A telephoto zoom is the first additional lens most DSLR or compact system camera owners go for. These can be pretty cheap but, unfortunately, the performance can be disappointing.
Cheap telephoto zooms tend to lose sharpness at their maximum zoom setting, which is where you most often want to use them.
Cheap telephotos also have a maximum aperture that shrinks as you increase the zoom. They might start out at f/4, for example, at their shortest zoom setting, but as you zoom in the maximum aperture typically shrinks to f/5.6 or even f/6.3.
To keep shutter speeds fairly fast for minimising camera-shake and motion blur, you can often find yourself having to combine the longest zoom setting with the widest aperture, which can really degrade image sharpness.
At the other end of the scale, fully professional telephoto zooms like the Canon EF 70-200mm f/2.8 L IS II USM, the Nikon AF-S 70-200mm f/2.8G ED VR II and the Sony 70-200mm f/2.8 G SSM II are much more refined. The relatively wide f/2.8 aperture remains available throughout the zoom range, and the physical dimensions of the lenses remain fixed at all zoom and focus settings.
But these professional lenses come with problems of their own. First, they're expensive – they can cost as much as some full-frame DSLRs. Second, they have large front lens elements to provide that f/2.8 aperture all the way through the zoom range, so they are big and heavy and typically weigh in at around 1.5kg.
But one popular compromise is to opt for a less expensive 70-200mm f/4 telephoto zoom. You lose one f-stop in maximum aperture but it's still constant throughout the zoom range. These lenses are smaller, lighter and cheaper – yet you can still expect robust build quality and premium-grade glass.
Wide maximum apertures aren't quite as important as they used to be. The latest DSLRs usually deliver excellent image quality at raised ISO settings, and you won't see much of a loss in quality if you raise the ISO setting by 1 stop to allow for an f/4 lens versus an f/2.8 lens. You can still get very shallow depth of field when shooting at 200mm at f/4, so it's still possible to get nicely defocused backgrounds.
Another bonus for Canon and Nikon shooters is that some of the latest DSLRs can autofocus when the widest available aperture is as small as f/8. This means you can fit a compatible 2x tele-converter to a 70-200mm f/4 lens without having to resort to manual focussing. Live View focussing is also possible at even narrower apertures but, on DSLRs at least, it tends to be very slow.
Another way to reduce purchase costs is to switch brands. Compared with 70-200mm f/2.8 lenses from the likes of Canon, Nikon and Sony, direct competitors from Sigma and Tamron can cost half the price or less. Even so, they still offer a very good standard of build quality, upmarket optical elements, and similar ring-type ultrasonic autofocus systems, as used on own-brand telephotos. The latter can be an important consideration, because many of us use this type of lens for sports, action and wildlife photography, where fast, accurate autofocus is vital.
The Panasonic Micro Four Thirds and Sony E-mount lenses are the only ones in this round-up to lack ring-type ultrasonic autofocus. Instead, they use stepping motors. (Canon DSLR users with the latest 18-55mm or 18-135mm STM kit lenses will be familiar with this technology.)
A plus point for video shooting is that this autofocus technology is practically silent, while also giving smooth focus transitions without the lurching effect of conventional systems. The downside is that, in DSLR lenses at least, autofocus isn't particularly quick. Our tests do reveal a slight performance deficit with the Panasonic and Sony lenses.
For hand-held telephoto shooting, image stabilization is another key factor. At long focal lengths, we've generally found that in-lens optical stabilization is more effective than camera-based, sensor-shift stabilization. Indeed, Canon and Nikon SLRs don't have in-camera stabilization at all.