"not everyone has found a 1600 ppi maximum with the 4990 or a 700 ppi difference between the 4990 and the 700/750 scanners."
True enough.
Other than Ted Harris, has anyone else demonstrated greater than 1600 effective spi with the 4990, or demonstrated that the 700/750 scanners deliver an effective resolution other than ~2300 spi ?
One could make the claim that both scanners deliver 9000 effective spi - but it would be an exceptional result - rather than the norm. We would have to consider it as an anomaly, rather than a vindication.
Since Ted's test images aren't available - and since he is no longer with us to elaborate on his methodology - we might want to consider his results, respectfully, as just that: an exception to what others have found and shared.
Here are links to my comparison tests:
raw scans and
sharpened. I have not shot a USAF target, but my tests on the 4990 accord with that of the others: resolution peaks somewhere between the 1200 and 2400 settings, and does not improve after that. In fact, the only compelling improvement among the scans, is seen when we go from 800 to 1200 spi. After that, you really have to squint.
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