Thanks to the new facilities in Seattle the Sony is the first super-telephoto 500mm that we’ve been able to test. However, we have recently assessed the Canon 300mm f/2.8L IS II USM, the first in the series of up-graded telephotos lens from the firm eschewing UD (ultra low-dispersion) glass and adopting a double fluorite element design instead.
With that in mind and no direct comparison for us to work with, the Canon 300mm f/2.8 is almost certainly out-resolving our test camera the Canon EOS 5D Mk II, scoring the maximum possible 21P-MPix in the sharpness test, and, seemingly, out-resolving the Sony 500mm. Also bear in mind the Sony Alpha 850/900 models adopt a sensor with a higher pixel count than the Canon.
In other respects, the Canon performs similarly, including a somewhat poor Transmission value, though the lack of distortion is notable, as is the very low chromatic aberration. The older 300mm f/4 IS USM is showing its age, with a low 13P-Mpix Sharpness score and disappointing Transmission value (of 4.6Tstops). Chromatic aberration, Distortion and Vignetting are close to the longer, more difficult to correct 500mm f/4G, all pointing towards the Sony performing well in these areas.
Overall, the Sony looks promising but it’s a pricey option. As well as using fluorite the new Canon super-telephotos have the latest image stabilization technology (offering up four-stops compensation, and including a new Mode 3 option to prevent disruption during composition). Not that, that is going to be of interest to Sony users, but goes some way to explain the cost of a lens like this. Without those features or the performance to match, the Sony lens looks over-priced.
Based on our P-MPix figures, the Sony is close to 20% sharper than the Canon 300mm f/4L IS USM, but some 20% less sharp than the new Canon 300mm f/2.8 L IS II USM. It’s worth noting that although the Canon lenses are shorter in focal length and easier to correct, the additional stop in ‘brightness’ or Transmission (actually 1.1TStops) of the 300mm f/2.8 makes that and the Sony similarly challenging to correct for aberrations during the design stage.
You could look at it another way. Teleconverters are regularly used to supplement telephotos, and while the 300mm f/2.8 matched with a 1.4x converter may be too short at 420mm f4, a 2x extender will give you a 600mm f/5.6. Both types are made for Nikon lenses but they also produce a 1.7x teleconverter that will adapt the AF-S 300mm f/2.8 to a 510mm f/4. If you’re more likely to find use of a 300mm and require longer lengths less frequently, supplementing that with teleconverters may be answer.