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Old January 23rd, 2006, 10:51 AM   #1
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Alternative Screens (thin-film, colloid, etc)

I decided to start this thread because the one about the slide viewer-based adapter has a different focus.

It was found that thin-film screens, while providing great light performance, had horrible diffusion of bright spots. I think I found the reason for this bad bokeh.

Some of the thin-film material (plastic bags or otherwise) shows that its diffusion is not rotationally invariant. This means that it diffuses light better in one direction than in others. While I may speculate that the phenomena has to do with the alignment of the polymer strands I think I also found that different materials have different invariances to rotation.

Below is a test using the LED display of a DVD player. The image on the left shows the screen vertically aligned, the middle image shows the screen horizontally aligned, and the final image shows the screen diagonally aligned.

Here you can see the variance very clearly, from one my "bag" screens:

http://mentemagica.com/35mmAdapter/RotVariance1.jpg

Here is another material with much less variance:

http://mentemagica.com/35mmAdapter/RotVariance2a.jpg

(the slight variations also have to do with me holding the screen by hand :) )


So the first material will produce bokeh that looks like streaks of light, while the second material will produce bokeh that looks nice and evenly diffused.

I will try the second material on my adapter (which is not very good but will do for the moment), to see if the second screen gives better bokeh (as I would expect).
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Old January 23rd, 2006, 11:56 AM   #2
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Here are some tests:

The first group were shot with the original bag thin-film screen:

http://mentemagica.com/35mmAdapter/AdapterTest29.jpg
http://mentemagica.com/35mmAdapter/AdapterTest29a.jpg
http://mentemagica.com/35mmAdapter/AdapterTest29b.jpg
http://mentemagica.com/35mmAdapter/AdapterTest29c.jpg
http://mentemagica.com/35mmAdapter/AdapterTest29d.jpg

The second group were shot with the new rotational invariant material:

(excuse the bad framing, I forgot to check that on the monitor, and the LCD screen on my camera is overscanned)

http://mentemagica.com/35mmAdapter/AdapterTest36.jpg
http://mentemagica.com/35mmAdapter/AdapterTest36a.jpg
http://mentemagica.com/35mmAdapter/AdapterTest36b.jpg
http://mentemagica.com/35mmAdapter/AdapterTest36c.jpg
http://mentemagica.com/35mmAdapter/AdapterTest36d.jpg

The second group, while a lot more grainy (not an issue with a moving adapter), have less light loss and seem to look better than the first group.

What do you think?
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Old January 23rd, 2006, 12:22 PM   #3
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Agreed. That's the kind of bokeh I can work with.

Where did you find this different, better bag material? Just another bag at the supermarket you picked up?
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Old January 23rd, 2006, 12:34 PM   #4
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Quote:
Originally Posted by Ben Winter
Agreed. That's the kind of bokeh I can work with.

Where did you find this different, better bag material? Just another bag at the supermarket you picked up?
This material didn't come from a bag. It looks just the same but is thicker. For some reason it has even better light performance than the bag material, go figure.

I got it from a "pouch" that holds some medical information stuff (insurance). But I am trying to locate a source that other people can have access to. I am making a trip to Office Depot to check it out.

So the good news is, better bokeh, even less light loss. Bad news is, grain... so it will require a moving adapter.
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Old January 23rd, 2006, 02:04 PM   #5
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This is consistent with what I'm seeing....more grain is required to get that "film" bokeh. I wouldn't call bokeh differeing from this to be "bad"...just different. If you were looking for a very unique look, this wouldn't be a bad think at all.
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Old January 23rd, 2006, 02:12 PM   #6
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I think the bokeh difference in this case is related to the rotational invariance, not the grain per se. It just happens that the isotropic (rotational invariant) material I found also has more grain.

But I don't see anything preventing an isotropic material having smaller grain too.

Keep in mind that these are polymers, not ground glass, so they are quite different. Your ground glass has the advantage that it will be isotropic (unless you sand it in just one direction).

Also, as you say, the bokeh is just a purely aesthetic and subjective situation. It is like the discussion about 24p versus 30p or 60i.
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Old January 23rd, 2006, 05:31 PM   #7
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Side by side comparison at almost the same angle (or pretty close)

On the leff is the original bag material, the out of focus areas look motion blured. On the right is the new material with an more evenly soft blur:

http://mentemagica.com/35mmAdapter/M...omparison1.jpg
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Old January 24th, 2006, 07:11 AM   #8
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I recently tried the following as a screen. It’s a HiMD case:

HiMD case

Here is what it looked like vibrated by a small disc motor. Now, the setup was not perfect as the screen was too rigidly mounted, but here is what it gives. A better motion would certainly help the grain disappear.

