Buy any DSLR that comes complete with a standard zoom lens and, chances are, your next purchase will be a telephoto zoom.
A typical 18-55mm 'kit' lens offers a fairly generous wide-angle perspective on life, equivalent to about 28mm, but runs out of reach at the long end of the zoom range. That's OK for portraiture, but when you want to close in on distant subjects, only a telephoto zoom will suffice.
Most of us use APS-C format camera bodies, so there's something to be said for buying a telephoto zoom specifically designed for an APS-C format camera. The focal length is generally around 55-200mm, so once you consider the crop factor or focal length multiplier, they're equivalent to about 80-300mm.
This effective zoom range is pretty much the same as conventional 70-300mm budget telephoto lenses, originally designed for full-frame cameras. But what if you upsize to a full-frame lens?
The physical size of a full-frame 70-300mm lens is likely to be bigger than an APS-C format telephoto zoom, but there are some distinct advantages.
Firstly, you'll get more telephoto reach, equivalent to 450mm (480mm for Canon APS-C bodies), giving you greater telescopic power in a package that's still fairly compact and lightweight. Secondly, because the image circle produced by the lens is cropped by APS-C cameras, you'll only be using the central region, where image quality is at its best.
You can expect greater sharpness into the corners of the frame, and better peripheral illumination, so vignetting (darkened image corners) is less of an issue. And if you ever decide to switch to a full-frame DSLR, you can carry on using the same telephoto lens.
Focal length gap
One thing that puts some people off going for a 70-300mm lens is the gap in focal length between a 18-55mm and larger lens. In practice however, this isn't a major problem.
Moving a few paces when composing, or a little creative cropping when using your standard zoom lens at its longest focal length, is all that's needed to offset the missing 55-70mm of zoom range.
Even so, out of all the lenses we tested here, Nikon bucks the trend. The Nikon AF-S DX 55-300mm f/4.5-5.6G ED VR gives the conventional long-end reach of a full-frame lens, but it's the only one on test that's actually designed for APS-C format cameras. As such, it picks up at the same focal length that a standard zoom runs out, while still offering the same maximum telephoto reach of other lenses in the group. But what price image quality?
The lenses in this group fall short of the fully professional qualities you'd expect from constant aperture 70-200mm f/2.8 lenses, which cost around £1,500 or more. But they fall into one of two main categories, either being built to a rock-bottom budget or aiming a few rungs further up the quality ladder.
Some of the older and cheaper lenses in the group were originally designed for 35mm film cameras. While they can still produce good results with DSLRs, they tend to be lacking in features, and optical quality isn't likely to be as good.
For £300 or more, you can expect better optical quality and state-of-the-art technology which, in most cases, will include optical image stabilisation. It's a great feature for shooting handheld, especially with zoom lenses, as camera-shake is more of a problem at long focal lengths.
The rule of thumb is that for consistently sharp handheld shots you need a shutter speed that's at least as fast as the reciprocal of the effective focal length.
So, if the effective focal length of your telephoto zoom stretches to 480mm, you'd need a fast shutter speed of 1/500 sec.
The largest available aperture at the longest zoom setting of a budget telephoto lens is normally f/5.6, so you may struggle to get sufficiently fast shutter speeds in anything other than bright, sunny conditions.
Sure, you can increase your camera's ISO setting to enable faster shutter speeds, but that's likely to impact on image quality.
The lenses that feature image stabilisation are signified as Canon IS (Image Stabilization), Nikon VR (Vibration Reduction), Sigma OS (Optical Stabilization) and Tamron VC (Vibration Compensation).
Most boast a four-stop advantage, meaning that you can expect sharp handheld results when shooting at just 1/30 sec (at a focal length of 480mm) instead of 1/500 sec. With a three-stop stabiliser, you'd need to increase the shutter speed to 1/60 sec.
Tamron doesn't fit its VC system to the Sony version of its 70-300mm lens because Sony bodies have sensor-shift stabilisation built in. Sigma takes a different view, fitting optical stabilisers to both the Pentax and Sony versions of its 70-300mm OS lens, so you get the option of using either sensor-shift or optical stabilisation on these cameras.
We've found that optical stabilisation generally produces more consistent results, so it's worth having.