Accurate Method for Comparing Grain of Various GG?

[Jim Lafferty]

Hey...

Today, I just put the finishing "polish" on a piece of glass with newly acquired 1.4 micron aluminum oxide. I'm hoping soon to have a sample 1 micron slurry shipped to me so I can work with that as well. At some point, as with CPU benchmarks, I'd like to devise a way to establish a system of standards, using e.g. 3 micron GG as the "base," and comparing other GG techniques against it.

My idea was to scan the GG using my flatbed scanner at a high resolution (say, 600dpi), then blow up a 1/2 by 1/2 inch section to 4 or 8x original size. Then, do the same with my 3 micron GG, and eventually my microwax glass.

Anyone have better ideas, or at the very least suggestions that would make my estimated numbers above more realistic and accurate?

There was a tangible leap from 5 micron to 3 micron glass, tangible to the nakid eye, even. Now, between 3 microns and 1.4 I'm at a loss to spot the difference.

At some point, provided we agree on some sort of standard method, we could post and compare these scans with our projects.

- jim

[Jim Lafferty]

47 views but none of you clever guys have any advice to offer? :D

I picked up a Nikon D screen yesterday and by comparison (to the eye) the grain looks finer than my 1.4 micron GG. I'll scan them over the course of the next week and post shots if it proves to be a good method.

[Glen Hurd]

My only concern with the scanning method would be that if you use a scanner in a reflective mode (ie. scanning photos, documents, etc.) that it may show different characteristics in a ground glass from what is seen when it's used as an adaptor, with light transmitting through it instead.
Could you use a digital still camera, and place the diffusion material close enough to the lens to completely block it, while still being able to get a sharp focus on the glass? This way all grain that would be revealed in a diffusion scenario would show up, along with any other characteristics it may have, but at a resolution that's significantly greater than SD video can handle. You could even try panning the camera slightly with each diffuser, and use that as a means of evaluating the diffusion properties themselves -- if the picture alters simply by panning the camera a little, we know it's not diffusing light 100% -- letting the outside scene affect picture.
I only point this out because it seems possible that grain structure may appear differently in reflected light than in transmitted light, and the transmitted version is the one we're all pursuing. (It would be great to have both, though, if you had the time ;)
my 2 c worth,
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[John McManimie]

I may be off base here... but how about actually shooting video (or digital photos) through each sample of ground glass (at a mostly solid, evenly lit background that would show mostly no detail other than the grain --- and under consistent conditions for lighting, zoom, etc.) and comparing the grain content of the video? I figure that it only has to be sampled at video resolution; if it isn't noticeable on video, does it really make a difference?

Perhaps you could use something like the Match Grain filter from Boris Continuum Complete, which is used to copy the grain signature from a source clip and apply it to a destination clip to create more realistic composites. The sampled grain can be stored for later use, so I assume it could also be used for comparison (The Generated Grain view option displays just the grain signature that the filter would apply). The BCC DeGrain Filter might also work, using the Removed Grain view to show the sampled grain that would be removed from an image. There are probably other (less expensive) tools like this available (BCC is just one I happen to use).

[Matthew Wauhkonen]

Not a bad idea, but I don't think it'll be possible to replicate conditions that precisely. I mean, it depends a lot on the camera, the amount and quality of light, how much the 35mm adapter itself rejects light (my earlier designs had lots of light spill) and even the optics that you're using from the macro to the condensor to the SLR lens itself (although I assume you'd measure without it) and the precise size of the focusing screen--which is hard to measure in camera--is one of the most important factors here, clearly. There are just way, way, too many variables, although that would seem initially like the simplest and most pragmatic approach.

If the scanning thing works, I think it's brilliant. I'm not convinced it will, though, but it's worth a try. In fact, I'm kind of curious what results you discover.

The only issue with comparing wax screens is that the rate of cooling affects grain every bit as much as the composition of the wax.

[Glen Hurd]

I think Jim's desire for a GG standard is brilliant. It could help all the static design guys evaluate which GGs hold the most promise. (This would probably be of benefit to the motion GG designs too, assuming mass doesn't play too much of a part there).
But part of that standard requires keeping it simple (no optics, condensors, etc. involved). Just evaluating the actual characteristics of the GG.
Not that they couldn't be evaluated separately, but they involve so many different approaches it may be hard to determine what gives any particular setup its strengths or weaknesses.
And because some are conent in SD, others in HD, and others potentially in EHD (can you imagine Oscar's dual camera setup with 2 HDs -- whoooh!) this information can be of interest to a great number of people. So the higher the resolution, the more accuracy, the more people affected.
One thing that would help would be to have some common, consistent, scale that could be photographed/scanned along with the GG, so each person's contribution could easily be compared with another's, regardless of which scanner or camera they use. Even differences in image resolution could be tolerated, as long as that single common element exists in each person's image.
For instance, with the perfect GG, we should see nothing. It would be nice if there was a strand of horse hair, for instance, across the middle to verify that all was in sharp focus! Carrying that example further, when you enlarge the image to explore its characteristics, you get an immediate sense of the scale you are at -- a slight showing of grain when a hair is measuring 1 inch across the screen is probably going to be an acceptable thing in HD.
The horse hair is just an example -- I don't know what we all would have easy access to, that would be consistent enough for a standard - 1 lb test fishing line? A strand of copper wire pulled from your favorite computer's power supply? A strand of wool?
I don't know.
And to Matthew's point, if someone comes up with a perfect GG, this is the way to show it. But if he can't remember how he made it (ie. cooling time, etc.) he's going to need to hide somewhere ;)

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[Oscar Spierenburg]

You are right. But...a good condenser setup like a Movie-Tube and I'm sure the G35 has allot of effect on the grain. I think the grain on the GG is lit from the center and create a sort of shadows from the grain/structure so it becomes more visible. If you have one or two good condensers, I think the light comes in straight or parallel on the GG and create much less 'shadows'. Maybe this theory is totally wrong, but it's the only reason I can think of that the grain is less with condensers.

[Bill Porter]

On the horsehair idea, can anybody confirm the difference between hair from different breeds? This might throw a monkey wrench into things.

I have access to hair from an Arabian and a Morgan myself. Maybe we can get other members to collaborate, perhaps Oscar you could inspect a Dutch Warmblood, Daves Spi can check out a Przewalski's horse, and Javier Gallen could provide that of an Andalusian.

[Glen Hurd]

Beautiful Ventura! I'm not surprised you have access to horse hair there ;) Was stationed at Point Mugu for 4 years (Navy photographer). May have flown over your place a time or two in one of the F-4s or a helicopter.
I miss California :(

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