The answer is that 'it depends'. As you will see from (early English books online) EEBO, conversion from microfilm to digital is not always terrifically successful. Film is an analogue process and resolution is therefore measured in lines per inch (lpi).
Digital resolution is measured in dots per inch (dpi). There is a conversion factor of roughly 2.2 x lpi = dpi which can be used but this is a very crude conversion. Also, microfilm can be at a variety of physical lpi resolutions and even colour. It all
depends on the speed of the film, the grain size and the origination. So the best thing would be either to use a dedicated microfilm scanner such as a Zeutschel which works things out automatically by sampling or to try digitising a single microfilm frame
at various resolutions and see which gives you the best result at (say) 75dpi, 150dpi, 300dpi, 360dpi or 400dp when you zoom in under (say) Photoshop. Then use that reference value for the rest of the scans of that microfilm roll.
You are correct that to scan a microfilm frame at a resolution higher than the film grain would be a pointless exercise because scanning at too high a resolution would just be scanning noise as you can only scan the film grains that are present in the
film gelatin suspension.
Regards
Tony Harris
Sent from my iPhone.
Dear Colleagues,
I was recently asked by a colleague about standards for digitizing microfilm of medieval MSS. Does anyone on this list have a recommendation as to what the optimal file format and resolution would be for digitizing black and white microfilm? (I am, perhaps
naively, assuming that because the source image is a b/w microfilm that there is a certain resolution limit after which point higher resolution would be of no value since the original image is limited).
Any advice?
Thank you,
DM
David A. Michelson
Assistant Professor of the History of Christianity, Divinity School
Affiliate Assistant Professor of Classics, College of Arts & Sciences
Vanderbilt University