | | |
16 | 16 | | | | |
17.32 | | 30 | 3 | | |
18 | | | | 32 | 8 |
20 | | 40 | 4 | | |
22.36 | | 50 | 5 | | |
22.6 | 32 | | | | |
25 | | | | 16 | 4 |
27.36 | | 75 | 7.5 | | |
31.62 | | 100 | 10 | | |
32 | 64 | | | | |
36 | | | | 8 | 2 |
38.7 | | 150 | 15 | | |
44.72 | | 200 | 20 | | |
45.2 | 128 | | | | |
50 | | 250 | 25 | 4 | 1 |
54.77 | | 300 | 30 | | |
63.25 | | 400 | 40 | | |
64 | 256 | | | | |
70.71 | | 500 | 50 | | |
71 | | | | 2 | |
100 | | | | 1 | |
-------------------------------------------------------------------
_Image Angle._--The image angle represents what is called covering
power. It may be expressed in terms of the focal length, and
doubtless this is the best method, but it is not customary. It may
be expressed as an angle, the angle formed when a line is drawn from
each extremity of a line equal to the diameter of the circle
covered, and caused to meet at a point distant from the base line
equal to the focal length of the lens. The angle where the two lines
meet is the image angle. But generally the covering power is
expressed more roughly, as the ordinary size of the plate that
sufficiently good definition can be obtained on.
_Tele-Photographic Lenses._--If a negative (or dispersing or
concave) lens is introduced between the ordinary lens and the plate,
the equivalent focal length of the arrangement is greater than that
of the ordinary lens alone, but the length of camera necessary is
not proportionately great. It is possible therefore to obtain an
image of a size that would otherwise require a lens of long focal
length and a corresponding and perhaps impossible length of camera.
But this is not the only advantage, for if the ordinary lens and the
negative lens are separable to a variable extent, the amount of
magnification of the image, or increase in the equivalent focal
length of the optical system, is adjustable at will. For further
details concerning tele-photographic lenses and their use, reference
should be made to Mr. Dallmeyer's pamphlet on the subject.
There are two other subjects connected with the production of images
by photographic lenses that must be referred to, though neither of
them is of great importance if we exclude the use of hand cameras
(which are separately treated of) and bear in mind the ordinary
practice of to-day. These are depth of definition and the distortion
due to the use of single lenses.
_Depth of Definition._--It has already been shown that the action of
the lens is to bring to a point in the image all the light that
falls upon it from the corresponding point of the object. Now it is
clear from fig. 2 that, if different parts of the object are at
different distances from the lens, and this must be the case with
solid objects, these different parts cannot be in focus at the same
time. Still it is possible to get them so nearly in focus that the
result is serviceable, and the ordinary method of doing this is to
examine the image on the ground glass, and if the whole subject is
not sharp enough, to reduce the size of the aperture. Depth of
definition is increased by using a lens of shorter focal length or
by reducing the aperture. If a large aperture has to be used, the
focal length must be short if much depth of definition is wanted, or
conversely, if the focal length must be long the aperture must be
small. It follows that very rapid lenses that have a very long
focus are of no use, for in portraiture, for example, this
combination of properties would lead to the ear in the image being
fuzzy if the eye was sharp.
If a lens were perfect and had a flat field, the depth of definition
would depend only on the aperture and focal length. But if the lens
gives inferior definition towards the edges of the field, it is
quite obvious that there must be less depth of definition there, if
a minimum of defining power is accepted. The definition at its best
may be inferior to the minimum accepted and then obviously there is
no depth. Depth of definition therefore at the centre of the plate
depends entirely on the focal length and aperture, but away from the
centre it depends also on the quality of the lens, and is much
greater in a flat field anastigmat than in a lens of an older type.
But depth of definition is not a quality apart, it depends entirely
upon other factors, and it is better in examining a lens to
determine these factors separately rather than to lump them together
as depth.
_Distortion_ produced by single lenses is due to the fact that the
diaphragm is either in front of or behind them. If the diaphragm is
in front, the image is drawn towards the centre of the plate to an
extent that increases as the margin of the field is approached. A
line along one side of the plate has its ends drawn in to a greater
amount than its centre, because they are further from the middle of
the plate, and therefore it becomes curved like the side of a
barrel, and this effect is called barrel-shaped distortion. If the
diaphragm is behind the lens, the displacement is outwards, also
increasing towards the edges of the field, and a straight line at
the edge of the plate becomes curved so that it is convex towards
the centre of the plate. This is known as hour-glass distortion.
Both these effects are illustrated (and exaggerated for clearness'
sake) in fig. 6, the central square representing the true figure.
