Camera scanning: conservator's perspective

[rdeloe]

There are many threads on the forum about "DSLR scanning", also known as "Camera scanning". An active recent example is this one: https://www.largeformatphotography.i...74#post1504274 I was going to add this material to that thread, but decided to open a new thread because I think there may be some value for the community in knowing about the perspective of conservationists (the museum crowd).

Many people on the forum, and elsewhere, have found ways of digitizing negatives and positives using cameras of some kind. Others do it with scanners. It's all good. This thread is about camera scanning, which seems to be the future of digitizing film.

In figuring out how to "camera scan" my 4x5 negatives, I've discovered lots of advice, much of it contradictory, and lots of it based on the personal experiences of other people who have figured things out on their own too... It turns out that photographers who are interested in hybrid workflows (film > digital) are not the only ones thinking about this! Thanks to a tip from a fellow photographer, I discovered the conservationist's perspective on all of this. Lo and behold, there are standards out there!

Long story short, I just purchased an interesting little book called DT Digitization Guide. Digitization Workflows: Transmissive. DT is a commercial outfit that sells equipment and services to the "conservation" world, in other words, museums. Their equipment is WAY outside of my budget! However, this books was only $5 USD, so worth it to me.

The book has two main parts: a general overview of issues relating to camera-based digitization of transmissive materials, in other words, negative and positive film; and instructions that are specific to using DT's own equipment, which includes Phase cameras and Capture One software. I bought it for the first part, which is a nice, not too technically complicated overview of the conservationist's perspective. But even the second part has good value if you're willing to extend the ideas to your own gear. There are also some useful tidbits in the appendix.

The knowledge this little book contains is probably out there on the Internet somewhere for free, but I have not encountered it before, and it's nice to have it all in one place. Here are some highlights:

Conservationists have some concerns that do not necessarily overlap with those of the photographer who is shooting film with the goal of digitizing and then editing of the digital file to make their own "interpretation" of what the camera recorded. Here are a few examples:

* Conservationists seem to view wet scanning (fluid mounting) as a Very Bad Thing. Makes sense: you're immersing the negative or positive film in mineral spirits or some other fluid. However, for someone like me, who is shooting the film to make the digital file, I'm not concerned about preserving the negative for someone to look at in 100 years. I wet scan for the quality improvement.
* Similarly, using tools like ICE (or just spotting in Photoshop) is a major no-no in the conservation world when the goal is to make a "Preservation Digital Object" (a faithful reproduction of the thing).
* One interesting difference between what I need and what a conservator needs is exposure. When making a "Preservation Digital Object", the conservator wants to make an image that is faithful to what the object would look like in real life, e.g., placed on a light table and viewed by someone. In contrast, I need an exposure that gives me high quality data at each pixel while preserving the full tonal range of the image. I don't care if the resulting file is remotely "faithful" to the way a viewer would see the negative on a light table.

Apart from those kinds of concerns, the basics are the same for the person in the museum who is making a Preservation Digital Object, and the photographer who is making a digital file to work on in Lightroom or Photoshop (or whatever software they use). There's lots of good basic technical advice in the book that is consistent with standards that are emerging.

Finally, the book also gets at a common topic of debate on forums, which is the resolution at which to digitize the film. I like their position: it's not about what we can do with the equipment we have, it's what we should do for our intended purpose. For example, in the conservation world, the goal of digitization is to create "a surrogate to the original object, replacing most needs for physical access to that original object". Thus, "the selection of PPI must be based on the content of the original transmissive material." In contrast, my own goal is to balance practical considerations such as the ability of my computer to handle the file with "how many pixels do I need to make a high quality print?" In case you're curious, the FADGI 2016 standard for conservation digitization of 35mm, 645 and 4x5 film is 4,000 ppi at 90% sampling efficiency (which is way more than I can make or need); it's 2,000 ppi at 90% sampling efficiency for 8x10 film.

Rob

[Peter De Smidt]

Good stuff, Rob.

[Chester McCheeserton]

It is interesting. Those ppi numbers are exactly where most experienced scan operators would land as well.

