View Full Version : Relationship between ground glass and lens
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
27-Jan-2014, 19:52
You may need a coarser, not a finer, ground glass.
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
28-Jan-2014, 02:13
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.
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
28-Jan-2014, 03:40
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 :)
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,
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
28-Jan-2014, 16:22
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?
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
My most recent photos with the lens turned out badly so I scrapped them. I can recall comparing them to the same shots with the 180 Sironar and a Heliar and not believing the results. (I blamed myself for not checking focus properly.)
I'd have to go searching for the actual image that I'm thinking of that was sharp (IIRC). Now that my ears have pricked up on this one I've remounted the lens about 5 times ! I'll be devastated to hear it is the lens. I paid good money for it and it is my special lens used on the odd occasion.
Leigh the off-axis abberations here are gross. Not even close to other lenses. The far trees are just a blur whereas other lenses are showing some leaves at least.
Cheers,
ps. I've sent an email to Schneider in California. Unfortunately for my impatience they are only open on Mondays, Wednesdays and Fridays so I'll have to wait a little longer.
Leigh the off-axis abberations here are gross. Not even close to other lenses. The far trees are just a blur whereas other lenses are showing some leaves at least.
OK. There's something seriously wrong.
My first guess is that the lens originally had one or more shims (front and/or rear) that were somehow lost during mounting or cleaning.
Shims, if present, are very important since they set the internodal spacing.
- Leigh
genotypewriter
28-Jan-2014, 20:47
My most recent photos with the lens turned out badly so I scrapped them. I can recall comparing them to the same shots with the 180 Sironar and a Heliar and not believing the results. (I blamed myself for not checking focus properly.)
Not sure if you missed my question earlier but what range of f-stops do you end up shooting at usually?
Now that my ears have pricked up on this one I've remounted the lens about 5 times ! I'll be devastated to hear it is the lens. I paid good money for it and it is my special lens used on the odd occasion.
I don't call a lens a special one until it performs the way I want it to perform :)
Geno,
I won't have been shooting at even f11. I would normally shoot at f16 or f22 or so.
Rgds,
genotypewriter
29-Jan-2014, 20:11
A couple of things have left me (and I'm sure some others) confused in isolating the problem. This is what I've gathered from what you've said:
1. On the GG, the image is sharp in the center but it's difficult to determine focus off-center.
2. The taken image, even at ~f/16-22 is also blurred all over.
3. There might be one image that came out sharp.
#1 is normal behaviour as we have established. #2 and #3 are contradictory. If we pair #1 and #2, the question is how the image became softer as the lens was stopped down. If we pair #1 and #3, it suggests that the GG and lens are fine but there's something else that blurs things up.
For me, going with 1+3 make a lot more sense than going with 1+2. Check to see if your camera shakes easily or if you have involuntarily applied a bit of tilt or swing.
For what it's worth, here's the 110 SSXL @ f/5.6 on a "Chinese" Shen-Hao 8x10 :)
http://www.flickr.com/photos/genotypewriter/9719316020
(Note the link in "see original size")
Sorry if there is confusion.
At no point is focussing straightforward. There is glow at the centre and at the edges. Focussing a distant tree at the edges of the image is impossible. The relatively recent results I had at f16 or f22 were blurry all over. The 'one sharp' image would've been shot 2 to 3 years ago. My thinking is that the lens has deteriorated somehow sitting on a shelf. Sounds weird but how else did the problem evolve over time like Oren Grad's post would indicate? Perhaps an internal element is held in place with an O-ring or something organic ?? Perhaps a coating breaks down with time ? Dunno.
Anyway, lots of packaging and big costs later and the lens is on a plane. I suspect we'll see whether it is the operator or the lens !
genotypewriter
31-Jan-2014, 03:08
My thinking is that the lens has deteriorated somehow sitting on a shelf. Sounds weird but how else did the problem evolve over time like Oren Grad's post would indicate? Perhaps an internal element is held in place with an O-ring or something organic ?? Perhaps a coating breaks down with time ? Dunno.
Wasn't aware that you had the lens for several years and that it worked back then. In that case you might be right about the possibility of something deteriorating. I'd still expect to see some tell tale signs. E.g. separation of elements might be visible and maybe one side could be worse than its opposite, etc.
Guess you'll find out and do let us know of the outcome too, if you don't mind.
