Differnce in zoom/focal length?

[Seekwence]

I have started off with both the Nikon 15-55 VR and 55-200 VR lenses for my d3000. Recently I purchased the Sigma 18-200 OS and have noticed that the 200mm focal length on the Nikon is noticeably "longer" than the Sigma at 200mm. It's not a big deal, but I am a little confused.

Sorry, I don't have any sample shots to post right now...

 

[icassell]

I don't know those particular lenses, but this is a known issue with many 3rd party as well as OEM lenses -- they're not always exactly as claimed.