Abstract: This invention provides a lens which operates in a waveband in the infra-red region of the electromagnetic spectrum and which has a variable focus or magnification. The invention relates particularly to an infra-red zoom lens. The lens has a front part and a rear part which are adjustable axially towards and away from one another and a middle part whose function is to provide different focal length or magnification settings. The adjustability of the front and rear parts of the lens assists in overcoming the defocussing effect of temperature variations, the front and rear parts being held stationary when a focussed image of an object is formed in a desired image plane. In a zoom lens the middle part is a zoom part with elements movable axially to vary the image size in the image plane, the front and rear parts being adjustable axially relative to each other and to the range of movement of the elements of the zoom part.

Abstract: An infra red lens of very high aperture greater than F/0.75 specially adapted to transmit a waveband in the infra red range comprises from front to back in the order given (a) at least one positive meniscus element convex towards the front, (b) a negative meniscus element concave towards the front, (c) a positive meniscus element convex towards the front and (d) a positive element having a front surface convex towards the front. In a preferred embodiment there is a single positive meniscus element (a) convex towards the front in front of the negative meniscus element (b) and in another embodiment there are two positive meniscus elements (a) convex towards the front in front of the negative meniscus element (b). The back element (d) which has a front surface convex towards the front may be a meniscus element or may be convex-plano or biconvex.