There are two descriptions of lenses in use. The single lens is used for views and photographs of inanimate subjects. This lens requires a longer exposure of the plate than the double lens, but the resulting photograph is very clear in the details. The compound lens used for portraits consists of two pairs of lenses, mounted in a telescopic brass frame, having diaphragms or stops, and provided with a turnscrew to regulate the focus to a nicety, after it has been roughly obtained by adjusting the camera. The interior of the brass tubes holding the lenses must be kept of a dull black colour; should this wear off, a coating of gum-water and lampblack should be applied when cleansing the lenses. It is important to replace the glasses in the lens in exactly the same order and position, after having removed them to clean, which is to be done with a piece of very soft wash-leather. The single lens is composed of an achromatic lens mounted in a brass tube, fitted with diaphragms or stops of various sizes. These diaphragms are simply flat disks of brass, each having in the centre a circular opening, and upon the size of the opening of the diaphragm used, depends the length of exposure necessary, and the sharpness of the resulting picture.
The larger the opening of the stop, the shorter will be the time necessary to expose the plate in the camera; but if a stop be used with a smaller opening, the picture will, be sharper and more distinct in the details. Thus, in working with the view or single lens, the operator can choose which point is most material for the particular picture he desires. The plate is of course more rapidly affected in proportion to the brilliancy of the light striking upon it. It is sometimes necessary to use a diaphragm with the compound lens, as for instance in photographs of groups, but the openings in these stops are much larger than those used with the single lens. Portrait lenses are usually provided with central diaphragms.
According to Bausch, the qualities which are involved in a first class photographic objective are as follows: Exact coincidence of visual and chemical foci; a formula involving such glass and curves as will give the utmost freedom from spherical and chromatic aberration; such a relation of glass and curves as will avoid internal reflections and consequent "flare" or "ghost"; length of focus; diameter of lenses; absolute homogeneity of glass, or freedom from striae or veins; grinding and polishing the various surfaces so that they shall be strictly spherical; absolute centreing of lenses before and after mounting.
We will assume that the optician desires to make a medium-sized objective, of the form which is principally used to-day (a system of two symmetrical combinations), and that he has laid out in advance the formula on which he intends to construct it.
He has in his possession four plates or disks of rough glass, two of which are crown and two flint. If he has any experience at all, he will use no other than such as is warranted by the glass manufacturer, because even then he will find considerable difficulty in obtaining perfect glass. The glass comes from the manufacturer with a guarantee or not; in the latter he assumes no responsibility, while in the former he does as far as the refractive index and most glaring faults are concerned. If they are of guaranteed form, the disks have facets' ground and polished on the edges on opposite sides, by means of which the maker has carefully tested the glass and by which means the optician may do the same.
Any variety of crown or flint may be obtained, but the light flints and crowns give the most satisfactory results. Flints are usually the outer lenses of double combinations, and when heavy are exceedingly soft and subject to oxidation. The optician now begins to abrade the glasses, giving them approximately the form he wishes them to have, and then begins the process of grinding and polishing. Although this is done on the same general principle, the details vary with different opticians; and although the same result may be and is accomplished in different manners, it is safe to say that, if one were to follow the plan of another, he would fail. There is a peculiar touch or feeling in doing this work which it takes years of practice to acquire, and even then a person must be adapted to it. Few persons have any conception of the amount of exactness which is required in this work. This remark of course applies mainly to an objective which is to be of the highest standard, and which to a certain extent varies with different opticians according to their skill, and not to such productions as may be called commercial, for the reason that the pecuniary return, and not excellence, is the paramount consideration.
A certain amount of care is observed even in the cheapest forms, but these are not subjected to critical tests, and any faults which are not apparent to the purchaser are allowed to pass, which is not the case in the highest grade, where the best is none too good; the result is of course that they- are inferior, varying, and unreliable, and while occasionally one may find an excellent lens among them,.no reliance can be placed upon them, It has been computed that an error or. variation from the spherical surfaces, for instance, so infinitesimally small that there are no mechanical means for determining it, say of 1/500000 of an inch, will have a noticeable and sometimes disastrous effect in the formation of an image. A first-class photographic objective requires as much perfection as that in the telescope or microscope, and no work spent upon it to accomplish this result is lost.
To return to our objective, we will suppose that all the surfaces have been ground and polished. The difficulty of doing the surfaces increases out of all proportion with the increase in size, as well as the danger of scratching. It is an ordinary event to scratch a surface during the process of grinding and polishing, and nothing uncommon to do so in the last stages of the latter, when the entire work must be repeated. No matter how good a surface may other wise be, a scratch is inadmissible; not that it does any particular harm, but an optician who takes pride in his work cannot bear it, and the customer who has to pay for it will not accept it. After polishing, the lenses are put into a lathe one by one, and centred, that is, the optical axis is found, which we know we have when on revolution of the spindle the two images which the surface gives are stationary. The edges are now ground down to the diameter of the cell in the mounting.