Light loss with 35 mm adopter, what are best solutions?

[Radek Svoboda]

How many f stops of light do you lose? 2 f stops like P+S?

If you use some bright screens, is the perceived DOF different? I mean, does it affect DOF?

[Daves Spi]

DOF depends on how close you zoom to GG and how big screen you see. I've tested Nikon GG versus Maxwell GG and I did not see any important difference. If I use Maxwell, Im able to take more then 36mm, so I reach more DOF on shorter distance...

Extreme DOF you get with 200mm, but set bigger distance between lens and GG.

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DOF závisí hlavnì od toho, jak moc zoomuješ na matnici a jak velký obraz z ní snímáš. Testoval jsem matnici z Nikonu a oproti Maxwelovì matnici sem nevidìl žádný vìtší rozdíl. Pokud používáš matnici Maxwel, jsi schopný vzít obrázek pøes 36mm, tím jít blíž ke scénì a dosáhnout vìtší DOF.

Extrémní DOF lze dosáhnout s 200mm objektive, který dáš dále od matnice.

[Bob Hart]

Again, P+S Technik seem to have come up with the best balance. If the groundglass texture is coarse, the image is slightly softer focus but more even in brightness across = less hotspot. There is much light loss. If the groundglass texture is very fine, say 3 micron finish, then you can see through it just. The camera sees two images, the rear projection image and some of the aerial image. This seems to give a sharper image, better low light performance but bright background highlights will show as a foggy sharp image surrounded by the soft image of the projection. I have used the finer finish and put up with the mix of the projected and aerial images as it intercuts better with normal video. If you take care to avoid bright pinpoints of light in the background it works well enough.

My method with the groundglasses, both plastic and optical glass was to finish to 5 micron texture, then backpolish a little to bring a hint of the aerial image. There is quite a bit of room for creative choices to be made just with the finish of the groundglass. Wax disk was the absolute best but had very bad flicker and some vibrated badly.

[Radek Svoboda]

OK guys, thanks.

Now I am looking to Optosigma ground glass:
http://www.optosigma.com/miva/merchant.mv?Screen=PROD&Store_Code=OS&Product_Code=pg211&Category_Code=Filters+%26+Apertures

They have several grades, from 240 till 1,500. How does correspond to the micron finish? Is there table or formula?

The visual spectrum is about 400-700 micrometers wavelength, looking at a book. Looking at Optosigma graph it seems that that 800 grade is still Ok but 1,500 grade has about 2x as much transmission, that means non-defused light that is probably doing the effect that you Bob talking about.

Bob, does that mean that 1.500 grade will give brighter image with the halo or whatever effect but lower garade is what P+S is probaly using?

I think #800, or even 1000 would be correct grade. Am I right?

I want to use it for FX1 HDV camera. The GG would make motion like in Pro 35. Would finer grade be needed because of the higher resolution?

What 1000 means, maybe 1 micron?

Hope someone can help with this.

Radek

[Bob Hart]

I don't know enough about this. I think I read here once that 5 micron aluminium oxide powder is similar grade to 1200 grit silicon carbide.

Silicon carbide is not favoured for groundglass making because it tends to chip flakes out of the glass rather than make smaller digs in the glass.

For the same particle size the aluminium oxide makes finer finish.

My cerium oxide part-backpolish technique over a 5 micron aluminium oxide finish does not make finer finish but polishes little transparent flats on top of each little high point on the groundglass and lets a little more aerial image through that way.

I also backpolish against a thin felt pad stretched over a thick glass disk with the felt wrapped over the edge ans glued down on the back. This disk is fixed in the bottom of a rolling drum and the groundglass glass disk rolls across it. I glue the groundglass glass disk to a carefully machined heavy brass rolling disk with melted wax.

I place the disk and brass in a pot of water slowly heat the water to separate the disk after the work is done.

Presently I am trying to invent a true orbital polishing machine using the tumbler method. The polish pattern from a rolling disk is elliptoid with a point at one end and the rate of abrasion varies from outside to inside of disk which is not good.

[Radek Svoboda]

Bob,

That is a lot work. Isn't more effective buy those from Opto Sigma? But anyway, thanks for interesting information.

Has anyone buy from Opto Sigma? What grade you buy?

Radek