GB817455A - Improvements in or relating to reversible lens attachments - Google Patents

Abstract

817,455. Lens attachments. KODAK Ltd. Sept. 25, 1957 [Oct. 1, 1956], No. 30063/57. Classes 97(1) and 98(1) A substantially afocal reversible lens attachment comprises a biconvex doublet 1, 2 of focal length f and a biconcave component (e.g. a simple lens 3) of focal length between-0.7f and-0.9f, the radii of the surfaces R 1 to R 5 being such that 0.7f < + R 1 < 0.9f, 0.6f < -R 2 < f, 1.1f < -R 3 < 1.7f, 0.8f < -R 4 < 1.2f, and 0.85f < + R 5 < 1.3f, the refractive indices N 1 and N 2 of the positive doublet being such that 1.48 < N 1 < 1.57 and 1.58 < N 2 < 1.67, the refractive index N 3 of the negative component or of each element therein being such that 1.58 < N 3 < 1.67, the two components being of substantially equal diameter and being mounted in a lens tube each end of which is relatively detachable to the mount of a standard objective. As shown, the lens tube 51 has a central thread so that either end of the tube can be screwed into a collet 52 having an expandable end 53 for gripping firmly the mount 50 of a standard objective (e.g. in a camera). The negative component may be an achromatic doublet made up of elements having the same refractive index but different AbbÚ numbers.