Monday, May 19, 2008

When is a 200mm lens not a 200mm lens?

A couple of weeks ago, I was switching lenses between my 18-200mm zoom and 105mm macro, and I was surprised about how little difference there was in the "size" of the image in the viewfinder. I made a mental note to check out what was going on when I had the time. Thanks to a day of rain today, I decided to compare the different lenses that I have to see how the field of view compared at "200mm". In the process, I found a couple of surprising things.

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.

Other surprises:

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