M. PETZVAL'S PORTRAIT COMBINATION.
The object of this instrument is to obtain an image well defined in its principal parts, when a large volume of light ic admitted. In taking a portrait it is evident that the time of exposure should be reduced as much as possible, because after remaining in a con strained position for a long time the features of the sitter betray an expression of the discomfort felt. A lens of large aperture must therefore be employed in portraiture, so long as photographic pro cesses remain in their present state of insensitiveness. As soon however as the chemist shall discover the means of rendering these processes more sensitive, the optician will be released in a corres ponding degree from the necessity of constructing lenses of large aperture, and the defects to which such lenses are liable, and which admit of no remedy, will be avoided by using smaller apertures. The nature of the defects of large lenses have, however, been mis understood and exaggerated by persons ignorant of mathematics The portrait combination of Professor Petzval -is that which has been generally adopted by opticians, and the arrangement of the lenses is exhibited in the following figure.
'The front lens A B is a compound lens, exactly like the common view-lens, but placed with its convex side to the objects. It is achromatic, but not entirely aplanatic ; this defect being remedied by the posterior lens.
The posterior lens is composed of two lenses separated by a small space ; that next the front len.s is of flint glass, convexo-concave, and divergent, being thinner in the middle than at the edges ; the other is biconvex, and of crown glass, being placed with its most convex side next to the concavity of the flint, as shewn in the figure.
The posterior compound lens is achromatic, and the object of separating the lenses and giving them curves so different from that of the front lens is to cure eph,erical aberration in the entire com bination.
Both lenses are therefore convergent, and the effect of the posterior lens is to shorten the focus of the front lens. It may be well to observe in this place that when a convergent lens is rendered achromatic by combining two lenses of different ldnds of glass, the concave lens must be of flint, and the convex lens of crown glass. On the other hand, when a divergent lens is achromatized, the con cave lens must be of crown, and the convex lens of flint glass.
The dimensions of this form of lens, given by measurement of one which we constantly use and which is now before us, are as follow : The curves depend entirely on the refractive and dispersive powers of the glass used, and as these vary with every " pot of metal," it might only mislead the reader were we to state the exact curves of the lenses of the instrument before us.
To return to the figure.
Let P QR be an object placed before the lens, P being- a bright point upon its Ends. A direct pencil from P covers the entire sur face, A B, of the front lens, and is refracted by it towards a point f. This converging pencil AfB is incident upon the portion hk of the posterior lens, and is by it refracted to a point p, at which the image of P is formed. It will be .observed that hpk is a cone of light having a very large vertical angle hpk ; both spherical and chromatic aberration must therefore be accurately corrected in such a pencil, and the focussing screen placed accurately at p, or the indistinctness of the image would be considerable. The optician has therefore greater difficulties to encounter in the construction of ire portrait lens than in that for views.
We have next to consider the case of an oblique pencil.
The central pencil does not cover the whole of the back lens, hich is the same size as the front lens) but merely the central portion, h k. The angle D Ah is about equal to 3°. If therefore an oblique pencil, as QAB, makes an angle Q CP of about with the axis of the lens, the upper ray of the refracted pencil will run along the tube A D, and the whole pencil will come to a focus at q. The image therefore between p and q will be equally illumi nated. But if a more oblique pencil, as that from R, be incident on the lens, only a portion of the refracted pencil can pass through the back lens, the remainder of it being cut off by the upper part of the tube, which reflects it in the manner shown in Fig. 3, page 59, This is one of the great defects of the portrait lens, and the mode of mounting it. Sometimes the back lens is made a little larger than the front lens, and then the equality of illumination extends over a greater space, but the outer portion of a large back lens does not perfectly cure spherical and chromatic aberration, and from this cause indistinctness is introduced. The fact is, the portrait lens with full aperture has many serious defects, and the sooner pho tographers are enabled to give up using it the better. It should never be used from choice, but only from necessity.