HiMD case (too rigidly) vibrated
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Old January 24th, 2006, 11:28 AM   #9
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That HiMD case is quite a reversion -- it looks exactly like the surface of the "frosted" CDR's Agus and everyone were scrambling for about two years ago :)

It also looks like, BTW, the frosted plastic you can get cut to spec from places like Canal Plastics in New York. At about $1 per 50mm circles, I thought I had the world figured out until I took it home and placed it next to my WAO5 ground glass -- there's a world of difference.

Has anyone looked into different types of frosted vellum? This was another thing I'd tried way back but, never stumbled across sheets with the right properties.

It's so odd how elusive "the perfect GG" seems to be. The Beattie people have a good design framework -- positive fresnel backed by a frosted, diffuse surface -- but despite how precise their manufacturing methods are, there's still evident grain. I wonder -- would a sub-1micron aluminum oxide grind taken to the frosted side of a Beattie get'r'done? Something makes me doubt it...
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Old January 24th, 2006, 11:30 AM   #10
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As a related aside, I spoke with one of the head optics people at 3M a while ago -- very eager to help and solve this riddle. But even he came up against a wall and couldn't suggest a (affordable) solution. He suggested contacting people at SPEI or OSA (major optics groups) and asking them questions along this line. I've started that up a bit but it's slow going...
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Old January 24th, 2006, 12:20 PM   #11
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I must admit that I did not search back two years ago on this. :) I just saw this on my desk and thought it would be worth giving a try in a vibrating setup. I’ll still properly test it when I finish my vibrating GG adapter. It’s not fair to compare, but the light loss seemed to be much less than the GG provided in the DIY spinning GG adapter kit that I used.

BTW, what’s the WAO5 ground glass?
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Old January 24th, 2006, 12:42 PM   #12
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White Aluminum Oxide 5 microns. I eventually went to 3, then 1.4 micron. Each step better than the previous but none so good as "grainless".
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Old January 24th, 2006, 03:55 PM   #13
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Here are some Laser Tests of several materials I have tried (mostly bags :) )

This is the original sandwich bag material which seems to have the most extreme rotational variance:
http://mentemagica.com/35mmAdapter/S...Bag-Laser1.jpg

This is the thicker Pouch material with no variance (looks pretty circular):
http://mentemagica.com/35mmAdapter/M...uch-Laser1.jpg

Now, here is a test with two layers of the sandwich material. As you can see the difussion is now circular, but there is a higher light loss:
http://mentemagica.com/35mmAdapter/D...Bag-Laser1.jpg

Finally here are some other bags which are pretty much all similar:
http://mentemagica.com/35mmAdapter/BagA-Laser1.jpg
http://mentemagica.com/35mmAdapter/BagB-Laser1.jpg
http://mentemagica.com/35mmAdapter/BagC-Laser1.jpg
http://mentemagica.com/35mmAdapter/BagD-Laser1.jpg

Of all materials tested, the sandwich bag is the most asymmetric of all. The pouch is almost circular, but provides the highest light transmission.

This is a simple and effective way of testing the materials. (even grain can be compared if some care is taken while taking the pictures.

So get a laser pointer and go test some bags ;)
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Old January 24th, 2006, 10:21 PM   #14
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More news...

Tested Magic tapes with the laser:

http://mentemagica.com/35mmAdapter/3...ape-Laser1.jpg

As you can see the 3M Magic Tape is not too good it has a very horizontal diffusion, plus it absorbs quite some light, has some striation (horizontal lines).

But...

http://mentemagica.com/35mmAdapter/O...ape-Laser1.jpg

This thinner (cheaper) tape from Office Depot brand is much better. It has almost the light transmission of the pouch material I am using, its perfectly round, plus is has very small grain!

So this is proof that there is a material that can do all: Little light loss, small grain and round diffusion. The trick now is to find it in a wider format.
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Old January 25th, 2006, 06:53 AM   #15
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Great work. I think the laser pointer is a great way of testing Alain! Wider is always going to be better, for covering a CD too!
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