Super-tele tech
These are technical marvels that shoot to infinity and beyond
Sigma and Tamron have been pushing the envelope over the years, stretching the max focal length to 600mm. The extra reach of current Sigma and Tamron supertele zooms is nice to have, but comes with a narrowing of the aperture rating, which shrinks to f/6.3 at the long end of the zoom. Both companies have also released 100-400mm lenses that use this aperture towards the long end of their zoom.
Mod cons tend to include fast and whisper-quiet ring-type ultrasonic autofocus systems, along with effective optical stabilization, which is crucial when shooting handheld, or with a monopod at long focal lengths. The rule of thumb for handheld photography is that you should
use a shutter speed that’s at least as fast as the reciprocal of the focal length of the lens (‘effective’ focal length for crop sensors).
That’s about 1/500 sec when shooting with a focal of 500mm on an FX body, or about 1/1000 sec when shooting at 600mm on a DX camera. This can be hard to achieve without ramping up your ISO, given that the widest apertures of most super-teles shrink to around f/5.6 or f/6.3 at the long end. Stabilization helps get sharp results, usually at shutter speeds up to four stops slower. Most super-tele zooms have automatic or switchable panning modes, ideal for when you want to create motion blur with slower shutter speeds.
You could use any of the lenses in this test with the same make of teleconverter, but we wouldn’t recommend this with a lens that shrinks to f/6.3 at the long end of its zoom range. In this case, you’d end up with a widest available aperture of f/9 or f/13, using a 1.4x or 2x teleconverter. This makes autofocus difficult or impossible, even with cameras that support autofocus at f/8, as well as forcing you to bump up the camera’s ISO setting to maintain fast shutter speeds.
By comparison, the 1.5x crop factor of a DX format body gives no loss of aperture, maintains full autofocus facilities with no limitations, and image quality is sure to be better than using a teleconverter. So using a camera like the D5600, D7500 or D500, the ‘effective’ maximum focal length of a 600mm lens would be boosted to a whopping 900mm.