Test seventh, for marginal definition, conducted with the newspaper as above, by focussing as sharply as possible the letters that came on the outside of the field. A was nowhere with B in this test.
Test eighth, for depth. This I have always, by theory, held to be a quality decided, other things being equal, by the ratio of the aperture to the focal length; and these trials only con-viaced me of the correctness of that opinion, as there was no perceptible difference. A finely engraved card was focussed, and then slid to and from the lens on a line, as nearly as practicable coin< Went with the optical axis of the lens.
The above seemed to include all the important qualities of portrait lenses valuable in prac-tice; and, to recapitulate, gave tests first, fourth, fifth, sixth, and seventh, in favor of B; tests third and eighth, neutral; test fourth, in favor of A so slightly that I formed in my mind the conviction that while the " best is the cheapest," the highest priced is not always so. - W. J. Baker.
117. The accompanying diagram will give the student a good idea of the action of the diaphragm on a portrait lens. The dark lines, a a, represent the depth and direction of the focus of the lens at full aperture. The curvature of the field is purposely exaggerated in order better to show the subsequent result. We will take the full aperture to be four and a half inches; it will be observed that in this condition the field is curved - which gives bad definition at the margins that the depth of focus is limited, whieh gives enlargement and distortion to all projections, and that the size of the picture is small. Now on placing a diaphragm of three inches aperture between the combinations, the result is represented by the dotted lines b B; it is then seen that the field is extended and depth of focus increased both before and behind the central point D the consequence of which is that a larger field of clear definition is shown in the picture, and features which before were fuzzy and enlarged in their form, assume their natural aspect. If, to include more objects in the composition, or from other motives, still greater sharpness is desirable, a diaphragm of one and a half inch opening is substituted; immediately the qualities recapitulated above are still more Improved, as seen in the lines c c. Light has now, however, been much diminished by the small remaining area of aperture, and very considerable addition becomes necessary to the time of exposure. - Lake Price.
118. By the use of diaphragms the definition of the picture is increased, and they increase the focual depth. This, however, is inexpense of light, and a consequent increase in the time of exposure. This latter drawback compels us, in portraiture especially, to give up some of the qualities we should like to secure, lest a too long exposure try the patience of our sitter too severely, or, if he keep still, cause him to assume an unpleasing expression. Thus it will be seen that in photography, as in life, each advantage gained is at the of some other, in a measure, and are must be content with a fair medium or measure of what is desirable.
118. A represents a section of a four and one - half inch double or portrait lens at its full aperture, with the manner in which the pencils of light from the subject pass through the combinations, and are refracted by them to the film, a is the front, B the hack lens, cc central rays, eE the lateral ones, Ece the line of focus at the film.
The attention of the reader must bo directed to the width of the pencils refracted from the lens at this aperture and the obliquity of the direction of the lateral ones, E. e, in order that he may observe the changes which the application of a diaphragm will at once effect in them. B is the same lens with of light drawn to scale; a diaphragm.
DD, of two inches aperture, a, is now added 1 the combinations, the effect of which has been to diminish the obliquity and give more parallelism proceeding to the film, and more depth in the focus.On contrasting the width of the pencils is EE with those last diagram, they will be found to be diminished in their diameter by one-half, with more accuracy of delineation, but at the loss of rapidity of
119. The proper use of stops can only be attained by careful observation and practice. It becomes, like exposure itself, almost a matter of inspiration - of feeling, at least, - and no rule can be laid down. A few hints, though, may not be out of place. Each lens is usually supplied with five or six stops of varying sizes. The light, the subject, and the quality of picture you desire are the three governing items in the employment of stops. Study all these, and experiment with all the stops. Usually focus with the open lens or the largest stop, and then shift for the one most desirable to use for the exposure.
120. Photography is as susceptible to the variations of time of exposure, the quality and conditions of light, the atmosphere, the chemicals, and the diaphragm employed, as sound is to the instrument, and the other means employed for its production. Hence, there is always much to learn, and great room for careful experiment and practice in all departments. In no other is it more so than in the application of the stop to the lens. Therefore, employ spare time in studying this department of your work, that when you are required to produce the best results, you may go about it most intelligently.
execution by the abstraction of a corresponding illuminating area. The length of the focal distance from the back lens to the film is increased, but with a well-corrected lens of long focus, the field, under such conditions of aperture, is very nearly flat. - Lake Price.
119. Enough has been placed before the reader to show him that area of aperture is the very helm which regulates and guides the photographic action; if too much diminished, not only the time of exposure becomes irksome, and the expression of the sitter's countenance suffers, but a harsh and unnatural edginess characterizes the picture. If, on the other hand, it is allowed to be too great, the oblique pencils, which it should have corrected, interfere with the perfection of the image, and distorted forms and misty outlines are seen. - Lake Price.
120. It is often necessary to know the focal length of a lens or combination of lenses, especially in photography; but if no plano-convex lens of known focal length is at hand, for the purpose of comparing the size of the image, the following way may be adopted: first, focus the lens for a very distant object, on a screen, and mark the position of the screen. Do not move the lens, but place a bright object, about twice the focus of the lens, in front of it, as near as you can suppose; now move the screen about the same distance from the lens as the object was placed, and focus thereupon. If you find the object and the image not of exactly the same size, move object and screen accordingly, and focus sharp, until the object and image are precisely the same size; mark the position of the screen again, and the distance of the first and second mark is the focal length of the lens, or the equivalent focus of a combination of lenses. - Joseph Zentmayer.
In conclusion, a few words yet about treating your lenses. Always keep them from light and dust when not in use; never drop them on the floor, and when dusty or soiled clean them carefully with a piece of old linen or old silk handkerchief. From sad experience I almost feel inclined to add to this: New r loan them out. - R. Benecke.