[Ill.u.s.tration: Fig. 5.]
_The Development of Photographic Lenses._--When photography was first practised the best lenses available were those made for use as telescope objectives, and they had to be used with a small diaphragm to get good definition over a sufficient field. With the slow processes then in vogue a more rapid lens was much desired, and Voigtlander introduced a "portrait" lens constructed according to the results of the calculations of Professor Petzval. This portrait lens is still very largely used, and figure 5 will serve to show its general character and will be a guide to the putting of one together correctly if it has been taken to pieces for cleaning. A rapid lens such as this could not cover a sufficiently large field for landscape work, so that single lenses were still used for work in which rapidity was not of very great importance. Single lenses were improved, and other kinds of lenses were introduced from time to time, but it was not till 1866 that the "rapid rectilinears" or "rapid aplanats," called later "rapid symmetricals," and by innumerable other names according to the fancies of the makers, were introduced. Probably no lens has been made in such large numbers as this.
At about the same time, Dallmeyer introduced his portrait lens in which the position of the convex and concave elements of the back combination is reversed, the concave lens being outside, and this gives the photographer the opportunity of s.c.r.e.w.i.n.g it back a little, and so introducing a measurable amount of spherical aberration which has the effect of modifying the otherwise exceedingly fine definition at the centre of the field, and giving a greater depth of definition.
In 1881, Messrs. Abbe & Schott began a series of experiments in the manufacture of optical gla.s.ses, and they were so successful in making new and useful varieties, that an optical gla.s.s factory was eventually established at Jena, by Schott & Co. By the use of these newer gla.s.ses the limitations that had previously restricted opticians were removed, and it became possible to correct astigmatism and secure a flat field at the same time. Zeiss of Jena, towards the end of 1890, introduced his first series of "anastigmats." The "concentric" lens of Ross was introduced in 1892, a lens which probably remains unsurpa.s.sed for flatness of field and freedom from astigmatism; but as spherical aberration is present to a notable degree, an aperture of about _f_/22 is the largest that gives sharp definition. The "double anastigmat" of Goerz of Berlin was put on the market in 1893. It is a symmetrical lens, and in this different from the Zeiss anastigmats that preceded it. It consists of two similar combinations, each of three lenses cemented together. The unsurpa.s.sed qualities of this lens stimulated other opticians to seek to rival it, and there appeared similar lenses with four and even five lenses in each combination, besides other lenses that are more or less a copy of the double anastigmat. One of the most notable of these is the "satz-anastigmat" of Zeiss, each combination consisting of four lenses cemented together and forming an excellent single lens. These combinations are interchangeable in the same mount so that with, for example, one mount and three lenses, six different focal lengths can be obtained, as the lenses may be used singly or any two together as a doublet.
The "Cooke" lens is remarkable for the simple means by which the various corrections are made, consisting as it does of only three single lenses separated from each other. Obviously it must be used entire. These lenses do not cover so large a plate in proportion to their focal lengths as most of the other anastigmats, but perform excellently over the plates for which they are constructed.
The "stigmatic" of Dallmeyer is the latest lens of general utility.
It gives good definition to the margin of the circle of light that it transmits, reduction of aperture being necessary, when its full field is employed, to get equality of illumination rather than to improve the marginal definition. Its two combinations are different, and either may be used alone as a single lens, giving focal lengths of approximately one-and-a-half and twice the focal length of the whole lens.
The "planar" of Zeiss introduced just as we write, is a symmetrical doublet characterized by a very large aperture, from _f_/3.6 to _f_/4 up to 10 inches in focal length, and a little smaller above that. It is therefore comparable with portrait lenses. Although it is symmetrical, a single combination cannot with advantage be used alone as a single lens. Telephotographic lenses are subsequently referred to.
The one aim of opticians in improving photographic lenses has been to get good definition all over a comparatively large flat surface without having to use small apertures. A defining power on the axis of the lens, that is, at the centre of the field, far exceeding what can be taken practical advantage of in ordinary photography, has long been possible. But until recently, the defining power always rapidly deteriorated as the distance from the centre was increased.
But to judge of the quality of a lens, or to compare one lens with another, there are other matters that must be understood, and these we shall proceed to consider. Focal length, aperture and image angle are the chief details concerning lenses, granting that the aberrations referred to above are satisfactorily corrected.
_Focal length._--The focal length or focal distance of a thin lens 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 pa.s.ses 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 gla.s.s held in front of the lens. A disc of light will be seen on the ground gla.s.s 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 gla.s.s makes a little difference, and at every surface in contact with air there is loss by reflection, but these and a.n.a.logous 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.
[Ill.u.s.tration: 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 gla.s.s, 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-gla.s.s distortion.
Both these effects are ill.u.s.trated (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.
[Ill.u.s.tration: 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._
[Ill.u.s.tration]
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 circ.u.mstances 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 a.s.sistant 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.