With convergent optics, the light beams enter the eye at the same angle as they would should the naked eye be focussing on the subject 30cms away, 30cms being the working distance of the colposcope.
This means that when the user looks away, for example to pick up an instrument, there is only a small focal change needed from the eye. This reduces eye-strain to practically zero.
With parallel optics, the light beams enter the eye straight in the same way as the eye focusses on an object very far away. Then the eye needs to make a great adjustment every time one looks away. This is what causes eye strain.
Sets of parallel optics are standard for many applications including standard binoculars and microscopes and compared to parallel optics they need less labour to produce and so are less expensive.

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There is a concensus that Leisegang optics are second to none -contact Genesis Medical .for a demonstration to see for yourself