I thought I’d write about where we are with the microfilm digitization part of our project. In particular, we had a minor setback with our scanning that proved (to me, at least) to be quite educational.
We enlisted the help of The Crowley Company, a commercial digitization lab that offered state-of-the art equipment and a clean room, and that, along with experienced operators doing the work, convinced us that we would produce digital copies of these collections of the highest possible quality. But what of the microfilm itself? Were the reels in good enough condition, and was the original photography done well? We were eager to view the results, and it fell to me to analyze the scans and assess their quality.
How’s it look to you?
I’ll admit to having been a little unsure of myself. I explored various reels, looking for poor exposure, blurry images, anything indicating an error or gap in the digitization process. Eventually, I got a sense of what the overall resolution of the images was. But something didn’t feel right. When I looked at the top of the page (and why wouldn’t you start there?), things looked fine, but occasionally I saw things that looked blurry, and this gave me pause. Was I looking at the maximal sharpness of the image, or could things be improved? Without the microfilm on hand, I couldn’t even determine whether it was the digital photography that was to blame or the original microphotography. If the latter, there was really nothing to be done, short of re-shooting the papers, and that wasn’t an option!
Fortunately, when we had an on-site visit from Crowley, I had a chance to raise the issue with their representative. He very quickly identified a curious trend: the top of each page was crisp and clear, while the bottom was less clear. A few minutes later, he was on the phone to the scanning lab, as they still had the microfilm in their possession. At his request, the film was examined under a microscope, and we confirmed that original microphotography was in focus. This can be done ad hoc, by looking at a frame selected at random. But, fortunately, there is a more systematic method for answering questions such as these.
Resolution Calibration Test Targets Are Your Friends
When microfilm is professionally manufactured, there is a calibration chart included in the material photographed. The photographs of the chart document the focal quality of the entire batch of images. Moreover, they are designed to exhibit the limitations of any optical lens used in photography: aberration. The target looks like this:
The four seemingly redundant charts at the corners tell you something very important: how the photography resolves an image at the extreme edges of the camera lens. It’s not enough to have good focus at the center, a lens must be designed and calibrated to produce sharp images at the periphery, and these charts document how the photographic lens performed for the microfilming session. Like an eye chart, finding the smallest lines that can be resolved (and are not just a grey blur) indicates the limit of your ability to focus in on a detail in an image.
As this is a standard, highly accurate printed chart, rather than pen ink on a hundred-year old manuscript, you can get a much more reliable sense of what the photography (and subsequent scanning) captured. And here we can see the variation between a top-corner and bottom-corner chart on the very same exposure:
I find that if you examine the bars labeled “2.0”, you can see a distinct difference. (Click on the image to expand.)
Once we’d established that at least some of our scanned reels resulted in partially out-of-focus digital images, more detective work was required. Our Crowley representative quickly established that there was a calibration error with the scanner that processed the samples we examined, and their records indicated the entire run was done with the same machine. They very quickly sent that scanner to be repaired and re-calibrated, and since they still had the microfilm on hand, they offered to rescan the material as soon as the machine was deemed fit to return to service.
We were more than happy with this arrangement. I’m hoping to use these images in an effort to extract transcripts and/or metadata from the documents photographed, and clarity is very important. Consider this side-by-side example:
The first scanning attempt:
And the results of recalibration:
I think the increase in clarity is pretty obvious (your mileage may vary!)
I’m glad we investigated this, and the turnaround time for the rescanning was excellent. So thanks again to Crowley for their diligence and expert eyes! I have a much higher degree of confidence in our digital images, and look forward to the next phase of our work.
UPDATE: The nice folks at Crowley have blogged about this project as well.