Jorge,
Apology accepted (but not really necessary - you'd have to try a lot harder than that to REALLY offend me).
Not sure what you mean by "defocused" in this context. Just keep in mind that t he fall-off will be worst with the lens focused at infinity. The size of the im age circle increases as you focus closer (increase extension). At 1:1 (extensio n = 2x focal length), the image circle will be twice as large as it will be when focused at infinity. So, if the wall was closer than infinity (or, more accura tely, your extension was greater than required for infinity focus), you will not see the worst case fall-off. This is one possible explanation for your test re sults.
When I shoot landscapes (approaching infinity) on color transparency film with t he 75mm Nikkor SW, there is definite fall-off in the corners. It is especially obvious when there is a bright blue, even toned sky. The fall-off from center t o edge is on the order of 1.5 - 2 stops. That is why I find a center filter nec essary with this lens.
FWIW, I have yet to find any lens under 90mm (on 4x5) that did not require a cen ter filter for MY needs. That includes the 75mm f4.5 Nikkor SW, 75mm f6.8 Grand agon-N, 65mm f5.6 Super Angulon and 80mm f4.5 Super Symmar XL.
Again, there is no "correct" answer to this question. The answer will depend on the user and the application. FWIW, I truly do wish I didn't require a center filter on my wide angle lenses. They are a pain - expensive, you lose 1 1/2 sto ps, and it makes it a pain to use other filters. If you find them unnecessary, count your blessings.
Kerry
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