This "curvilinear distortion" is absent in all cases in the middle
of the plate and generally for a considerable area, and if single
lenses of only long focal length are used, say of a focal length
equal to at least one and a half times the length of the largest
side of the plate, it may be neglected. Wide-angle single lenses
should never be used except on a suitably small plate, so that the
above conditions hold. The nearer the diaphragm is to the lens the
less is the distortion, and some of the most modern single lenses
have the diaphragm so near that the photographer is even more safe
in the use of them.
[Illustration: MELTON MEADOWS. A. HORSLEY HINTON.]
_The Comparison and Use of Lenses._--The optician when he tests
lenses looks for each fault individually, but this the ordinary
photographer is hardly able to do, nor is it particularly desirable
for him, because if a lens is inferior it matters little to him why
it is so. On the other hand occasion may arise when he wants to
identify a fault, then the information already given will probably
be sufficient to enable him to do so, if to it is added that a small
pinhole with a flame behind it is a convenient point of light, and
that if the image of this luminous point is examined with a good
eyepiece, without the focussing screen, at various parts of the
field, the character of the defect may be discovered.
The main things that the photographer needs to look to in judging of
a lens or comparing it with another, are (1) that it works to focus,
(2) the quality of its defining power especially towards the edges
of the plate. There must also be taken into account the focal length
and aperture, and if both these are not the same in the lenses to be
compared they should be nearly the same, and the proportion that the
aperture bears to the focal length should be exactly the same. A
special diaphragm may have to be cut out of card for one of them.
The best test object that is always at hand is a newspaper pinned
flat against a flat wall. The camera must not be moved during the
work. Each lens is very carefully focussed and a negative made,
using the same aperture, time of development, and in all ways
similar treatment for both. If the focal lengths are different, the
images will be of correspondingly different sizes, and then the same
detail must be compared, not the definition at the same distance
from the centre.
All good lenses work to focus, but some of the cheaper ones do not.
To test this, any series of small objects arranged side by side, but
at distances varying by intervals of say two inches from the camera,
is photographed after carefully focussing on the middle one. If any
other than the middle one is the best defined, the lens is at fault.
But in this, as in all similar tests, it must be remembered that
ordinary dry plates are not quite flat, and the error of the plate
may make an appreciable difference.
The use of lenses comprises the whole art of working with the
camera, it is therefore not our province to say much about it. But
so far as lenses themselves are concerned it may be remarked that,
if a lens has a round field, it may be advantageous to tip up the
lens with regard to the plate when only a part of the plate is being
used, as for example sometimes in taking a landscape. But in using
the modern flat field lenses special care should be taken to keep
the lens and plate exactly true to each other, the plate exactly at
right angles to the lens axis. The image and plate must coincide or
definition will suffer. If the image is rounded and the plate flat,
then in any case the result is only a compromise, but to take full
advantage of the larger apertures when the field is flat, much more
care than has been usual must be devoted to this matter.
_Simple uncorrected lenses_ such as _spectacle lenses_ or
"_monocles_," suffer from the defects that have already been
described, and are valued on this account by some workers because
they give blurred or "soft" images. With a small enough diaphragm
they will give good definition, and generally it may be stated that
reducing the aperture lessens the effect of any fault that a lens
may possess. To get the best definition that a simple lens will
give, the plate must be brought nearer the lens after focussing by
about one-fiftieth of the focal length of the lens, so that it may
be brought from the best focus of visual light into the best focus
of the photographically active light. If the object photographed is
nearer to the lens than about one hundred times its focal length,
the amount of movement after focussing must be increased. If four
focal lengths distant, the correction is nearly one-thirtieth of the
focal length, at three focal lengths distant, nearly one-twentieth,
and at two focal lengths, about a thirteenth.
_Pinholes_ give an image that for all practical purposes may be said
to be equally blurred or "soft" over the whole plate. Much has been
written about pinholes and their use, but it is not definitely known
yet whether the exposure should be longer or shorter than the
exposure required when a lens is used, allowing, of course, for the
smallness of the aperture. The following short table and exposure
rules from the writer's "Science and Practice of Photography," will
probably prove useful:--
+----------------------------+------+------+------+------+------+
| Pinholes--diameters | 1/16 | 1/22 | 1/32 | 1/45 | 1/64 |
+----------------------------+------+------+------+------+------+
| Distance from plate for | | | | | |
| sharpest image ... | 64 | 32 | 16 | 8 | 4 |
+----------------------------+------+------+------+------+------+
All the above figures are in inches. Whatever pinhole and at
whatever distance, estimate the exposure for a lens at _f_/16,
_f_/22, _f_/32, _f_/45, or _f_/64, as the case may be, and multiply
it by the _square_ of the number of inches that the plate is distant
from the pinhole. But if the distance is as given above for any
hole, it is sufficient to expose for as many minutes as the plate is
inches distant from the hole, for a subject that would require one
second with an aperture of _f_/16.
