I have three zoom lenses that cover 200mm: an 18-200mm zoom, an inexpensive 70-300mm zoom, and an 80-200 f/2.8 lens, all Nikon lenses ranging from consumer to professional quality. I compared all three lenses outside, set at 80mm then at 200mm. The 80mm results were quite similar, and although there was a noticable difference at 200mm, it was not as big a difference as I expected. I then tried the same test inside, at much closer focus. At four or five feet focus, there was quite a big difference, both at 80mm and at 200mm. Here is an example at 200mm (please, no comments on my book collection!).
The first image is from the 18-200mm lens at 200mm:
The next image is from the 80-200mm lens at 200mm:
The last image is from the 70-300mm at 200mm:
As I suspected, the 18-200mm lens is wider at "200mm" than the other two lenses. The same result holds at 80mm.
I don't mind, it's just a fact about the lens that is good to know. Modern lenses have quite a complicated design, especially a lens that ranges from 18 to 200mm, an 11x range. No doubt this is one of the comprimises that must be made to achieve this kind of range.
Along the way, I made a couple of surprising discoveries. First, the consumer-grade 70-300mm lens performs surprisingly well compared to the professional 80-200mm lens. I wasn't making a comprehensive lens comparison, but at first blush, the 70-300mm lens produced a sharp and constrasty image quite comparible to the 80-200mm lens.
A second surprise was the problems I had trying to focus the 80-200mm lens when it is set to 200mm. Using autofocus at close range, the focus was clearly off. Manual focus was successful, thankfully. This is another behaviour that I will have to take into account.
. . . Rob Williams