Last week, in some comments to Mike's "Touch Once" post, I mentioned that I was on my fourth round scanning some of my film and that re-scanning my film seems to be something I have to do about every five years despite my best efforts to make scans so good that I could never imagine needing better ones. (Apparently, my wildest imaginings have a tangible expiration date.) This, of course, raised questions about how good a film scan really needed to be, and so here we are.
I'm close to wrapping up the task of scanning all the undigitized negatives of the photographs in my dye transfer portfolio. After several weeks of work, I've got a folder of image files that's about 100 GB in size. That's what happens when almost all your work is medium format negative and you're scanning it at 16 bits per channel color depth at a pixel pitch of 4800 PPI. My average file size tops three quarters of a gigabyte. Shouldn't this be enough for anybody?
Well, I really hope so. I don't look forward to repeating any of this five years from now; I truly can't imagine why I would have to. The problem is that I'm pretty sure these aren't the best of all possible scans. That leaves the door open to the possibility that at some time in the future I will come up with some unimagined need that demands even better scans. Be afraid, be very afraid.
The problem lies in the resolution. Forty-eight hundred PPI crisply images film grain, but it doesn't actually resolve it. What's the distinction? Point your digital camera at the night sky and open the shutter for a few seconds. You'll photograph stars, but the stars themselves are thousands of times smaller than an individual pixel in your camera. The stellar image expands to fill a pixel with some average level of brightness, but it's not resolved.
1200 PPI scan (click on these images to see them larger; the auto-resizing of the blog software obscures the sharpness of the images a bit). You can image film grain at remarkably low scan resolutions, as these comparison photos of TMAX 100 film scans show. An image of film grain, though, is not the same as resolved film grain.
Film grain is extremely small, even in coarse-grained films. Forty-eight hundred PPI scans don't resolve it. What you get are little square pixel-sized blobs in place of the original grain that mimic the pattern of the grain.
So what? Well, there are some subtle kinds of image degradation that occur when you don't sharply resolve grain. Good darkroom printers are familiar with the effects that occur when a print is just a bit out of focus or made with a less than a top-notch enlarging lens. Edge acutance gets subtly altered. More visibly, tonality and gradation in the extreme highlights and shadows gets distorted or muddied, because those tiny individual film grains (or spaces between grains) have a disproportionate effect on tonal rendition at the extremes.
The situation is not quite the same for film scanners, but it's close enough that there are issues to worry about. The losses are small; I think they may never matter to me. But there's that demonstrated lack of future imagination.
How much scan resolution would it take to satisfactorily resolve the film grain? I don't really know. The late Bruce Fraser and I discussed this on occasion, and we guessed the scans would have to resolve in the 10,000–15,000 PPI range, but neither of us ran any tests. I'd be looking at 5–10 GB scan files, and that would give pause even to Lloyd Chambers' uber-Mac.
There are other practical issues. Scan pitch is not the same as scan resolution, It's only the upper limit on the resolution. The real resolution is somewhat less, due to losses in the optical train. (A cautionary note there for any readers who want to report on their ultra-high-resolution scans. Do you know what resolution you're really getting?) I'm fortunate. I've tested my Minolta Dimage Multi Pro scanner, and it resolves 4200 PPI in the center of the field and 3800 PPI at the edges. 80–90% of theoretical resolution at a scanner's upper limit is very good performance.
Thing is, if I had a scanner that delivered 10,000–15,000 pixels per inch, I have to come up with new measurement tools to find out what was actually resolved. Nothing in my experimental arsenal is good enough. For all I know, there aren't even any film scanners of any type out there that can resolve 10,000 PPI, manufacturers claims notwithstanding.
If the gods choose to smile on me, I'll never have to answer these questions. All I can know for sure is that there is information in the original negative that I'm not capturing in the scans, and all I can do is hope that I never need to care.
Ctein's regular weekly column appears on TOP on Wednesdays.
Note: Links in this post may be to our affiliates; sales through affiliate links may benefit this site. More...
Original contents copyright 2011 by Michael C. Johnston and/or the bylined author. All Rights Reserved.