I have the Cannon 28 - 200 USM lens. It seems that the range between 135 and 200 is quite short. In fact, I compared the lens at 200 with the 70 - 300 lens at 200, there is a significant difference in terms of the zoom. Is this a defect or a common issue.

-- John Chen (upupdown01@yahoo.com), May 28, 2002

Answers

I don't know what the exact answer in this specific case is, but it's probably one or both of the following:

The focal lengths are approximate; lens manufacturers round them to the closest standard focal length, so it's entirely possible that the 200mm setting on the 28-200 is really 203 or 189 or some other number. Also, the intermediate markings on a zoom are approximate - where the 200 line is might not be exactly 200.

Also, focal length is measured at infinity, and often changes as you focus closer - particularly with internal focus lenses, for reasons I don't understand but someone with a decent understanding of optics probably would. If you were focusing on something reasonably close to you, that could cause the two lenses to shift their focal lengths by different amounts.

-- Steve Dunn (steved@ussinc.com), May 28, 2002.

Steve's probably right on both counts. All the wide ranging zooms that I've tried, and that have internal focusing, loose a great deal of their focal length when focusing at less than infinity. Less ranging zooms don't seem to be as effected. The Canon 28-135 gets blamed all the time for being no longer than the 28-105, but when you compare them at infinty you see the difference.

-- Jim Strutz (j.strutz@gci.net), May 28, 2002.