Why doesn't Canon 18-200 mm lens show any visible difference from 170 to 200 mm?

Question

Not that it's very important but I'm wondering why doesn't a Canon 18-200 mm lens have visible change in framing between 170 and 200 mm? Why is it declared as a 200 mm lens if it stops zooming at 170-something millimeters?

Or is it something subtle that I'm missing? 30 mm on a lens is usually not negligible so I'd also like to know is it the same throughout the focal range or is it less noticeable at telephoto end?

Answer

The focal-length of lenses is measured while focused at infinity. If you were inside a store you likely focused at less than infinity and got a different focal-length than 200mm at the long end. The closer you focus, the more likely you are to be off.

At infinity, there should be a small (probably 7% as @mattdm computed) but noticeable difference. Otherwise the lens was most likely defective.

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