_Chapman Jones._
_Portraiture._
[Illustration]
The photographer who may be expert at landscape or architectural
work, will find himself at a loss when he essays portraiture. For
apart from the art of managing the sitter (a most important element
in producing a successful result), he will soon find that the kind
of plate that is suitable for outdoor work does not answer well for
portraits, unless the developer is greatly modified, for quite a
different kind of negative is required. As a general rule it is
advisable to use very rapid plates for portrait work; and in this
respect, at the present day we are much better supplied than even
five or six years ago, and with an extra-rapid plate it is possible
to secure a fully exposed negative in half a second, in weather and
under lighting that was quite impossible ten years ago. The best
expression and pose are generally secured when the sitter is unaware
of the actual moment of exposure; and for this purpose a silent
shutter working inside the camera is best. The sitter should never
be _asked_ to keep still unless, in groups, and when circumstances
necessitate a long exposure; and nowadays an exposure of five or six
seconds is a long one. Every effort should be made to put the sitter
quite at ease.
A head-rest should not be used unless absolutely necessary, and few
photographers are aware how easily it can be dispensed with, and
fail to realize how strong an objection nearly every sitter has to
it. It is far better to have an occasional plate spoilt by working
without the rest than to make every sitter uncomfortable by its use.
In fact some portrait negatives are actually improved by a slight
movement. In a special kind of lighting when the face is in _shadow_
relieved against a light background, a slight movement which
produces the effect of diffusion of focus greatly improves the
result.
Great care must be exercised in choosing the background even when it
is only plain or graduated, and it is well worth exposing three or
four plates on the same sitter, in the same position and lighting,
and with the same exposure, but with different backgrounds, and then
carefully comparing the resulting prints. Even if only one
background is at hand its depth can be varied by placing it nearer
or farther from the source of light. The background must also be
selected to suit the lighting of the sitter, as a background of
medium tint suitable for what is called "ordinary lighting" would be
quite unsuitable for "Rembrandt" effects, or where strong contrasts
of light and shade are used, when part of the face is in dark
shadow. For such effects a dark background is usually best, as it
gives luminosity to the shadow side of the face. But such dark
grounds are not suitable for "ordinary lighting" where the face
should be full of delicate half-tone, all of which would be killed
by the strength of the dark background.
For the Rembrandt effects a much longer exposure is necessary as
less light is reflected from the face on to the sensitive plate;
they will often need twice or three times as much as for ordinary
portraits.
When pictorial backgrounds or accessories are used it must be
remembered that the object of the photograph is to secure a portrait
of the _sitter_, not to show what a large stock of accessories the
photographer possesses. It is best to use as few accessories as
possible; I have heard a lady complain bitterly of a well-known
photographer, who having posed her in a very difficult position,
kept her waiting for five minutes while he arranged a screen, a
palm, a footstool, a tiger-skin, etc., so that she felt positively
ill before the exposure was made. The sitter should not be kept
waiting in the pose to be photographed any longer than is absolutely
necessary. If accessories must be used they should be simple and
suitable.
When portraits have to be taken in ordinary rooms it is advisable to
get a friend or assistant to experiment upon, if possible
beforehand. Even a few minutes spent in studying the possibilities
of light and arrangement of furniture will save a great deal of
worry when making the actual exposures, and nothing upsets nervous
sitters more than having all kinds of experiments and arrangements
made with them. But it is sometimes well worth wasting a few plates
on exposures which the photographer thinks will be useless, in order
to give the sitter time to get accustomed to the room; it must not
be done in a fussy, irritating way, but rather to show that it is
not such a very dreadful operation and really "doesn't hurt." This
plan often works well with nervous children, who soon become
accustomed to the room and the photographer. There is a great deal
to be said in favour of the maxim "leave your sitters alone." The
photographer must cultivate quick observation so that he sees at
once a good pose, and secures it; and here again quick plates are
essential, as many of the most charming poses are caught
unexpectedly. It has been well said that the best poses the
photographer secures are those he _observes_, not those he
_creates_. But a spontaneous pose may not be quite perfect and a
slight alteration may be easily made without disturbing the rest of
the figure. It will generally be found that a pose that takes a
great deal of arranging is not a success.
When taking portraits in an ordinary room it is usual to place the
sitter near the window, so that one side of the face is strongly
lighted and the other in deep shadow, and then use a white reflector
to light up the shadow side. It is often better, when the window is
a large one, to place the sitter farther back in the room almost
facing the window, and put the camera near the middle of the window
looking into the room; a softer lighting will then be secured. For
outdoor portraits a shady corner is best, and if possible, one where
the side light is much subdued on one side; a light head-shade may
be used with advantage. A large grey rug out of focus makes a good
background; a blanket is too light.