Center versus Corner Sharpness of the 18-200mm DX
Prepared 2006-03-18 (146/55282) by Bill Claff

I got thinking about the issue of center versus corner sharpness for the 18-200mm DX so I thought I'd try to do some "objective" measurements.

I set my 18-200mm DX to 50mm f/4.8 (the minimum at 50mm) and took a series of shots around the ideal center AF position.
(Using an Adorama focusing rail at 5mm increments.)
I used A Technique for Measuring Relative Sharpness (2005-08-04 86/7071), archived on my web site; to measure the center and corner sharpness of each image.

This chart summarizes my results:

The green line and top row of 100% crops are for the center of the images.
This result is exactly as expected.
(I'm still amazed at how accurate the AF is since I achieved AF Sensor Nerdvana.)

The red line and bottom row of 100% crops are revealing.
Perhaps not surprisingly, the data shows that even at it's sharpest the corner is not nearly so sharp as the center.
The real surprise is that sharpness improved at the camera was moved away from the target.
Given the shallow Depth Of Field (DOF), I had expected corner sharpness to improve as I moved the camera toward the target.
(Since the target was flat and the corners further than the center from the entrance pupil of the lens.)

Make of it what you will.