Why is it GROUND PLASTIC instead of GROUND GLASS?

K

Kholi

Section Moderator
It's probably here somewhere, but why? We call it a ground glass, but it's really just plastic.

I'm no adapter manufacturer nor engineer but is it safe to guess that actual glass would produce all around better results than plastic?


For light loss, sharpness and bokeh it seems like the perfect aesthetic solution with these adapters would be using actual glass but why does it seem like it's impossible to get one in these adapters?
 
easier to get the slight opaqueness right? I'm not sure how scientific frosting glass is or any of the techniques used in making a GG. Good question.
 
Where there's sand blasting, hand ground, and I've seen some machines that grind the glass as well. For most higher optics its laser frosted.


Kholi, simply it's the term used the wrong way.

It's actually

- Focusing Screen

or

- Ground Glass



"For light loss, sharpness and bokeh it seems like the perfect aesthetic solution with these adapters would be using actual glass but why does it seem like it's impossible to get one in these adapters?"


Yes some will increase bokeh, but light loss hmm well not so much. The lower the grit used to matte the GG, the better bokeh. The main problem with using GG is, there's more visible grain, weight, and cost. Screens can be found at a cheaper price (well some) and to have GG ground perfect everytime is going to be hard to find. I've used anything from optosigma, edmunds, anchor optics, to a private company in Japan. My experience is you'll always find a few in the bunch that are scratched or just 86.











:)
 
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Ah that's what I thought. So, what about manufacturers giving us a GROUND GLASS? actual glass on a spinning platform instead of plastics?

Why hasn't it been done yet?
 
"Ah that's what I thought. So, what about manufacturers giving us a GROUND GLASS? actual glass on a spinning platform instead of plastics?

Why hasn't it been done yet?"



Well I have a few adapter units (spinning & vibrating) that I'm going to be putting together here in the next couple of months for testing. My vibrating adapter is just about finished (Wow it's only taking me almost two years to complete.... It's a curse being a perfectionist) , but can't seem to shake the vignetting from one of the testing screens (beattie screen). I'm cutting out another screen case for a smaller format adapter in a few days, well see how it goes.


To tell you the truth, I have to give it Wayne with the SG pro. Most underrated adapter, but produces amazing footage from what I've seen !!!

This has actually inspired me to make a spinning adapter, and can't wait to compare the two I'm making.



Oh and yes it has been done before with spinning ground glass.

Chris
 
Kholi said:
When has Glass been used with success? I'm interested.
Click to expand...


Grinding a glass disc and drilling a hole in the center is far more time consuming and hence, expensive, than applying a matte laminate to a piece of clear acrylic.

I actually took the time to hand grind a real glass "cine screen" for the M2, and I can tell you, it would probably take at least 10 times longer to produce this kind of ground glass even with an automated grinder. It's a very slow process to do it right.

.
 
What were your results, Chris? I can imagine it being a very difficult process in comparison to, but the difficulty would pay off, right?

Isn't that why P+S uses Glass in a vibrating format instead? I could see a spinning Glass destroying the P+S.

What kind of image rendition could you expect when comparing Spinning Ground Plastic versus Spinning ground glass? Time aside and image to image.
 
Most camera manufacturers just use plastic for the focusing screen in their SLR's these days right? Was there a point at which SLRs actually used a ground glass?
 
"What were your results, Chris? I can imagine it being a very difficult process in comparison to, but the difficulty would pay off, right?"



My results were ok, only because I made that adapter with caveman tools. When I made the adapter (about three years ago) I didn't own cnc machinery, so I was very limited on what i could have done. As they say the proof is in the pudding. I'll have some stuff to show in a few months. :)

I had an idea to try wax dipped glass, but the company wouldn't make small quantities. I've tried a few things around the house with wax, but didn't get really good results. Plus it takes too long to make it.





"Most camera manufacturers just use plastic for the focusing screen in their SLR's these days right? Was there a point at which SLRs actually used a ground glass?"


yes, focusing screens. Lager format cameras in the past used GG and I believe some still do, but I'm not sure if smaller & medium size did. I'm sure at one point time they probably did.
 
Kholi said:
What were your results, Chris? I can imagine it being a very difficult process in comparison to, but the difficulty would pay off, right?

Isn't that why P+S uses Glass in a vibrating format instead? I could see a spinning Glass destroying the P+S.

What kind of image rendition could you expect when comparing Spinning Ground Plastic versus Spinning ground glass? Time aside and image to image.
Click to expand...


I really never tested it properly. I put it in for a short I was shooting a year ago but then realized I couldn't get focus on infinity because I hadn't used spacers to position it further forward (since the original gg was thicker) so I took it out and never put it back in.

I'm going to do some real tests soon with it compared to the original M2 ground glass. I'd really like to know if it is in fact optically better than the plastic, which I assume must be the case. I'd expect ANY optical grade glass to be better than any kind of plastic, not to mention the (I assume) better ground glass face on the glass as a result of using the super fine abrasive to grind it.

.
 
I also assume that it's going to be better than plastic, how much is the question.

Bokeh to Bokeh
Light loss to Light Loss
Grain Pattern to Grain Pattern
Sharpness to Sharpness

Theoretically, an SGpro type unit with GLASS instead of plastic being put out could solve everyone's issues with all of these things. Edge Sharpness would all be dictated by Achromat and Camera Lens still.

I think the best part would be Sharpness and Bokeh rendering.
 
The Brevis GG seems to be glass to me. I just put one of the extra interchangeable ones up to my teeth and it feels like glass.
 
Being an adapter designer, I have tested many different types of 'focusing screens'.

My first prototype adapters used hand ground glass using 320 grit aluminium oxide and backpolished using the 'nevea cream' method. Results were ok but the bokeh was hazy even though the GG was diffusing properly with no 'ariel image pass through'. This is going back to 2005.

The SGpro currently uses an optical grade plastic which is much like acrylic, with the diffused surface formed from a glass master that we, shoot35, own. This results in a unique type of diffusion which gives the SGpro it's excellent bokeh rendition.

The cost of producing a real glass disk is about 8x more expensive. Indeed, I have 3 samples here, 1mm toughened glass with a 40 micron grind produced by a well respected optics company here in the UK. Better? No. Twice the weight, more lightloss and the bokeh rendition is not even close to as good.

I don't personally feel it is the material, either glass or optical plastic, that makes a difference, but the grain size and structure of the diffused surface.
 
Hey Wayne! Thanks for chiming in.

Would you conclude that there's no way to produce a Spinning Ground "Glass" adapter then due to the weight, lightloss and bokeh? Or, is it just more R & D? Of course that equates to more money, however.
 
Kholi,

Yes it is certainly possible, I was more trying to point out that we shouldn't assume that simply because its glass then this will result in better bokeh or lightloss. I used the SGpro's current 'focusing screen' as comparison. I know for a fact, that you cannot replicate its diffusion characteristics with glass.
 
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Thanks for educating me. It's definitely an assumption, but now that I have the info I won't assume that it's better.

The Pro35 uses glass and I think that the Diffusion characteristics are great but then again I dunno how much light loss is included.

It's still an intriguing idea, i wish there were examples up.
 
Wayne has it right. Glass is heavier, meaning it won't spin as fast in a rotating adapter, and the battery won't last as long. In terms of practicality, that is a huge drawback when the rotating adapter's biggest bonus is that it is able to spin the grain fast enough that it won't show up as easily as on a vibrating adapter.

Also keep in mind that it isn't 'just plastic'. It's optical grade plastic - the stuff in the lenses in your glasses.
 
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