LENSES.
[Sidenote: Optics.]
We do not intend to incorporate in this chapter elementary optics, as
the subject is well known to most educated men, but in case any reader
should know nothing of light and optics, we recommend him to get
[Sidenote: Ganot’s Physics.]Ganot’s Physics, and thoroughly master at
least the paragraphs of Book VII., on “Light,” that we enumerate
below.[10] This may seem a little formidable, but our reader will find
that with a very simple knowledge of mathematics he can easily
understand all the sections marked, and it is our opinion that light and
chemistry should be studied directly from systematic text-books that
treat of those subjects. In the Appendix we shall refer to some
additional books which we consider advisable for the student to read,
but for the present we strongly recommend him to thoroughly master the
parts of Ganot that we have cited, and to avoid all other desultory
reading until he has done so.
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Footnote 10:
Namely, paragraphs 499, 500, 501, 502, 503, 504, 506, 508—the Laws of
the Intensity of Light, 509—Photometers, Rumford’s and Bunsen’s, 510,
511—first proof only, 512, 513, 514, 518, 519, 524, 525, 528, 533,
536, 537, 538, 539, 540, 542, 543, 544, 551, 552, 554, 555, 556, 558,
564, 565, 566, 567, 568, 569, 570, 571, 572, 573, 574, 575, 576, 579,
580, 581, 582, 583, 584, 602, 604, 612, 615, 616, 617, 618, 619, 620,
621, 625, 626, 627, 628, 629, 631, 632, 634, 635, 636, 637, 639, 640,
641, 645, 646, 650, 652, 655, 656, 659, 661, and 664.
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Far too much time has been given, and far too much importance has been
hitherto attached, to the subject of optics in connection with
photography. Much time and expense would have been saved had the
pioneers of photography had good art educations as well as the
elementary knowledge of optics and chemistry which many of them
possessed, for without art training the practice of photography came to
be looked upon purely as a science, and the ideal work of the
photographer was to produce an unnatural, inartistic and often
unscientific, picture. It is, indeed, a satire on photography, and a
blot which can never be entirely removed, that at the very time the
so-called scientific photographers were worrying opticians to death, and
vying with each other in producing the greatest untruths, they were all
the while shouting in the market-place that their object was to produce
truthful works. At length, when the most doubly patented distorting
lenses were made to meet their demands, they, with imperturbable
self-confidence, presented a sharp, untrue photograph, insisting upon
its truth. “A truer picture,” said they, “than drawing;” “truer than the
eye sees,” some said. In short their picture was absolutely perfect.
When a lens giving a brilliant picture, with all the detail and shadows
sharp, and the planes all equally sharp, was at last produced, the
scientists were _in excelsis_. But, alas! they proved themselves as
unscientific as they were inartistic! Had they but taken up their
simplest form of lens and used it as a magnifying-glass, they would have
seen immediately that all was not right, and instead of clamouring for
the artistic falsities of “depth of focus,” “wide-angle views,”
“sparkle,” and the other hydra-heads of vulgarity, they might have set
to and made the lens which was required. It was but a simple thing that
was required.
[Sidenote: Dallmeyer’s long-focus landscape lens.]
The question then arises—What is the best lens for artistic purposes?
That lens is _Dallmeyer’s new long-focus rectilinear landscape lens_.
This summer (1888) we used one of these lenses and were delighted with
it.
[Sidenote: Why this lens is the best.]
Why is this the best lens for our purpose? is the question that
naturally arises. It is the best because being what is called a
long-focus lens, it cannot be so ignorantly employed as can lenses of
shorter focus, there is no appreciable marginal distortion, and with
open aperture the outlines of the image are softly and roundly rendered,
and in addition the relative values seem to us to be more truly rendered
by it.
[Sidenote: Best focal length to use.]
This lens then being, as we think, the best for artistic work, the next
question that arises is what focal length of lens must we use to get the
best results. The student will be told _ad nauseam_ that if he places
his eye at the distance of the focal length of the lens from the
photograph he is inspecting, all will be well. Such, however, is _not
always the case_. He may prove it for himself by taking a lens of short
focus and photographing any suitable object placed too near to him, and
he may then place his eye at the distance of the focal length, and if he
be an artist, he will immediately detect that the drawing is false, and
the distance is dwarfed and pushed together as compared with foreground
objects, whilst in a true drawing the proportions must be true between
the foreground objects and distant objects. This misuse of the lens is
what leads to the production of so many photographs false in drawing,
and it is evident that since many of these falsely drawn photographs
have been and are a basis for many scientific purposes, the deductions
based upon them will have to be reconsidered.