Doubtless some conservators have extensive experience as photographers...but rarely do they actually have the experience of physically making another print or object, their equipment and methods are I bet 99% are for the screen only. Another group of people who would might know something about this (at least working from prints, not negatives) are the people operating the medium format copy cameras for reproduction at high end book places, like Meridian in Rhode Island.

One test result I posted a while back, kind of buried, but maybe relevant.
https://www.largeformatphotography.i...40#post1454940

[invisibleflash]

Good rundown....Thanks!

Has it been determined which method produces better results? Got any photos of the conservator's setups?

[rdeloe]

Got any photos of the conservator's setups?

This link takes you to a story on the National Geographic blog. It's about digitizing "autochromes" (early colour photographs). Doug Peterson, who is the focus of the article, works for DT Cultural Heritage. He provides links in his write-up back to DT's site so you can see the gear they provide.
https://nglibrary.ngs.org/public_hom...of-Autochromes

[Peter De Smidt]

http://www.gigamacro.com/gigapixel-m...maging-system/

[Tin Can]

Whoa

Bugs!

http://www.gigamacro.com/gigapixel-m...maging-system/

[Oren Grad]

In case you're curious, the FADGI 2016 standard for conservation digitization of 35mm, 645 and 4x5 film is 4,000 ppi at 90% sampling efficiency (which is way more than I can make or need); it's 2,000 ppi at 90% sampling efficiency for 8x10 film.

These standards are inadequate if one seeks to preserve the grain structure of the film. In fact, for ISO 320/400 films like Tri-X, 4000 ppi will produce aliased pseudograin which increases the apparent size and obliterates the native character of the grain.

Yes, I know that many users don't care about the grain structure for their purposes, which is fine.

I did note in the DTI piece on copying autochromes that faithfully recording the "grain" structure was an explicit objective for that project. Not surprisingly, the level of resolution required even for that relatively crude medium was moderately high.

[Peter De Smidt]

I agree with Oren. Scanning at 6000 spi leads to significantly finer grain than scanning at 4000 spi with my Cezanne, but I don't make big enough prints for that to matter with large format.

[rdeloe]

I wanted to understand the point Oren was making, so I did some digging*. The results may be of interest to others, so I’ll share them here. I’m also curious if I’m correct in my understanding (so please jump in if you think I’m wrong).

First, we have to distinguish between silver particles and grain. I’ve encountered people who think their high resolution scans are capturing the silver particles in their black and white film. Silver halide particles average 0.2 to 2.0 microns, depending on the film. Assuming 1 pixel per particle, we’d have to be scanning at 25,400 ppi. Double that to capture some detail in the shape of the particle. Even the best drum scanner can’t pull that off. That’s scanning electron microscope territory.

What we think of as “grain” is the clumping of the silver particles in the emulsion. Those clumps range in size from 15 to 25 microns. At the large end of this range, a scan with a resolution of 2,032 ppi will cover each 25 micron grain clump with 2 pixels. At the small end, you need 3,387 ppi to cover each 15 micron grain clump with 2 pixels.

More pixels per grain clump is better, if your objective is to record grain clumps, so if you’re scanning a fine grained film (say 15 micron grain clumps), a resolution of 5,080 ppi will allow for 3 pixels per clump. At the other end, at 6,000 ppi, you’ll be able to cover each 25 micron grain clump with around 6 pixels. So yes, it seems that 6,000 ppi not only can preserve the grain structure of most black and white films, but do it with more useful detail.

However, depending on the film and how much detail you want in recording the grain clumps, lower resolutions might do the trick too. From this perspective, it seems the people who came up with that FADGI 2016 standard of 4,000 ppi are happy having two pixels cover each 15 micron grain particle – because that only needs 3,387 ppi.

In my own setup, I am definitely not preserving grain structure! Anything that looks like a grain clump is actually sensor noise even at my current 2,667 ppi. But that's a whole other issue.


* Very useful basic information, including information about the size of silver particles and film grain clumps, came from this document written by Tim Vitale: http://vashivisuals.com/wp-content/u...resolution.pdf