Good luck!
Doremus Scudder
31-Jan-2014, 04:16
Sorry if there is confusion.
At no point is focussing straightforward. There is glow at the centre and at the edges. Focussing a distant tree at the edges of the image is impossible. The relatively recent results I had at f16 or f22 were blurry all over. The 'one sharp' image would've been shot 2 to 3 years ago. My thinking is that the lens has deteriorated somehow sitting on a shelf. Sounds weird but how else did the problem evolve over time like Oren Grad's post would indicate? Perhaps an internal element is held in place with an O-ring or something organic ?? Perhaps a coating breaks down with time ? Dunno.
Anyway, lots of packaging and big costs later and the lens is on a plane. I suspect we'll see whether it is the operator or the lens !
Steve,
Any fogging, fungus, coating degradation, etc. will be easily visible if you open aperture and shutter and look through the lens.
If the lens has been dropped or bumped hard enough, a lens element may be displaced, causing your focusing problem.
Even if the ground glass is in the wrong position in your camera in relation to the film plane, you should still be able to focus a sharp image on it from a lens in good working order, even if the resulting neg is out of focus due to misalignment. If you cannot get a sharp image on the ground glass with just one lens, then likely the lens is at fault. Do some more focusing and changing lenses and compare image sharpness in the center. A lens like the 110XL should perform superbly in the center. If it isn't, there's definitely something wrong with it.
Best,
Doremus
Back again to fill in ...
I've heard back from Schneider. Apparently, there is an element in the rear cell that is out of alignment. They'll repair and clean the lens for a fair price.
Has anyone experienced this ?
I swear to you there were no bangs or bumps put onto the lens. Nor was there anything loose when it was shaken. I'm thinking the only way for this to occur must be through the expansion and contraction of metal. Could this be possible ?? Is it possible for a lens to lose its alignment by being left in the sun and then shade etc over time ?? How else could a retaining ring or whatever is used to hold the elements in place become loose ? How do I prevent it in the future ?!
Is it possible that there was a seal of some sort that has degraded inside the lens cell ?? Schneider wouldn't use an O-ring of sorts to hold anything in place would they ? I thought the insides of cells were just paint, metal and glass.
Anyway, I'm hoping it all goes well and that I'm glad I sent it to Schneider.
Hope this helps anyone in the future.
Cheers all.
I would not attempt to second-guess the manufacturer.
The defect might be known, as they described.
It might be known but something they don't want to admit, so they're feeding you a bunch of smoke.
Who cares??? You just want the lens to work right, and they've agreed to make it right.
- Leigh
hoffner
22-Feb-2014, 05:58
Back again to fill in ...
I'm thinking the only way for this to occur must be through the expansion and contraction of metal. Could this be possible ?? Is it possible for a lens to lose its alignment by being left in the sun and then shade etc over time ?? How else could a retaining ring or whatever is used to hold the elements in place become loose ? How do I prevent it in the future ?!
Hope this helps anyone in the future.
Cheers all.
Yes, it is possible for a lens to change its performance due to the thermal stress you speak about. Direct sunshine can have a devastating effect on the alignment be it of a single lens element. Especially if the lens is heated unilaterally (sun/shade on opposite sides) and with quick temperature changes. Often, the effect is accumulative - a lens could have been knocked several times without any bad consequence - then comes thermal stress and becomes the proverbial last drop. Lenses should not be exposed to direct sunshine for a longer period of time. Dust often enters lenses also due to the thermal changes.
Cheers guys,
I do hear you Leigh in that all I really care about at this stage is that it comes back to me in tip top condition but I would like to know if I contributed to the problem.
hoffner, I'll be taking even more care I assure you !!
Over and out.
Lens arrived yesterday ...
I can focus it without issue now. In all my dreaming of how it would be able to see a fly's armpit hair at 50 ft I'm still not quite sure ... but I think I'm at the stage of sayin, "If it has passed Schneider QC then it passes for me".
I'm glad the little drama is over and it has arrived back without incident.
I will say in a discussion recently with another of our forum members he reckoned it was a outgassing of a sealant or something. In my correspondence with Schneider there is a reference to a sealant being used ... so there is a possibility. But in the end, no more blurry / hazy pictures hopefully.
Ciao
Thank you for letting us know the end of the story (which is not always the case with OPs). Be happy with your lens!
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