Patent GB-514441-A (GB)
Title: Improvements in or relating to objectives
IPC: G02B13/18
Filed: 1938-05-05
Abstract: 514,441. Lenses. TAYLOR, TAYLOR, & HOBSON, Ltd., and WARMISHAM, A. May 5, 1938, No. 13462. [Class 97 (i)] An objective has the air-exposed surface or surfaces immediately next the diaphragm position concave and in the form of a surface of revolution 'whose meridian section is represented in polar co-ordinates (with respect to an origin at the centre of curvature at the vertex and an axis of reference through the origin and the. vertex) by an equation of the form r = ro + a #<4> +a #<6> where ro is the radius of curvature at the vertex, and a and b are constants of which a is negative, the remaining surfaces of the objective being spherical. The form shown in Fig. 1 comprises a single compound meniscus lens. The coefficient a is preferably greater than 1¢ per cent. of the equivalent focal length, the axial thickness of the lens greater than 6 per cent. and the radius of curvature greater than 15 per cent. Data are given for examples with apertures of f/8 and f/7.3. An alternative form comprises a divergent meniscus doublet with its air-exposed concave surface facing the diaphragm position and its convex surface facing a convergent component. Data are given for an example with an aperture of f/4. The form shown in Fig. 3 comprises two compound convergent meniscus components enclosing the diaphragm position and one or each of the concave surfaces may be aspherical. By making the system. symmetrical with each half self-connected, the whole system or one half thereof may be used as alternative lenses. In one such arrangement, with An aperture' of f/4.2 for which data are given, all the air-exposed surfaces have radii greater than 25 per cent. of the focal length, the thickness -of each component is greater than 10 per cent., and the coefficient a is greater than 25 per cent. An asymmetrical modification is identical except that the asphericity is confined to one of the concave surfaces. Another asymmetrical form with an aperture of f/2 has two asymmetrical surfaces, all the air-exposed surfaces having radii greater than 25 per cent. of the focal length, and at least one component having a thickness greater than 12 per cent. The axial radius of curvature of the concave air-exposed surface is less in the front component and greater in the rear one than the radius of the corresponding convex air-exposed surface. In the form shown in Fig: 4 two compound divergent meniscus components enclose the diaphragm position and are situated between two convergent components. Data are given for four examples, three with one aspherical surface and one with two. The latter example has an aperture of f/1.1. The numerical sum of the radii of the convex airexposed surfaces of the divergent components exceeds 90 per cent. of the focal length and of the concave surfaces at their vertices 55 per cent. Another example has an aperture of f/2. The coefficient a is less than 1 per cent. of the equivalent focal length, the sum of the radii of the convex surfaces of the divergent components greater than 75 per cent and of the concave surfaces at their vertices 52 per cent. Specification 435,149 is referred to.