...if we have a focal length of 100 mm and are focused at a
distance of 10 meters, then an misplacement of the focuing screen
of 0.01 mm will cause the object plane of focus to shift by about
100 mm!
No wonder Canon has shims whose thickness varies by 0.002 or 0.003
mm steps over most of their range (11 shims altogether).
Please critique the following discussion:
Screen position is critical, as Doug has explained above.
Manufacturing tolerances(Canon's) are such that the camera/screen pairing may produce a "fit" anywhere between two extremes, ..camera/screen "too tight", ...camera/screen "too loose".
"Anywhere between" becomes the issue.
Thus, a range of shim-sizes, to be used to fit the PARTICULAR screen and PARTICULAR body being discussed.
Assume(momentarily) that the Haoda screen has a manufacturing tolerance of zero. Unless its thickness exactly matches that of the Canon screen being re-placed, using the original shim will NOT "match" the camera/screen fit of the original factory assembly.
If Haoda has a "manufacturing tolerance" greater-than-zero, which is certainly the case, the Haoda screen would need its own "matched to this PARTICULAR body" shim, ...or there will be a mis-match.
When both screen and shim, IN COMBINATION, must match a particular body, to a design-point fit,...then shim must be custom-fitted to screen.
Instruction from Haoda to "just use the original shim" can easily be incorrect., if screen thickness is different from original Canon screen.
Corrections/comment welcome, ;-)
Larry
