Alongside the 100-500mm Canon unveiled two very unusual lenses, both of which employ collapsible mechanisms and DO – Diffractive Optics – elements in their designs, to reduce their size and weight. The first is the RF 600mm F11 IS STM. Canon claims that the use of a new material for the gapless dual-layered DO element allows for smaller and more cost-effective designs compared to the EF equivalents.
Compact and lightweight
The 600mm F11 IS STM weighs less than a kilo (930 g / 2.05 lbs), features a perfectly reasonable 82mm filter thread and measures only 200mm (7.9 in) when stowed. Considering its focal length, these figures are impressive. The DO optical technology certainly helps, but the main reason for its unusual compactness is an innovative retractable design.
Retracting / collapsing designs have been around for as long as photographic lenses have existed, and the basic principle (and benefit) has remained the same – extend and lock the mechanism when you need to shoot with the lens, and retract / collapse it when you don’t, for compactness. This isn’t possible with complex multi-element primes (let alone zooms) but for a long tele like the RF 600mm, where most of the glass is clustered together, in a design with a lot of empty space between groups, it becomes an option.
To unlock the 600mm’s extending mechanism, just turn the collar, and pull (or push) to put the lens into either the extended shooting position, or collapsed storage position. When extended, the lens measures 270mm (10.6 in).
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.
Image stabilization and autofocus
The 600mm F11 is also equipped with an image stabilization system which can deliver up to 5 stops of stabilization. Canon did not claim an increase in performance when paired with the EOS R5 and R6 IBIS systems, and this makes some sense given the limited ability of IBIS systems to effectively stabilize very long focal lengths. Autofocus is handled by an STM motor, of the kind that has become common in Canon’s more enthusiast-centric lenses and those optics intended to be used for video, as well as stills. It’s not as fast – or usually as silent – as Canon’s Nano USM motors, but still quite effective.