Current variable image magnification methods present problematic issues for high precision metrology instruments. This paper describes a novel approach to providing variable system magnification while avoiding those shortcomings. An optical zoom generally has multiple lens groups moving in concert to simultaneously adjust the effective focal length and maintain focus. Inevitably, the motion of the elements results in image drift and small variations in focus over the zoom range. An alternate approach using turreted fixed focal length lenses can have higher image quality but still suffers from image drift unless great care is taken in alignment of the individual lenses.
The variable-zoom method presented has a detector array with at least M*p pixels in each axis, where M is the maximum magnification and p x p is the size of the image array at each zoom. Pixels are binned based on the current magnification such that the final image size at each zoom magnification has a constant pixel array size. The fixed focal length imaging lens utilizes a variable aperture stop which is adjusted to maintain diffraction limited resolution. At low magnification the lens has a large field but requires lower resolution. As magnification increases the variable stop increases in size resulting in a smaller diffraction limited spot over a smaller FoV. The optical system can be much less complex than a traditional zoom and because there are no moving lenses, the image does not drift on the detector and
there is no focal shift as the magnification changes.

Figure 1. Interferogram and resulting phase map for small (~1") optic which subtends ~200 pixels at 1x system zoom (top row). When the system is zoomed by 5x, the surface subtends 1000 pixels (bottom row).

Keywords: Phase-shifting interferometer, optical zoom

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