It works with a maximum aperture of about 8 and is therefore four times as rapid as the lens described last. It does not, however, include quite so wide an angle as the wide-angle variety of the single lens. Still, the angle which it will include is enough and more than enough for all ordinary cases.
The Wide-angle Rectilinear and Wide-apgle Symmetrical Lenses.
• We here illustrate the wide-angle rectilinear lens, which may be taken as typical of a number of lenses which are made to include a very wide angle. They are all constructed of two combinations placed opposite each other, with the concave sides facing one another.
Such lenses should only be used when it is absolutely impossible to get far enough away from the subject to use a lens of longer focus—that is to say, they should never be made to include all the angle which they are cap able of doing, unless it is unavoidable. There is no harm in using for ordinary circumstances a long focus lens of this type so as to take in only a narrow angle, but then the special property which the lens possesses, and to obtain which other properties are sacrificed, is not utilised at all, and another lens might he used instead.
The subjects for which a very wide-angle lens are most useful are chiefly interiors, when it is impossible to get far away from the object to be photographed.
The lenses of this type give no distortion. They are not rapid, having a maximum aperture of about Indeed, they are the slowest lenses which are made, except certain old-fashioned forms of the single lens.
The Portrait Lens.
This lens is the one on which the optician has expended his greater ingenuity. It was the outcome of working the slower photographic processes, which are now things of the past. In it everything has been strained to get rapidity, so that the exposure for portraits might be as short as possible. Rapidity has certainly been gained.
The largest apertures of portrait lenses vary from in 4 the ordinary portrait lens to —f in the extra rapid portrait 2.&
lens. It is thus from four to ten times as rapid as the rapid landscape lenses. At the same time that rapidity has been gained the qualities which are required in a lens to be used for portraiture only have not been sacrificed. The definition given through a very narrow angle is exquisite, and the field is fairly flat. The great difficulty in the portrait lens is that, especially in large sizes, the depth of focus is very slight, unless a small stop is used, in which latter case the sole advantage which a portrait lens possesses—namely, rapidity—is sacrificed.
The Group, or Universal Lens.
This lens may be considered as a compromise between the rapid landscape lens and the ordinary portrait lens.
It is not so rapid as the latter nor so slow as the former. It may be considered as a slow portrait lens, whilst on the other hand, if it be used with a small stop, it will include a moderately wide angle, and may be used for landscapes. It works at about and is therefore about twice as quick as the rapid landscape lens and twice as slow as the ordinary portrait lens. We do not consider it necessary to illustrate this lens, as in construction it does not materially differ from those already described.
The Use of One of the Combinations of a Double Combination Lens.
One of the combinations of a double combination lens may be unscrewed and removed, and the other com bination being left in its place may be used alone. In this case we get a lens of double the former focal length of the instrument complete, which is often useful if our camera will open wide enough, but we cannot expect to get any but a very slow lens by this device. The com bination not being specially ground to work as a single lens will probably not work at a larger aperture than about or even .
20 30 Either combination may be used, except in the case of a portrait lens, in which case the front combination is the only one which will do.