It’s like comparing chalk with cheese. Lightweight budget telephoto zooms, the sort sometimes bundled as ‘twin lens kits’ with DSLRs, are fine for casual use if you’ve got a DSLR with an APS-C sensor. But they have significant disadvantages, quite apart from the low-cost build. One is the variable maximum aperture; typically, a 55-200mm zoom will go from f4 at the 55mm end of the zoom range to f5.6 at the 200mm end. It’s bad enough that the maximum aperture should be so restrictive in the first place (f4 is hardly ‘fast’), but even more of a nuisance that the aperture range available depends on the focal length you’re shooting at.

This won’t affect the average snapshooter much. If the camera’s left on program AE mode it’ll sort it all out and leave you free to concentrate on what’s in the viewfinder. But if you like to take control over the lens aperture yourself, or work out the exposure manually, the changing aperture value will be a handicap.

The Tamron, by contrast, has both a faster maximum aperture and a constant one. At the shorter end of its zoom range it’s around a stop faster than the average telephoto zoom, while at the longer end it’s a full two stops faster. This means you get a brighter viewfinder image and you can shoot in dimmer light without having to increase the ISO.

Having said that, though, the advantages of fast lenses like these are being eroded by image stabilisation mechanisms and improving high ISO performance. You might find, in fact, that the Nikon 55-200mm VR, say, is steadier than the Tamron even though you have to use slower shutter speeds because of the smaller maximum aperture. The latest DSLRs are so good at high sensitivities that you can up the ISO to 800 or 1600 without having to worry too much about picture quality.

The Tamron is more likely to appeal to serious enthusiasts and professionals who still value optical speed above all else and frankly you are going to have to be pretty enthusiastic to put up with the penalty in weight and size. A budget 55-200mm lens might be barely larger or heavier than your kit lens, but by comparison this Tamron is quite a monster. One reason is the sheer amount of glass needed to provide this maximum aperture over the whole zoom range. Another is that this is a full-frame lens – and this makes it look even better value. On a regular DSLR it’s roughly equivalent to a 109-310m zoom, but it produces a full-size image circle so it can also be used on a full-frame DSLR like the EOS 5D Mk II or Nikon D700. This makes the Tamron an even more attractive purchase at a time when the DSLR market is undergoing such rapid change.

You might have an SLR with a smaller-sized sensor now, but who’s to say you won’t go full-frame next year, or the year after that? By then, full-frame SLRs may be as cheap as good APS-C SLRs are today.

This is something that’s worth thinking about, because if you buy a DSLR lens, you’re tying yourself down to an APS-C SLR for some time to come. With the Tamron, though, you’ve got both bases covered – as long as the optical quality is up to scratch, of course. There’s no need to worry about that here, though: the Tamron’s test results were very impressive indeed. There was a noticeable loss in definition at full aperture at longer focal lengths, but this is only to be expected, and the results wide open were still pretty good. By f4 though, it sharpens up considerably, and it was good to see that there’s almost no drop-off in sharpness at longer focal lengths (common on less expensive designs).There is a small degree of chromatic aberration at 70mm, but the levels are low compared to most other telephoto zooms. There’s no visible distortion at 70mm, and only the faintest hint of pincushion distortion as the focal lengths get longer. The performance is remarkably consistent from the centre to the edge of the frame, too. All in all, the optical performance is excellent.

The autofocus is less impressive, though. It’s a little noisy but, worse than that, it’s quite slow. It’s pretty poor at keeping up with subjects moving in and out of the plane of focus, and in these situations you’re probably going to be better off focusing manually – to do this, you just slide the Focus ring back to disengage the AF.

As previously mentioned, this lens is more likely to suit pro-level photographers. The Tamron does demand a certain amount of skill and patience, and the focusing precision required doesn’t just apply to moving subjects. The f2.8 maximum aperture allows for very attractive depth-of-field effects, but it also requires pinpoint focus accuracy. This is the main point about this lens and others like it – it’s going to take a good deal of patience and skill to get the best results. On paper, a fast maximum aperture that’s constant across the zoom range looks ideal. In reality, though, making proper use of it might not be quite that easy. Anyone who’s used fast telephoto zooms before will know this already, but if you haven’t, and you’re weighing the Tamron up against a cheaper, lighter budget zoom, it’s important that you know exactly what you’re getting into.

Final Verdict If you need a constant-aperture telephoto zoom and you can put up with the bulk and weight, you won’t find a better bargain OVERALL

Our lens reviewer, and technical expert, Rod is a veritable photographic encyclopaedia. His illustrious CV has seen him write for many mags, websites and journals.