The 18-200 gives distinctly mixed results in our studio tests; sharpness is perfectly reasonable in the normal to moderate telephoto range, but dips alarmingly at the 135mm setting before rallying at 200mm (indeed we spent a lot of time verifying the 135mm results, as they were so unexpectedly low). Also distortion is extremely marked across the entire focal length range.
|Sharpness||Sharpness is very good between 18mm and 70mm, giving decent result across much of the frame at optimum apertures. However the telephoto end is a whole different story; the lens really struggles at 135mm, although performance returns to respectabability at 200mm, especially in the centre of the frame.|
|Chromatic Aberration||The results show red/cyan chromatic aberration at wideangle, shifting progressively towards blue/yellow at telephoto. As is usually the case, changing the aperture has little effect. Overall this is not a bad performance, considering the zoom range.|
|Falloff||We consider falloff to start becoming a potential problem when the corner illumination falls to more than 1 stop below the centre. The 18-200mm performs commendably here; falloff is only an issue at the two extremes of the focal length range, and in both cases essentially disappears on closing down one stop.|
|Distortion||Distortion is a problem at almost all focal lengths. The 18mm setting displays an extremely high level of barrel distortion (2.8%), which changes rapidly to pincushion by 24mm, and becomes extreme from 35mm to 70mm (peaking at 50mm and 2.3%), before diminishing again towards the telephoto end.|
Specific image quality issues
As always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests.
The distortion shown by this lens in our studio tests was so marked that we felt it merited illustration with some real-world shots. Barrel distortion at wideangle isn't unusual on zooms, but this lens is worse than most, and it has a complex 'wave' pattern with recorrection towards the corners, making software correction relatively tricky. It's also rare that we find pincushion distortion so pronounced as to be visually disturbing, but that can be the case between 35mm and 70mm; the good news is that the pattern is simpler, and can be readily corrected in software if desired.
Now it must be said that distortion only really causes problems on shots that feature lines which clearly shound be straight, and many subjects (e.g. landscapes, portraits, wildlife) simply don't have these to a great extent. But the bottom line is that if you like to photograph architecture or other regular structures, and want your straight lines to remain that way, you will need to consider a different lens (or lenses).
|18mm - barrel distortion||35mm - pincushion distortion|
Image quality at intermediate telephoto settings (135mm)
Again, this is an aspect of the lens's performance which bears elaboration with real-world examples. The MTF graphs show a big dip in performance at 135mm, and this is reflected in real-world photos between about 100mm and 150mm. The issue also appears to depend on distance, with image quality falling further at close focus, where it's not helped by decidedly inelegant handling of even slightly out-of-focus areas. Indeed the image quality at 135mm and F5.6 can visibly impact even small prints (6" x 4"/10cm x 15cm), and whilst the performance at F8 is better, it only really sharpens up at F11, just prior to the the onset of diffraction.
The problem here is that for many telephoto subjects (e.g. wildlife or sports) the use of large apertures is highly desirable, both to keep shutter speeds high and to help isolate the subject from the background. This means that photographers who are planning on shooting at telephoto much of the time, e.g. for wildlife, will probably be better served by a telezoom such as the 70-300mm F4-5.6 VR.
However it's also worth pointing out that some compromises are inevitable in a superzoom with an 11.1x range. Ideally the strongest focal lengths of the lens should be those which are going to be the most-used, and we might expect typical shooting of this lens to be mainly in the normal to short-telephoto range (around 18-70mm) for 'everyday' shots, peaking again at 200mm for long shots; so having the 135mm region as weakest is perhaps an acceptable compromise.
|135mm F5.6, long distance||100% crop - edge of frame|
|135mm F5.6, close-up||100% crop - edge of frame|
Flare is overall rather well controlled, and certainly better than the 18-55mm VR we tested in parallel. The lens copes pretty well with the sun placed in the top corner of the frame, and due to its complex optical construction, can show intricate flare patterns at small apertures - quite pretty if you like that sort of thing. It's also far less fazed by strong sidelighting than its cheaper stablemate.
|18mm F16, sun in corner of frame||48mm F8, strong sidelight|
Background blur ('bokeh')
One genuinely desirable, but difficult to measure aspect of a lens's performance is the ability to deliver smoothly blurred out-of-focus regions when trying to isolate a subject from the background, generally when using a long focal length and large aperture. This lens can allow you to achieve quite substantially blurred backgrounds, especially at 200mm F5.6.
The appearance of the background blur is not entirely smooth, with hard-edged specular highlights giving a harsh appearance to defocused regions (in fairness this is rarely a strength of this type of zoom). But with a little thought plus careful choice of background, it's possible to coax some quite appealing results from this lens.
|200mm F5.6||100% crop|
|200mm F5.6||25% crop|
The 18-200mm features Nikon's second generation 'VR II' vibration reduction system, which claims to allow handholding at shutter speeds 4 stops lower than usual before blur from camera shake becomes apparent. The mechanism is near-silent in use, with just a very quiet whirring noise when operational, and with distinct clicks when it activates and deactivates from the VR group moving in and out of the 'at rest' position.
We've generally found the stabilisation units in SLR lenses to be pretty effective in real-world use, and to quantifiy this, we subjected the 18-200mm to our studio image stabilisation test, using the wideangle and telephoto settings plus one mid-range focal length (50mm). With its effective focal length range of 27-300mm, we'd normally expect to be able to get good results handheld at 1/50 sec at wideangle, and 1/400 sec at telephoto without image stabilisation. The subject distance for these tests was approximately 2.5m.
We take 10 shots at each shutter speed and visually rate them for sharpness. Shots considered 'sharp' have no visible blur at the pixel level, and are therefore suitable for viewing or printing at the largest sizes, whereas files with 'mild blur' are only slightly soft, and perfectly usable for all but the most critical applications.
|18mm VR OFF||50mm VR OFF||200mm VR OFF|
|18mm VR ON||50mm VR ON||200mm VR ON|
It's no great surprise to see Nikon's VR system performing well here. Whilst it doesn't quite seem to deliver the 4 stops which Nikon claim in our hands, it comes pretty close, especially at 200mm. The key difference here is not so much the yield of critically sharp shots at slow shutter speeds, as the hugely increased chances of getting usable shots with only mild blur, which would be completely impossible in the absence of stabilisation.
At 18mm, therefore we see an 80% chance of getting usable results at 1/6 sec; at 50mm, an 80% chance of getting usable results at 1/13 sec; and at 200mm, an 80% chance as low as 1/25sec. Even at slower speeds a stop slower than these, you'll get 'keepers' hand held if you're prepared to take multiple shots. This therefore greatly increases the lens's versatility in low-light conditions (or indeed any other situation where a slow shutter speed would be desirable).
The VR system on this lens also features automatic panning detection, which disables stabilisation in the direction of movement when it detects the photographer is following a moving subject. A few informal tests in the field showed that this seemed to do a decent job, and worked as advertised when tracking moving vehicles or birds in flight.
Finally the lens also has an 'active' mode for shooting from moving vehicles, but we've not had the chance to test this in any meaningful fashion, so can't really comment on its performance.