Relationship between ground glass and lens

[swmcl]

Hi,

Is there a relationship between the ground glass and the lens ? eg. If you had the sharpest lens out there would that mean you'd need an exceptionally finely ground glass on the camera to focus the lens perfectly ?

I've got a lens that doesn't seem to easy to focus. I'm trying to determine whether it is a lens issue or whether I should get out the carborundum paste or something. I do have a Chinese camera so I do appreciate there is room to move in grinding the screen some more.

I also would appreciate an opinion on image 'glow'. Some lenses seem to glow more than others. No not the soft focus Kodak Portrait ... I'm talking modern Schneiders and Rodenstocks. The Sironar S 180 seems exceptionally sharp and clear. The 110 SS XL seems to glow on the ground glass to an extent. This glow makes focussing harder too.

The lenses themselves look to be in exceptional condition. There are no marks nor damage that I can see. So I wonder if the ground glass could be the issue. Perhaps it is the loupe ?? A Silvestri in this case. How is a lens of this vintage affected by minute differences in where it is sitting in relation to the shutter ? Could it be that they aren't seating correctly ? Maybe a dimensional issue ?

The thoughts and experiences of the LF community is needed !! Please dont mention the operator. I realise there are limitations in operators !

Cheers,

Steve

[Bill_1856]

You may need a coarser, not a finer, ground glass.

[swmcl]

I've been looking at it today and I'm almost convinced that I have a lens with some kind of issue. I think I need to talk to Schneider. It's a bit hard to explain. It seems impossible to focus at infinity - there is no snap. The edges of everything look a bit blurry - certainly at a distance. I can focus at shorter distances but it still feels like a Kodak Portrait. Sort of a sharp edge with some 'glow'. And this is from a 110 SS XL ?? It is way different to the 180 Sironar.
How do I go about contacting Schneider ??!

[Doremus Scudder]

A finer ground glass may or may not help. Finer may show up more detail under higher magnification, but it will likely be darker, i.e., not capturing and diffusing the image light as much. there is a compromise between brightness and fineness of the grind.

Unless you are working with two-dimensional subjects, image sharpness is affected by a lot of other factors such as aperture and depth-of-field. For focusing, you only need to find the right position within the limitations of your ability to finely adjust the focus knob. More resolution than is needed for that is overkill. I find I have plenty of resolution with a regular ground glass (with or without Fresnel) and an 8x or 10x loupe. A good lens will resolve a lot more than we can see.

As for "glow"; you may be getting more diffusion from your wide-angle lens due to the angle of incidence of light from the lens. A 180mm lens will project much more straight-lines from lens to gg than a 110mm. You didn't say whether you have a Fresnel screen installed; than might be affecting your image as well with the 110, since Fresnel screens are lenses with a focal length and often don't work well with shorter lenses.

You don't say which camera you have, but if you bought it used, you might simply try removing and cleaning the ground glass. And, yes, some ground-glass screens are better than others and there are a lot of replacements available and you can make your own. If you are not happy with what you have, try some of the alternatives: search here for how to grind your own or try one of the screens available. If you don't have a Fresnel screen, you may want to try one of those as well.

Hope this helps a bit,

Doremus

P.S: I was typing when you posted the above about your possible lens issue. Before you do anything, make sure the lens is properly mounted on the lens board. I had a similar problem years ago, and found that a too-thick lens board was preventing the rear lens element from screwing in completely.

[swmcl]

Thanks Doremus,

I've got a Shen Hao purchased new - nothing but standard. The lensboard is standard Shen Hao issue. I have again looked at the physical setup and can't see how the lensboard could be getting in the way. THere is a reasonable gap to the rear element. I should say that during my testing this afternoon, I did unscrew the front cells and the rear cells to see if it made much difference but I think it is much the same. This one has me beat. The lens looks to be in excellent order there is nothing loose either. For a minute I thought maybe a glass element might have slipped or something ... It'll be really embarrassing to send it away to find some simple operator issue !

Cheers,

Steve

[genotypewriter]

I've got a Shen Hao purchased new - nothing but standard.

Hi Steve,

I think the question we all have on our minds is what format size the camera is

Ultra wide lenses like the SSXLs (I can speak for the 110 SSXL but the designs are similar across all the f/5.6 ones) have lots of aberrations as soon as you go off-center at the maximum aperture... this is what you generally perceive as a "glow" instead of a pin-point.

As for the GG thing... you don't need a finer definition (coarser grained as Bill said) with good lenses. If anything, it should be the opposite because soft lenses require higher precision from the GG to allow the user overcome the ambiguity of focus... with heavy aberrations, the points go from one type of blob to another type of blob and you should be able to tell which is the better type of blob to go with. A sharp lens you can focus on a piece of paper and shoot... and I have

[swmcl]

Hello geno,

I've re-fitted the lens to a small Shen Hao board to fit onto the 4x5 but it came off a recessed Sinar size lensboard that was on my 5x7. I'm not sure how the wide angles is much of an issue as other wide angle lenses seem fine.

I do admit it seems less of an issue wide open at the centre and I was trying to get an infinite focus on the top of some distant trees that were at the top of the photo. I can't believe I've not seen this before on this lens. The last issue is that my eyesight has changed fairly dramatically in the last couple of years - at close distances - hence the queries about the loupe. The only other magnifying type glass of higher power that I have is a fabic analyser (for looking at warp and weft) I could try that.

Cheers,

[swmcl]

Hello again,

I've just found something disturbing. A post by Oren Grad in another thread that says,

"The other thing to watch out for in shopping for a used SS-XL is that some early-production lenses developed some sort of fog or haze. You can probably find a couple of threads about it if you search here. By all accounts, Schneider figured out what the problem was and fixed it and has been exemplary in replacing defective lenses, but it's something to ask about with respect to early used samples."

I can't see any other mentions of this problem but it looks as though it could be me ! I purchased the lens a while ago. I'll bet Schneider don't want to know me by now although it does say 'developed' a haze because I'm sure I've taken one good sharp photo. Maybe its a coatings thing coz glass couldn't change over time ...

Ach!

[genotypewriter]

Hi Steve,

Your observation about center being easier to focus the way you want is consistent with what I've said. I can't speak for all wide angle lenses (which other wide angles do you have btw?) but the SSXL is not sharp off-center when wide open. I say this comparing 110SSXL @ 5.6 as a 110 degree lens against the Nikkor 65/4 @ 4 as a 100 degree lens. Relatively speaking the Nikkor is sharper when wide open (anyone else reading who's about to jump on this comment please save yourselves the trouble... the context here is sharpness when focusing on GG ).

Haze is unlikely to be your issue... if it's that bad you'd see it with your eyes when you shine a light through the lens. Also the center would also be affected... I've had very hazy lenses that were crazy difficult to focus and they don't fit your description.

Have you taken any shots with your lens? If so, at which f stop?

[Leigh]

I always center-focus, even if that requires re-framing the image afterwards.

All lenses have off-axis aberrations, some worse than others.
Standardizing on a center-focus method eliminates some sources of error.

- Leigh