[Sidenote: Experiment for
finding a
rough
rule for
the use of
lenses.]
The next question is, what proportion, as a rule, should the focal
length of the lens bear to the base of the picture to give approximately
true perspective delineation? This proportion should be as two to one,
that is, the focal length of the lens should be as a rough working rule
twice as long as the base of the picture. We arrived at the result by
making a series of drawings on the ground glass of the camera, and
comparing them with a perspective drawing made upon a glass plate.
Opticians have arrived at the same conclusion, for we find this is the
rough rule stated by Mr. Dallmeyer in his “Choice Lenses.”
[Sidenote: Comments.]
The falsity of the statement that photographs are always true—a
statement that has been in vogue from the earliest photographic days—is
then apparent. [Sidenote: False drawing producing false tonality.]It
will now be obvious why some lenses make ponds of puddles, and otherwise
falsify the landscape. This fact would have long ago been noticed had
artists always seen the landscape from which the photograph had been
taken. Another thing which a wide-angle lens, if wrongly used, does, is,
in the case of a picture with clouds, to draw down and crowd together
the clouds, and define them more sharply than the eye sees them, so that
when the negative is printed they appear too strong in value, and the
whole picture is thrown out of tone, and is therefore false and
inartistic, even if the lens be correctly used; this fault is generally
present in pictures taken with these lenses.
[Sidenote: Lenses recommended.]
It will be seen from our remarks, therefore, that the only lens we
recommend for artistic work is Dallmeyer’s new rectilinear landscape
lens. At least two of these should be obtained of different focal
lengths, one of which is advertised to cover a plate a size larger than
that used by the photographer, and the second to cover the same sized
plate that he uses. In addition a rapid rectilinear lens as advertised
to cover a plate of the same size as his camera, will be found very
useful for quicker work. [Sidenote: Lenses for special purposes.] For
special purposes, for example in photographing beetles, or fish, or
flowers for scientific manuals, the finest lenses procurable must be
used, and sharpness, brilliancy, &c., are vital qualities in such cases,
for the work desired is diagrammatic and not artistic, but in these
cases also the greatest care must be taken to use the lenses properly,
so that the drawing is correctly rendered. Ignorant critics and
enthusiastic partisans alike have claimed for photography, as its chief
merit, “truthfulness.” As has been shown, a photograph may be very false
indeed.
[Sidenote: Composite photography.]
Another chimera is that of “composite photography,” to which we shall
again refer. When Mr. Galton tells us he uses an ordinary portrait lens
for his work, and gives no other details, that is quite sufficient, in
our opinion, to seriously impair the value of his “composites,” even
were there no other considerations.
[Sidenote: Portraits taken with rapid rectilinear lens.]
The only really artistic series of photographic portraits we have ever
seen, namely, those by Mrs. Cameron, were taken with the next best lens
to that advocated, namely, a rapid rectilinear lens, but even they would
have been improved by the use of the new lens. We have besides seen here
and there really artistic portraits by others (but these were the result
of chance, as no second picture was ever produced by the same worker),
and they were taken by a rapid rectilinear lens. Mrs. Cameron, though
not an artist, had knowledge enough to see that the portrait lenses of
the day were undesirable for her work. And here it may be remarked that
a great ignorance of optics is as harmful as wasting too much time upon
its study. One industrial portrait photographer, who has very
occasionally succeeded in producing an artistic picture, prides himself,
we are told, on not knowing what lens he uses. Such a man can never be
an artist, for he cannot know whether his work be true or false. To
appreciate falseness in drawing requires considerable training. An
average judge of photography might discover gross distortion of limbs,
due to violent perspective; but how many would notice the false drawing
in a face which is taken with a portrait lens?
[Sidenote: Diaphragms.]
Supplied with his lenses, the student will find “stops,” or diaphragms.
