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18-200mm zoom lens issues

As super-zoom lenses with ranges of 10x or higher become increasingly common, an interesting optical issue will be experienced by more photographers. Many thanks to Hang Chen, Professor of Mathematics and Computer Science at the University of Central Missouri who contacted our Forums following his experiences with a Nikkor 18-200mm VR zoom.

Under certain conditions, Professor Chen noticed the Nikkor 18-200mm at 200mm delivered approximately the same field of view as his older Nikkor 35-135mm did at 135mm when fitted to the same D200 body. Surely the longer focal length of the former lens should deliver a field approximately 1.5 times smaller. Some investigation was clearly in order.

Below are images taken with the Nikkor 18-200, 35-135 and 75-300mm lenses at focal lengths of 200, 135 and 200mm respectively. All lenses were fitted to a D200 mounted on a tripod to ensure a consistent position. As you’d expect, the 35-135mm at 135mm delivers a wider field of view than the 18-200mm does at 200mm. The latter field is also roughly the same as that from the 75-300mm set to 200mm. So far so good.

Nikkor 18-200mm at 200mm
(focused at infinity)
Nikkor 35-135mm at 135mm
(focused at infinity)
Nikkor 75-300mm at 200mm
(focused at infinity)
18-200mm at 200mm
(300mm equivalent using D200)
35-135mm at 135mm
(203mm equivalent using D200)
75-300mm at 200mm
(300mm equivalent using D200)

Where things get strange though are at closer range as seen below. Again, all images were taken with a D200 mounted on a tripod in a consistent position. Here the 35-135mm at 135mm appears to deliver pretty much the same field of view as the 18-200mm does at 200mm. In contrast, the 75-300mm at 200mm delivers a tighter field, which is what you’d also have expected from the 18-200mm at 200mm.

Nikkor 18-200mm at 200mm
(focused at 3m)
Nikkor 35-135mm at 135mm
(focused at 3m)
Nikkor 75-300mm at 200mm
(focused at 3m)
18-200mm at 200mm
(300mm equivalent using D200)
35-135mm at 135mm
(203mm equivalent using D200)
75-300mm at 200mm
(300mm equivalent using D200)

While our reviews reveal lenses with the same focal length ratings often deliver slightly different fields of view, none have resulted in such a big difference. Under certain conditions, could the 18-200mm at 200mm really have no advantage over the 35-135mm at 135mm? We contacted Nikon UK’s professional support department to find out.

Many thanks to James Banfield from Nikon UK for the following explanation:

“What the reader is experiencing is due to an anomaly that happens with all lenses but is more pronounced with zoom lenses that cover vast focal ranges.

As you may know a lens focal length is calculated with the lens focused at infinity. The focal length on any lens however varies depending upon the point of focus due to the element groups shifting. On a fixed focal length lens this subtle variation is normally unnoticeable but zoom lenses amplify this occurrence and this variation becomes increasingly more obvious as the focal range of the zoom increases.

With lenses like the 18-200mm VR the focal range is so large that the effect can be quite visible - on this specific lens it is more noticeable when focusing on near objects close to minimum focusing distances, as proven by the reader, where the field of view is much wider.”

So there you have it: the focal length quoted on your lens is with it focused at infinity, and will vary to some degree as the lens is focused at closer distances. As explained by Nikon, this effect is most noticeable with zoom lenses sporting long optical ratios when focused at their closest distances. It’s not a new anomaly by any means, but one which has become more obvious as super-zooms have become more common.

Ultimately super zoom lenses like the Nikkor 18-200mm VR still deliver genuine 11.1x optical ranges and genuine 200mm focal lengths when focused at infinity, but it’s worth knowing their power will be reduced relative to lenses with smaller zoom ratios when focused at close range. For more details on this lens, check our Nikkor 18-200mm review and video tour

Gordon Laing




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