A Photography Question: when is 200mm really 200mm?

I use two lenses with my DSLR: an 18-50mm zoom and a 55-200mm zoom, both Sigma. Before I went on holiday in June, I bought one lens to cover the same range – another Sigma, 18-200mm.

When I got home, I wanted to compare the new lens quality to my old lenses. I set it to 200mm and took a picture; then did the same with my 55-200mm.

The quality wasn’t an issue; but the two pictures had quite different fields of view – despite both being “200” mm. I estimated that at its full extent, the new lens was equivalent to c. 130mm on the older lens.

I wasn’t happy about this – having just forked out a couple of hundred quid, I had failed to duplicate the full range. I took the lens back to the shop, where we duplicated the test, and the salesman agreed that the field of view was significantly less than I had expected. He explained that this was because the 10-200mm lens was not a zoom but a “versatile” lens. This sounded like saleman’s flannel – surely 200mm should be 200mm? (He was good enough to refund my money, and I would recommend them for good service and prices.)

My brother was visiting this weekend, and he has a Nikon 18-200mm zoom, so we repeated the experiment – with exactly the same results. The image from the Nikon lens had a much wider field of view, equivalent to about 130mm on the Sigma.

Here’s the image from my original lens, set at 200mm:

Here’s that from the Nikon 18-200mm, set to 200mm:

Here are the two overlaid, the Sigma image resized (not a perfect fit):

And here are the outlines of the two, for comparison – the Nikon 18-200mm in red, the Sigma 55-200 in blue:

The metadata recorded by the camera says that both lenses were shooting at 200mm.

Clearly, 200mm is not always 200mm. I don’t know which lens is truer to 200mm: I don’t have access to a fixed 200mm lens.

Does anyone have an explanation for the difference between the lenses?

Any views appreciated!

(crossposted)