The name, “stop,” suggests its use. By making the light pass through a
contracted hole, the weak marginal rays are cut off, and the image is
therefore made sharper all over, spherical aberration is reduced, and
the depth of focus is increased. But though diaphragms are used to
correct an error, yet the ignorant use of them is as great a source of
error. One of the causes of sharply defined and false heavy shadows in
the much-vaunted “sharp photographs” is due to focussing sharply, and
“stopping down,” that is, to using a small diaphragm. This is the
invariable practice of most photographers.
[Sidenote: Modified diaphragms.]
Some ingenious workers have suggested modifications in the construction
of diaphragms, with a view to improving the picture; one of these being
a paper diaphragm, made translucent with castor oil; but we have not
found any advantage in these novelties. It is, however, a legitimate
field for experiment, and translucent diaphragms might be tried in
indoor work and bright out-door effects.
[Sidenote: Intensity of lens.]
The student will often see in photographic papers that a lens works at
F/8, or F/32, or some other number. This simply expresses the ratio
between the working aperture and the equivalent focus of the lens, and
is obtained by dividing the equivalent focus by the working aperture.
F/8 then means the aperture is one-eighth of the focal length of the
lens referred to. The rapidity of lenses are compared in this way by
squaring the denominators of the fractions thus obtained; when the
results will give the ratios of rapidity. [Sidenote: “Depth of
focus.”]By “depth of focus” is roughly meant the sharp rendering of the
different planes of a landscape, or any object with more than one plane
in one plane. Needless to say, this quality, greatly sought for in
lenses by photographers, is a thing to be carefully avoided in artistic
work, as we shall show later on.
[Sidenote: Flare spot.]
By a flare spot is meant a circular spot on the focussing screen, which
receives more light than the surrounding field; it is said to be caused
by the diaphragms being wrongly placed. The same effect is produced when
the sun shines into the lens, the light being then reflected from the
brass tubing of the lens, and it is for that reason that the lens must
be carefully shaded during exposure, when the sun is directly in front
of the camera.
[Sidenote: Angle of view.]
The angle of view included by a lens is an important consideration, and
we shall refer to this later on; here we shall only show how this angle
may be determined when the student wishes to do so. The angle depends on
two factors, the length of the base line of the picture, and the focal
length of the lens. This is practically determined by ruling a
horizontal line the actual length of the base line of the picture, and
drawing from the centre of this line a perpendicular equal in length to
the focal length of the lens. Completing the triangle, we have in the
angle contained by the two sides of the triangle the required angle,
which can be measured by an angle measurer. Experience shows that if the
base of the picture is greater than or equal to the focal length of the
lens, the angle included will vary between 53° and 90°; but if the base
is less than the focal length, these angles will vary between 44° and
19°, or less. It will be seen, therefore, that the long-focus lenses
give more suitable angles of view for pictorial purposes.
[Sidenote: Hints on lenses.]
Delicate optical instruments, like lenses, must, it is needless to say,
be carefully protected.
A good lens should be free from scratches, striations, dull patches, due
to imperfect polishing, and veins; but air bubbles do not affect its
value, for it must be remembered that the shape of the hole through
which the light passes does not affect the image, save only by cutting
off some of the light. Thus, if a wafer be stuck to the centre of the
lens, the image will be found unimpaired. Dust and dirt, however, though
they do not seriously impair the definition of the image, yet cut off
much light, as will occur to any one when he thinks of the difference
between the light of a room, when the windows are dirty, and when they
are perfectly clean. Lenses should not be left in bright sunlight, for
this causes a change that slows them, the dark also injures them in
certain cases, for, as all microscopists know well, darkness causes a
change in Canada balsam, with which lenses are cemented together.
Mr. Dallmeyer insists that lenses should be kept dry and free from
sudden changes of temperature, otherwise they may tarnish or sweat, as
it is called. Any one who has been troubled with this sweating will
never forget it. Our experience is that the best way to keep lenses is
in small leather, velvet-lined cases. We generally keep with them a
piece of soft chamois leather, or an old silk handkerchief. No compound
of any kind should be used to clean lenses, if anything appears to be
going wrong with them, they should at once be sent to the maker.
[Sidenote: View-meter.]
A valuable little tool is a view-meter. The handiest and compactest we
have seen is that supplied in telescopic form.