Fixed aperture of F11
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Fixed aperture of F11

Another way of keeping this lens small is its fixed aperture of F11. This is the kind of aperture that traditionally, we would have associated with mirror lenses, which were a devil to work with on D/SLRs because of the dim viewfinder image associated with such a small working aperture.

However, on a mirrorless camera with a decent EVF, that's much less of a concern, especially now that the Dual Pixel CMOS AF systems in Canon's EOS R-series cameras can autofocus at working apertures down to F22 (which - not coincidentally - allows for these lenses to be used with the new RF teleconverters). Yes, F11 (and remember that's fixed - you cannot stop down as there is no multi-bladed iris) is still a limiting aperture for exposure, but it's a much more practical working aperture now than it would have been back in the days of SLRs.