GG and Bokeh

[Wayne Kinney]

Originally Posted by Ben Winter

I actually find what they call "bad" bokeh pleasant.

I was actually thinking the same myself. I guess thats subjective.

I guess to conclude, the GG will effect the resulting bokeh, and we need to match it the best we can to what it is on a film camera, for any given lens.

[Frank Hool]

i agree with David:
http://www.dvinfo.net/conf/showpost....03&postcount=9

[Andy Gordon]

I spent a small fortune on a 50mm f1.2 Nikkor. I took some shots on film and it produces bokeh like fig 1 - bad!

Another thing with GG and diffusion - a less diffusing gg can produce good bokeh and solid discs with a longer focal length and/or smaller aperture. If you look on Dan's site he has an example of solid discs taken with a 200mm f3.5 on a beattie.

The tricky thing is to get a gg that can produce solid discs on a 50mm at f1.2 or f1.4.

[Frank Hool]

Originally Posted by Andy Gordon

The tricky thing is to get a gg that can produce solid discs on a 50mm at f1.2 or f1.4.

The tricky thing is to get exactly that image that drew your photolens. If gg diffuses too little then the image will be mixed with non-processed(by lens) rays. Which will be part of the image on camcorders ccd but it's not part of the image on the gg.

[Andy Gordon]

Originally Posted by Frank Hool

The tricky thing is to get exactly that image that drew your photolens.

That's physically impossible due to the nature of what is essentially a rear projection screen. With full diffusion no light would pass through the gg, so you have to compromise if you want an image that is going to be bright enough to film.

[Donnie Wagner]

So it seems that we have a good understanding of the tradeoff between transmittance and diffusion. But back to one of Dave's main points:

The "in focus" parts of the image that are being projected onto the gg is also being diffused. Even though it does not have the circle of confusion that the out-of-focus spots have, they are unavoidably diffused when they hit the gg. This is probably why the "in-focus" part of the image never looks as sharp when using an adapter vs. when using your camera alone. We have to componsate by using sharpening filters etc.

I think gg technology is going to advance quickly in the near future. But then again, affordable full frame 1080 24p 4:2:2 detachable lens cameras are problably not that far off either. What will I do then? Maybe go out and shoot something rather than reading this forum and working on my adapter to no end.

[Tim Johnson]

id agree the bokeh is much nicer on the second one! in photography people preffer smooth, circular booky - not pentagons.

[Frank Hool]

Originally Posted by Andy Gordon

That's physically impossible due to the nature of what is essentially a rear projection screen. With full diffusion no light would pass through the gg, so you have to compromise if you want an image that is going to be bright enough to film.

Andy, i agree with You 99%. But i'm actually, maybe too indirectly, trying to say that direct unprocessed rays are cause of image artifacts which sometimes rise up in forums as topics of bokeh. Those have no connection to bokeh really. Those just smear image what is drawn by lens. Besides those rays are illusory on gg. So whole bokeh thing from point of gg is always conversation about diffusion.

About retained 1%. It's that i'm just not too sure physical impossibilities :)

[Francois Poitras]

Tim, the shape of the out of focus lights reflects the form of the 35mm lens diaphragm. When the iris is wide open, you get circles; as you stop down, the blades of the diaphragm form a polygon, which is reflected in the bokeh.

You also get polygons (the number of sides depends on the number of blades in the diaphragm) in photography and, depending on the lens used (and the bokeh it produces), they may or may not be distracting.

[Tim Johnson]

i know im just saying, the edges are very defined - i wouldnt consider that good bokeh personally.

[Dennis Wood]

Interesting thread...and after a lot of experimentation, there's little question in my mind that diffussion/bokeh/sharpness are inherently related. I don't believe we'll ever see true film emulation with respect to diffusion because we are not dealing with a film emulsion layer and it's subtle inter-layer reflection properties, as Andy alluded. That's why belabouring the point of perfect film diffusion is simply beating a dead horse (apologies to horse lovers there). Getting film-like diffusion at higher f-stops is fairly easy, but a match at F1.2 is likely impossible...we can only approximate it. At that level of diffusion, I don't believe the focal plane can be kept as sharp. Hence our swappable diffuser setup. Focused film images are not soft, nor are they all that grainy at 100 ASA. Try a test, however, at 400 ASA and you'll see that the bokeh is extremely grainy. The look would likely be dismissed outright by the adapter community.

From what I can gather, very high diffusion screens have grain issues at fairly low f stop values which limit their usefullness. Unless someone can design an adaptive GG, you simply cannot have a GG that does it all. You can, however, chose the GG for the look you want, or vary your focal length/distance/f stop values to achieve shallower DOF. What you cannot do is take a highly diffuse adapter and make it sharper or grainless at higher f stops...you're stuck with its properties.

Bokeh is an entirely subjective topic. Most agree though that soft geometric shapes, absent of internal distractions constitute "good" bokeh. I find growing halation on specular highlights unacceptable..however many adapters show this.

What's good for SD is of course a whole other issue when HD enters the fray.

[David MD Smith]

Originally Posted by Andy Gordon

That's physically impossible due to the nature of what is essentially a rear projection screen. With full diffusion no light would pass through the gg, so you have to compromise if you want an image that is going to be bright enough to film.

I'm not sure this is quite correct. If you imagine a very very thin gg surface and consider a very very small element which scatters incident photons in all directions with uniform angular intensity, then you would have 100% scattering and 1/2 of the incident light would be scattered one side of the gg and the other towards the slr lens (with a very little in the plane of the gg). As such, the image recorded by the camcorder would be exactly that as captured by a film cell. Ie there woud be no aerial image (unscattered rays) which is what we are after.

The point was though, if you constructed an adapter such that the camcorder recorded off the front surface (as would be perfectly possible but perhaps a little silly) then one could just use any white surface (something that scatters incident rays in all directions). If this was the norm, would we be discussing the properties of the surface used in conjunction with bokeh. I think not.

[Tim Johnson]

i had a reply but it was somewhat foolish - but i dont think the idea of recording off a front surface is silly at all

[Francois Poitras]

Originally Posted by David MD Smith

So, the question is: has anyone actually tested varieties of ground glass and compared bokeh in like for like circumstances and if so what was the result?

I did test a variety of GGs. Here are some results showing the difference between a less diffusive GG and a more diffusive GG in the exact same lighting conditions. The diffusers used in this case are engineered diffusers which have a 10° difference in angle FWHM.

Camcorder Panasonic PV-GS400
Vibrating adapter
Canon 50mm F1.4

Less diffusive GG, camcorder at F4

More diffusive GG, camcorder at F3.4

By the way, both these setups are unusable because of grain showing in good lighting.

[Ben Winter]

Originally Posted by Tim Johnson

i dont think the idea of recording off a front surface is silly at all

It was discussed at length earlier on the forums. It was concluded that the technical aspects involved were unrealistic.

I also did some focusing screen tests, this time with varying types of diffusion gels for theatrical lighting. I will post the clip of the highest diffusor soon. You can see the bokeh in the video, and it is bokeh I find very desirable.