A really important test that we could conduct with a few common lenses would be to see how flat the focal plane is. In other words, if you focus on an object at a specific distance from the lens and swing the camera on a tripod does the specific object stay in focus or go out of focus. I guess I have assumed that anything that stays the same distance from the lens will stay in focus. I have never done any controlled experimentation to find out.
Sounds interesting. A challenge is deciding 'standards' for such testing. For example, if you use a detailed wall as your target for field flatness, how do you deal with the issue that many people have heads without marks for pan angular control, or that the angle marks are too coarse? And how do you ensure that everyone would be absolutely perpendicular -- both horizontally and vertically -- to the wall. And how far away from the way?
I'm all for a test, but standardizing the above (and maybe a few more parameters I hadn't though of) is the issue, as well as the precision of such observations.
For the test of the tolerance for focus-recompose-shoot process, a standard needs to be established there, too, because past tests have at times failed to define distance-to-subject or FL (or Angle of View).
a. on a zoom lens perform the test at shortest FL and also at longest FL.
b. the distance to subject...I assumed 3' on the test Thornfield had done (since I did not see it stated in his test) but maybe 5' is more reasonable distance (or not!) This distance DOES affect the DOF amount and also the angular displacement from edge of frame to the center, so is a critical factor in standardization of a test!
c. angular displacement...we could assume that you start at center of frame and move the rotationally horizontally, until the object is positioned 3/4 of the way between the center and the edge of the frame.
