The object of using stops is to increase the defining power of the lens. If we examine the image thrown by the lens on the ground-glass, we shall find that, although some por tions of the picture appear sharply defined, or, as a photo grapher would say, "in good focus," the rest of the picture is more or less blurred. To remedy this, we insert a medium-sized stop, and we now see a great change in the appearance of the image, which, instead of being blurred and fuzzy, appears sharp and distinct. I shall have more to say upon this part of the subject when I assist the reader in taking his first negative.
The aperture of a lens is the diameter of the stop used in relation to the focal length of the lens, and the size of the diaphragm is usually expressed by dividing the focal length of the lens by the diameter of the stops. For example, we will suppose a certain lens has a focus of 8 in., and we find, upon measurement, that the largest stop measures 1 in. in diameter. 8 1 = 8, therefore the aperture of the lens with that particular stop will be f/8, and so on. The stops of nearly all modern lenses are marked according to this system ; therefore, although it is necessary that the be ginner should understand its meaning, he will not have the trouble of making the calculation for himself.
Distortion is the defect in single lenses, and improperly constructed doublets, which causes straight lines in the object photographed to appear curved or distorted in the picture.
By the term width of angle is meant the amount of the subject photographed which the lens will include upon the plate. The rapid rectilinear lens which we have advised the student to purchase is what may be called a medium angle lens, and the pictures produced by it, when it is used on plates of the size which I shall recommend, do not suffer from that appearance of exaggerated perspective which is sometimes seen in photographs taken with wide-angle lenses. The width of angle of any lens is determined by the length of its focus in relation to the length of the base line of the picture. For example, an angle of 28° is obtained on a plate one-half the focal length of the lens; an angle of 37° on a plate which is two-thirds the focal length of the lens ; an angle of 53° on a plate the same length as the focus of the lens ; and an angle of 74° on a plate which is one and a-half times the focus of the lens —and so on.
I have already intimated that it is most important that the lens selected should be a good one, and I strongly advise the reader not to attempt to economise by buying a cheap and possibly inferior instrument. His safest plan will be to deal direct with an English lens maker, not a dealer, of whom there are several from which to choose. He should also be informed that there are vast numbers of foreign lenses imported into this country by photographic dealers, who engrave their names upon the mounts, and the novice may easily but erroneously infer that the instruments are manufactured by the firms whose names they bear. Now I
do not wish for a moment to disparage instruments of this class; many of them are most excellent, but, on the other hand, some are utterly worthless. I have frequently met with cheap foreign lenses which, for practical purposes, were equal to the best of English manufacture; but there is always the chance of getting a bad one, whereas if an English maker of repute be dealt with, he will take care, for the sake of his own reputation, that he does not issue an inferior instrument. As a rough guide on the question of cost; I find that the average price, less cash discount, of a half-plate rapid rectilinear, with Iris diaphragm, charged by either Beck, Crouch, Swift, Taylor & Hobson, or Wray, is about £3 10s. With such a lens the beginner may rest content until he has acquired some proficiency, for with it he can take either landscapes, buildings, or even his "sister, cousin, or aunt"; though, if lie be wise, he will not attempt either of the latter until he can with some degree of certainty produce presentable reproductions of the two first-named classes of subject. He will then probably begin to find that there are certain occasions when his rapid lens does not give him a sufficient amount of the subject on the plate; as, for instance, when circumstances compel him to approach very near to a building. For this purpose, and for photographing interiors, it is often necessary to use a short-focus, or wide-angle, lens, and in buying such an instrument all the foregoing observations made in reference to the rapid lens apply with equal force. Therefore, if a second lens can be afforded, it should be, for use on a half plate, one of the wide-angle type of about 5 in. focus.
Being in possession of a good lens, the amateur should take proper precautions to preserve it from injury. Every lens as sent out by the maker is provided with a leather cap with which to make the exposure; this, however, in the case of a doublet, only serves as a protection for one of the glasses, and to preserve both from injury a little wash leather bag, made double and padded with cotton wadding, should be made to contain each lens. The glass of which the lens is constructed is very highly polished, and although it is necessary, for the success of the photograph, to keep the surfaces free from dust, yet great care must be taken not to use any material which would cause scratches. If the lens is fitted with the Iris diaphragm it will seldom be necessary to unscrew the glasses to clean their inner sur faces, as dust will not easily find an entry through that form of diaphragm ; and when it becomes necessary to clean the outer surfaces, a piece of the finest chamois leather should be cautiously applied, taking care to apply it very lightly and with as little rubbing as possible.