Among the various incentives for ditching the long- serving reflex configuration for interchangeable lens cameras is what can be achieved with lens designs once the mirror box is out of the way. The shorter flange distance – or back focus distance – makes it easier to achieve more uniform centre-to-corner sharpness and consequently there is more flexibility with what lens designers can do with an optical construction, including making them more compact.
When Canon first launched its RF mount full-frame mirrorless system, there were a couple of early lenses that looked like its optical engineers had missed that memo… the RF 28-70mm f/2.0L USM zoom for one which, at the time, we described as “truly massive, but magnificent”. Since then Canon has really started to exploit all the potential of mirror lenses, perhaps most notably with the RF 70-200mm f/2.8L IS USM zoom, which is three-quarters the weight of its EF mount cousin and delivers better optical performance in a number of key areas.
Now, Canon has done it again with the RF 100-500mm f/4.5-7.1L IS USM telezoom for which there isn’t a direct EF mount equivalent, but the comparisons with the EF 100-400mm f/4.5-5.6L IS II USM are telling.
