18-250mm Superzoom Lens Group Test: Verdict

Sharpness and Resolution

With only one exception, the MTF curves for all of these lenses reveal that their long focal-length settings are more heavily challenged by the massive zoom ranges that are on offer. Sigma's lens differs slightly in that its minimum focal-length appears to drop to the lower side of the cluster of curves at about f/11 but in fact what has happened is the 145mm focal-length has suddenly become particularly strong.

Nevertheless, it is always true to say that the shorter focal-lengths correspond with the highest MTF figures. In fact the peak resolutions recorded are very good indeed, particularly for the Pentax, Sigma and Sony lenses. The first two have somewhat untidy curves but the trends are believable and there is no reason to doubt the quality of the test samples: the Sony zoom's curves are beautifully neat and well-behaved.

Internal Focusing

MTF figures are far from the whole story, however, and the use of internal focusing is a huge bonus when it comes to using long focal-length lenses. Pentax's lens and both of the Tamron zooms are described as having internal focusing yet their focusing rings all rotate in AF mode. Non-movement of the focussing ring is one of the main benefits offered by IF and it is sorely missed here.

For the record, there is no movement of the lens barrel during focussing in any of these lenses, including Sony's and Sigma's zooms which are not specifically designated as being equipped with IF systems.

Image Stabilisation

Only two of the lenses feature image stabilisation technology: Sigma's 18-250mm has an Optical Stabilizer and Tamron's 18-270mm features Vibration Compensation: these systems proved invaluable during field testing when the zooms were set to longer focal-lengths. The Pentax K200D and Sony α900 cameras both incorporate anti-shake technology in their bodies but this approach does not steady the viewfinder image, which means missing out on a major advantage when the user is attempting to hand-hold the full-frame equivalent of a more-or-less 400mm lens in blustery conditions.

Although the image-stabilised lenses are generally more costly than the others, the price range is relatively narrow and Sony's non-stabilised lens actually falls in between the two stabilised lenses. The image-stabilised lenses are, however, clearly and without exception heavier.

Conclusion

As to the big question of whether or not one of these zooms could truly be regarded as an all-purpose lens, the answer depends on your definition of "all-purpose". None of the lenses has a wide maximum aperture so creative selective-focussing techniques will be hard to apply and low-light photography will be limited; none of them offers true macro capabilities; none of them has a static focussing ring in AF mode; none allows manual focus adjustment; none can maintain quite the same level of high MTF performance at their longer focal lengths as they do at shorter settings.

Turning to the lenses' strengths; travel photography looks like being the top choice with undemanding portraits, sports and nature (except macro) photography following on. Technical and architectural photography will probably prove more challenging owing to aberrations and image distortion, which must be near-zero in these applications but have a definite presence in all these lenses.

That is important because as time passes and super-zooms push the focal-length boundaries still further they will inevitably have to improve in other areas too. And as these lenses get better and better their pictures will be examined even more closely, pushing the bar higher still. None of these lenses could yet serve as the sole lens for a demanding photographer but who knows what the future may bring?