Just a month after the Canon R5 came out in 2020, Canon filed a patent application – published only this morning – That attempts to make a smaller 600mm f/4, without resorting to fresnel DO optics and avoiding catadioptric designs that use a central mirror that blocks some light and causes doughnut bokeh.
The new design places two optical tubes atop each other, with a couple figure-four mirror arrangements that bump the light from the top, larger aperture tube eventually down to the lower tube that contains most of the focusing and aberration-correcting lens groups. The movement of the light between the two tubes takes up 575mm of the 600mm focal length, allowing for significant length savings.
This results in a two-tubes-squished-together look seen commonly among...
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