Analysis of Light Falloff 18-200mm versus 18-70mm
Prepared 2006-01-07 (146/47007) by Bill Claff

The next installment in my series of comparisons between these two lenses.
I took flat field images with each lens at each marked focal length at the maximum aperture allowed for that focal length (widest open).
(Flat field images are out of focus images of a uniform field; in this case a white target.)

The full frame Signal to Noise Ratio (SNR) ratio is dramatically affected by light falloff.
(The center SNR for these shots were between 98 and 102, very respectable.)
The less the SNR drops the less light falloff.

Here is a chart of the results:

The new 18-200mm outperformed the 18-70mm at all focal lengths.
(Those 3 aspherical elements must be worth the money )

Note that the best focal length for the 18-70mm is 50mm while the best focal length for the 18-200mm is 70mm.

BTW, the differences are evident in the images but I prefer to present my results in this more "objective" fashion.