Analysis of Focal Lengths 18-200mm versus 18-70mm
Prepared 2006-01-09 (146/47202) by Bill Claff

I measured the "true" focal lengths of these lenses at each marked focal length on the lenses.
(OK, actually I missed 24mm on the 18-70mm and measure 60mm instead )
(I used the technique outlined in Measuring Focal Length)

Here is a table:

	18-200mm		18-70mm
	infinity	nearest	infinity	nearest
18mm	18.8	17.8	18.2	16.7
24mm	24.2	22.8
35mm	35.0	31.8	32.3	27.5
50mm	51.8	43.9	52.8	39.4
70mm	72.4	56.9	70.8	45.8
135mm	147.5	81.7
200mm	215.0	87.9

And a graph:


summarizing my findings.

I know the values for the 18-200mm for 135mm and 200mm look high, but I double‑checked my findings.
(This involves a linear fit of several data‑points and the fit is quite good.)
Perhaps it's a sample variation as I also found that I could get a 1:4.09 reproduction ratio and the spec says 1:4.5

Note that despite concerns that the 18-200mm loses so much focal length during close focusing that in fact the 18-70mm loses even more.
I have confirmed, as the chart implies; that a shot at 70mm with the 18-70mm lens has about the same Field Of View as a shot at 50mm with the 18-200mm.