GG and Bokeh

[David MD Smith]

So recently there have been a lot of posts talking about acheiving good bokeh (nature of out of focus point sources of light) and which ground glass is best suited for the purpose in 35mm adapters.
I'm of the opinion the bokeh is a property of the slr lens used. When using fast lenses as is the necessity with these adapters, out of focus point sources of light can generate quite a large image on the gg. Surely, if the diffusive properties of the ground glass could in anyway distort this image, then it would useless for its primary purpose which is to relay the 35mm image as accurately as possible enabling the video camera to capture it.


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?

[Wayne Kinney]

Hi David,

I think its mainly down the the level of diffussion across different types of GG. I have been testing this alot and it has been posted many times that the more the GG diffuses, the better the bokeh, but there is more lightloss.

Testing a 1000 grit GG against a 400 grit GG, for example. The 400 grit would render the sharp disks from the out of focus points of light, which would be the same shape as the aperture when the lens is stopped down (pentagon shape). With the 1000 grit glass, this would not happen, the bokeh would be much more hazy.

So yes, the Bokeh is down to the 35mm lens, but also in the GG's ability to capture it.

[Tim Johnson]

So, which one gave better bokeh? 1000 grit GG? When you say hazey do you mean smoother?

(o/t:

Wayne Kinney
Inner Circle

Join Date: Jun 2005
Location: Brighton, East Sussex, UK
Posts: 666 <--)

[Wayne Kinney]

I had better reply to make it 667 ;)

The 400 grit glass gave better bokeh, but more lightloss. Hazy due to the ghosting mainly (aerial image).

[David MD Smith]

Hi Wayne,


But if the gg disffuses the unwanted sharp halos (the dreaded bad bokeh)
would it also not diffuse any other detail in the 35mm image as well?


cheers

Dave

Love the new look website by the way!

[Wayne Kinney]

Well, the points of light turning into disks when out of focus is what we want (good bokeh).

[David MD Smith]

When they have a distinct edge or halo that is bad right.
So if the gg softens away this effect then surely any detail in the rest of the image that we want to preserves will have to be diffused away.

I'm just not convinced that the bokeh can be positively influenced by a good gg.

[Ben Winter]

This is good bokeh: http://www.the-digital-picture.com/I...keh-Sample.jpg

This is bad bokeh:
http://www.frozenphoenixproductions..../LetusTest.bmp

The G35 has a focusing screen that produces results very similar to example 1. Good GG means good bokeh.

You see bokeh like example 1 in movies all the time. If you think that is bad bokeh, you are in disagreement with the majority of the industry.

[Wayne Kinney]

Ben,
The first image says what I mean, thanks. If shooting straight to film, this is what we should get.

In an adapter, the lower grit GG renders this better then say a 1000 grit glass.

[Wayne Kinney]

Originally Posted by David MD Smith

I'm just not convinced that the bokeh can be positively influenced by a good gg.

Well, i guess we are comparing good ground glass and bad ground glass, and the bokeh produced between the 2 will be different.

[David MD Smith]

I agree the first image looks great.
But what I don't get is this:
If we were shooting straight to film that is what we'd get right so the bokeh is a product of the slr lens.The gg should be such that it diffuses this image and what we get on video is identical to what would have been on the film cell.
If the gg modifies this then surely it is distorting what would have been on the film cell thus degrading the image we want to capture.

Beyond merely softening the image (which isn't really a good thing) and hence the edges of the out of focus lights I can't see what the gg can acheive.

[Ben Winter]

So what you're saying is if the out-of-focus areas don't look right, the in-focus areas might not either? It's possible, but the issue is usually that the gg isn't diffusing the image enough, not distorting it. As a result it's usually that more things are in-focus than they should be. I'd personally like to believe that a focusing screen that diffuses properly lends itself to a better image, but there are lots of people on these boards who cry themselves to sleep over light loss.

[David MD Smith]

That's it. Beyond merely softening the image (which isn't really a good thing) and hence the edges of the out of focus lights I can't see what the gg can acheive.

here's what I mean: on this site about 1/2 way down are 3 images of bokeh as a product of the lens
http://www.kenrockwell.com/tech/bokeh.htm
so if the gg can turn fig 1 to fig 3 then it must be doing something weird to the light all over the cell, which isn't quite what we're after.

[Wayne Kinney]

David,

Imagine this: We take an adapter and imagine using a clear peice of glass as the GG. Your shooting complete aerial image. DOF is very wide. Now, we begin to grind the glass, ground a little, we are diffussing very little, and still seeing the 'aerial' image. grind more, diffuse more. Thats 1 factor. Then the grit size will also influence diffussion.

This is just an example of how the GG can influence the Bokeh.

The 3 images in your example is good, I have had a GG which gives bokeh like the third image, more like a gaussian blur.

I think that example is dealing with good and bad bokeh between lenses, our job is to match the bokeh of a given lens to what it would produce if attached to a film camera.

[Ben Winter]

I actually find what they call "bad" bokeh pleasant. I like how the G35 has switchable focusing screens that give you "hi-lux" and "lo-lux" type bokeh, which is basically figure two and figure three of those examples. At the bottom he talks about bad bokeh examples, and how one picture had bad bokeh yet the photographer knew what he was getting. So while they may classify bokeh as technically "good" or "bad", it's still artistically subjective.