Johannes Winterot

Afocal zoom lenses and their applications

Afocal zoom lenses are state of the art modules for magnification changing in instrument optics, especially in the field of scientific and surgical stereo microscopy. Carl Zeiss implemented many types of afocal zoom lenses in the microscopy instrumentation in more than 30 years. Mechanically compensated zooms with two moving members for magnification range between 4x and 20x are the most widely used zoom lenses so far. As the instrumental optics gradually moves towards the digital imaging, some modified schemes have been created in order to meet new requirements for extended zoom range and for accessibility to the system pupil by an aperture stop or by a shutter. A structure and particular aberration analysis of a four member afocal 20x zoom lens is discussed. It is pointed out that a modular approach for instrumental digital imaging systems with afocal interfaces around the zoom module is one promising application of afocal zooms in the future. This approach contains an afocal zoom with an accessible central stop surrounded by focusing lens groups placed on the object and / or at the image side of the zoom. The advantages of schemes like that are the easy scaling to high magnification range, good balancing of aberrations and diameters of lens elements and the high flexibility in adaption of video systems to different applications. Two examples of modular video zoom lenses are given: an object-side telecentric 20x zoom lens with a four member afocal zoom and a telephoto-type 30x zoom lens with a three member afocal zoom.