The field of a portrait lens is not nearly so flat as that of a view lens of equivalent focus ; and the angle of view properly included by a portrait lens is much narrower.
When the light is sufficiently good, a stop may be used with a portrait lens ; and the question has arisen as to which is the proper place for it—in front of the front lens, or between the lenses. This question we will discuss with the help of the following figure :— The lenses are represented by the lines AB, as before.
is the origin of an oblique pencil, the axis of which, RC, makes the same angle with the axis of the lens, as if it were the extreme oblique pencil of a view lens, i. e., PR is about equal to one-third of PC. Now CA is equal to one-third of AD, and the line RC suffers little or no refraction on passing through the front lens, therefore the axis of the refracted pencil between the lenses passes very nearly through D, as shown in the figure.
Now we may suppose that a whole pencil, RAB, comes accu rately to a focus at r, for if it does not, the lens is worthless, inas much as it ought to cure spherical aberration in the oblique, as well as central pencils, when the focus is merely examined by unassisted. vision. We may assume therefore that every my of the pencil R A B which really passes through the back lens, passes also through r. This being understood, it follows that by placing a stop rnidway between the lenses, at In, a small excentrical portion, Rn, of the large pencil R A B is incident at n, passes through the stop at m, the back lens at o, and then comes to a focus at r ; while if a small stop is placed at c, the central portion of the large pencil RAB passes through it ; then through the back lens at D, and thence, as before, to r.
When, therefore, a very small stop is used, it scarcely matters whether it be placed at c or m ; but if a larger stop be used, it would obviously be better placed at m, because if placed at c, a portion of the rays of the refracted pencil would be cut off by the tube, and there would not be equality of illumination 'in the image. It must be remembered, however, that the focus at r is not abeolutely per fect, and that, theoretically, the rays incident at C go to r with less spherical aberration than those incident at n.
But a portrait lens is not intended to be used with a small stop.
When a small quantity of light will do the work required, it is far better to use the orthoscopic lens. Nevertheless, stereoscopic pictures are sometimes required to be taken instantaneously, and sometimes with the ordinary exposure for views ; in this case a pair of small portrait lenses, furnished with stops, will answer both pur poses, inasmuch as stereoscopic pictures need not include so wide a field as ordinary views.
The best arrangement for taking instantaneous pictures, incluclin,g a tolerably wide field, with a lens of large aperture, is as follows :— Reverse the portrait lens, that is, turn it with the posterior lens to the objects ; and let the other lens, which is a view lens, be made larger than usual. Then place a stop immediately in contact with the lens that is presented to the objects. The arrangement will then be as represented in the figures at pages 60, 61, and 67 ; and this will give a much flatter field than the portrait lens used in the ordinary way, and the picture will be more uniformly covered with good definition, and more equally illuminated, although possibly it may not be quite so good as before in the centre. The focal length of the combination is a trifle shorter when used in this way.
A portrait lens, therefore, is only intended to be used for sub jects which must be taken quickly, or instantaneously. Tt does not give so flat a field as a view-lens ; nor does it give equality of illumi nation ; nor can objects at different distances be all brought into good focus ; nor does it include a wide angular field ; nor does it give images free from distortion. In short, everything is sacrificed to the centre of the picture, and the lens should only be used when no other will answer the purpose. As for using a stopped portrait lens, instead of a view-lens, for ordinary out-of-door pictures, the principle is incorrect for the reasons which have been stated, and the plan only answers when a narrow field of view is included.
The next lens to be described is the best of the three, and every photographer should possess one. It has been called the " Ortho scopic lens," or " Compound View-lens," and the following is a description of it, given by us in the " Photographic Notes," No. 49, April 15, 1858, shortly after its introduction into this country.