Sigma 18-200

I have had the Sigma 18-200mm lens (non OS version) for some time now and have been very happy with its quality, handling and zoom range. However, since getting my Pentax 50-135 DA* lens I have discovered something strange with the Sigma Lens.

I was interested to see the difference between the 135 and 200mm end of the zoom, how much extra was the Sigma giving me? So I set the zoom to about 135 on the indicator and checked the Info which indicated 133mm then looking through the zoom I zoomed to 200. There was hardly any difference! So I decided to run a few tests to see, the following pictures are the result and are not cropped or adjusted in any way, just resized to 800pixls wide (sorry the photos are not great, just to illustrate field of view):
Sigma 70-210 @140 (Exif)



Sigma 18-200 @133 (Exif)



Pentax 50-135 @135 (Exif)



See what I mean!

I tested the field of view at 18mm against the 18-55 standard and it looked the same. I also took the lens to 200 and compared with the 135 on the DA* it seemed similar close (about 10ft) but better for a further subject! Weird!
Sigma 18-200 @133 (Exif)



Sigma 18-200 @200 (Exif)



Pentax 50-135 @135 (Exif)



So is my 18-200mm lens really only an 18-135mm zoom? Does anyone also have this lens and are you seeing the same thing? Also, what about the 18-250 (Tamron or Pentax)?

correct me if I'm wrong....but I believe that the focal length actually changes depending on the focussing distance..... it's the mechanics of the lenses that cause this... lots of parts moving inside the lens....so closer focus actaully does change the focal length....

good explanation:

http://en.wikipedia.org/wiki/Zoom_lens

I had the same initial worry with the Tokina 24-200. At close focus the 200mm wasn't
It's just the complex lens design.
Matt