- Jon Tarrant
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Perfect Lenses (part 2) - Features
The left side of this composite was taken using image stabilisation whereas the right side was taken simply trying to hand-hold the lens as steadily as possible against a strong wind. It is clear that the image stabilised section looks sharper and exhibits better detail. Photograph (c) Jon Tarrant.
When Nadia Comaneci scored the first perfect 10.0 for gymnastics (at the 1976 Montreal Olympics) people realised they were going to have to have to rethink the scoring system. One option might have been to scale the existing scores downwards but in fact a new two-part scoring system was introduced and the perfect 10.0 is now gone for ever.
There is a lesson here for lens testers and it makes sense to think about the features that a perfect lens might possess if it were score 20/20. Some of the obvious candidates include;
- a fast and constant maximum aperture setting
- a wide zoom range
- internal focussing (with no movement of the focussing ring)
- a tripod platform / carrying handle (for longer focal lengths)
- the option to rear-mount filters (for ultra-wide angles of view)
- an effective lens-hood (built-in if possible)
- a protective pouch / case in which to store the lens
- both APS-C and full-frame compatibility
- full-time manual intervention in AF mode
- image stabilisation technology
- compatibility with multiple camera systems (lens mounts)
- a genuine macro focussing capability
- the ability to pass distance information to the camera body
- visual focused-distance information
- full weatherproofing
- integral high-speed AF drive system
- low-dispersion elements
- aspherical lens profiles
- apochromatic focussing
- a near-circular aperture
Astute readers will spot the fact that there are 20 items in this list and, although some of them are mutually exclusive, it would easily be possible for a lens to be given perhaps just one or two marks out of 20 if it were docked one point for each appropriate (imaginable) feature that the lens lacked. More importantly, despite lacking many of the features on this list a lens might still offer a reasonable balance of features for its given price point and intended market segment. Clearly, therefore, a more refined approach is needed to determine a truly meaningful score.
Consider two everyday lenses that are more-or-less at opposite ends of the lens spectrum; a 100mm fixed focal-length macro lens and an 18-250mm super-zoom. The former is intended for one very specific purpose whereas the latter is meant to be an all-rounder and the features that they offer are correspondingly different.
Readers who have access to printed copies of What Digital Camera will find a super-zoom group test in the current (January 2010) issue and will see that Tamron's 18-270mm comes top for Features with a score of 19/20. All it really lacks is an IF system that leaves the manual focussing collar undisturbed. Obviously it would be great to have an f/2.8 maximum aperture that is maintained right across the zoom range but the inevitable increase in cost and mass would count against the lens as an all-rounder so no marks can be deducted on that count. Similarly, we will doubtless have 18-350mm (or whatever) zooms in the future and they might be expected to score more highly for Features but if you still have to keep your fingers clear of the focussing ring in AF mode then the perfect score will remain elusive.
By way of contrast, Nikon's Micro-Nikkor AF-S VR 105mm f/2.8G IF-ED scored 20/20 in last autumn's macro group test because it already has everything that can reasonably be asked of a lens of this type. In fact it may already be over-specified but the Features score is not the place to deduct marks on that basis: lowering the Value mark slightly would be a more appropriate tactic (as was done in that instance).
Although there is a certain amount of subjectivity in the way in which the Features and Value scores are determined I hope that this brief explanation will have thrown a little light on the matter.