A quick glance through a couple of makers’ catalogues (or our database) will confirm the plethora of 70-300mm f/4.5-5.6 zoom lenses available. These aren’t particularly fast, and while there are a number of 28-300mm f/3.5-5.6 lenses, the variable maximum aperture will likely be in the same f/4.5-5.6 range over the 70-300mm focal lengths, so there’s nothing to be gained in speed (brightness) for available light.
We can see from the DxO Mark Score results for our whole lens database plotted on the graph above that, even for primes, it’s very difficult to achieve the same image quality at 300mm than it is at shorter focal lengths. As an example, take Nikon’s two recently introduced 85mm lenses. Both have a higher DxO Mark score than the new $6,600 Canon EF 300mm f/2.8L IS USM (we’ve yet to assess the Nikon AF-S NIKKOR 300mm f/2.8G ED VR II). In fact, the pricey $2,000 AF-S Nikkor 85mm f/1.4G was recently ousted by the newer, cheaper AF-S f/1.8G version.
For those that are interested, we’ve adopted the DxOMark Score system instead of the recently introduced P-MPix metric, as that’s calculated under optimal/ideal shooting conditions. DxOMark Score, on the other hand, is measured according to our low light scene (150 lux and an exposure time of 1/60thsc), a more realistic metric for lenses intended for capturing elusive wildlife.
As the DxOMark score is an average of the whole focal range, zooms with a wide-range could be artificially higher in the ranking. For wildlife, the image quality at 300mm or greater is what we’re particularly interested in, but as images may be captured at any focal length, we believe the average DxOMark score is still relevant for ranking one lens above another. Even so to see how a particular zoom lens performs at a specific focal length, then the reader should check the DxOMark score map.
If money is no object, then the Canon EF300mm f/2.8L lenses easily outperform the more versatile 70-300mm f/3.5-5.6 zooms, and can be paired successfully with 1.4x and x2 teleconverters, or extension tubes (to reduce the minimum focus distance) adding to the flexibility. Adding teleconverters to 70-300mm f/3.5-5.6 lenses is possible but with the loss of one to two stops, autofocus operation is likely to be impaired or even impossible. The other downside is the effect teleconverters have on image quality. At 300mm, the optical performance of telephoto zooms will be at its lowest, and adding a teleconverter, or multiplier will only make this more noticeable. Our graph above shows the results for full-frame cameras, but APS-C cameras deliver slightly lower scores, however the ranking is very close.