Testing ground glass register

[andrewch59]

I have a Shen Hao 4x5, which I have not used for a while, I bought a Maxwell screen for it due to a really dim image on the gg. I can't praise this screen enough, but suddenly I was getting soft slightly OOF images? It was retired for a time. I recently took it for a three hour trip down to the coast to shoot some long exposures of the coast, old boats, piers and the like. I returned home excited to develop these pics, only to find them all soft, thinking it was my sironar 210mm lens I set the tripod up in my little studio and tried to focus on the following image, well the smallest writing on the paper, which was the internet address. It focused fine, so it wasn't the lens, which only left the registry. After countless exposures I finally got a decent picture with two slices of film and two strips of thin paper on each side of the GG to push the GG back to the film registry. Pics are now sharp.
So I got out my 8x10 which has never had a film through it, to test my new-to-me fujinon w 250mm f6.7 (arrived today), once again I have the soft image problem, back to the studio, and the lens actually produces a sharper image than the sironar. Rather than spending lots of film doing the same registry alignment test, I thought I might ask the more experienced if there is an easier way?? I don't mind zeroing this way, but if there is another method I would be interested to here about it.
180316

[mpirie]

There is an ISO standard measurement for the distances between the depth of the film holder front edge and actual film plane.

This should tie up with the location of the ground glass surface in the focussing screen.

I'm not sure if the measurements are the same between 5x4 and 10x8......but I'm sure someone who does know will be along soon to explain in detail :-)

Mike

[Jim Jones]

The standards Mike mentioned has been discussed at length here: http://www.largeformatphotography.info/forum/showthread.php?143185-ANSI-Standard-for-14x17-holder-seems-odd&highlight=film+holder+dimensions.

[Tin Can]

Wood camera, maybe it dried out and shrunk a bit.

Your shims seem to be only 0.010". The spec is 0.014" wide or +- 0.007".

I can think of other variables, but that's for others to introduce.

[Doremus Scudder]

I take a photo of a detailed horizon line (e.g., hill with tall trees) at infinity focus and a wide aperture every now and then to check my ground-glass alignment. If it's off, then a test with a ruler at an oblique angle to the camera is done. Shoot at a wide aperture and focus on a specific marking on the ruler. If the focus is off one way or the other, then you at least know which way you have to move the ground glass. Shim a bit and test. When I need to re-adjust the ground glass, I do the tests when I'm set up for printing. Shoot the neg, 3 minutes in Dektol, stop and fix, rinse and inspect wet with the magnifier. Re-shim and repeat as needed.

I have a good depth micrometer too, but find the practical tests to really be the fine tuning I need.

Best,

Doremus

[Drew Wiley]

You're wasting your time unless you have a precision vacuum or adhesive film holder, because your film is not otherwise laying flat. Ordinary holders do not do that. At a minimum get some thin removable double-faced Scotch tape to tack down the center of the film where it bows the most. As an option you could try viewing the aerial image using a good loupe with no film involved. But fresnels or screen intensifiers make this second approach quite difficult, because they might be part of the problem. Ordinary holders cannot do that, especially with large sizes of sheet film. Filmholders as well as camera backs can also become warped or misaligned. You have to check every variable step by step in a process of elimination. Besides a truly trustworthy film holder, get a straightedge which is precisely ground flat - not some mere aluminum extrusion or cheap level.

[ic-racer]

You only need one full sheet of film. Exposing film in the film holder is the only way to really know what is going on. When you determine how far off your negative image is from the target you can use the simple lens equation to estimate the GG correction needed:
1/(intended target distance mm) + 1/(intended film plane mm) = 1/(focal length mm)
1/(actual focus distance mm) + 1/(actual focal plane mm) = 1/(focal length mm)

Solve for "intended film plane" and "actual focal plane" and subtract them to estimate the ground glass error.

You can wrap ten turns of tape around a rod and measure the thickness divided by 10 to find out the thickness of your 'shim' tape, if needed.

For example with your 210mm lens, if you focus at 2 meters and the processed film image is sharp at 1.9 meters your ground glass is 1.5 mm too close to the lens.
If you wrap ten turns of tape around a pencil and measure it to be 4mm then you could use 3 or 4 layers of that tape to shim the ground glass back.

If you wonder if you could just "stop down" take note that f16 gives 1mm of depth at the film plane and f22 gives 2mm.

[Randy]

Brrrr - all this talk sent a cold chill down my back - I am almost done building an 8.5 X 15" back (including a slide-in ground-glass frame with home-made gg) for my 8X10 camera so that I can use a Vageeswari film holder I purchased. I wonder if there is any chance that I'll get lucky :(

[Drew Wiley]

I have a long bar of Starrett precision-ground flat stock with a hole drilled in the middle to accept a digital depth micrometer. I have yet to encounter a view
camera where the film plane is entirely correct; and I've got some Swiss Sinar Norma, F, and P gear. So don't expect any wooden camera to be precise and
without need of a bit of tweaking. But in most cases you should be able to achieve "precise enough". Certain old roll film holders and warped sheet film holders are hopeless unless being somewhat out-of-focus is just part of your game; but in those cases, expect light leaks too.

[Greg]

Adding in my 2 cents... Orientation of the camera is important. Years ago was using an 8x10 Sinar Norma, mounted on a Linhoh Heavy Duty Copy Stand, above my Nikon SKe microscope to take some photomicrographs. Focusing was tricky (couldn't reach the focusing knob on the microscope where at the same time viewing the GG). First few negatives turned out to be out of focus. Figured that because the film back was horizontally (pointed face down), the sheet of 8x10 film had to be sagging a few mm's. Configuring an adhesive back to the film holder surely would have worked, but never thought of that at the time. So I exposed 5 sheets of film lowering the back 1mm at a time. Turned out that lowering the back 4mm solved the problem (compensated for the sagging of the film) in the center of the image. In the end lowered the back 3mm to get the best overall focus across the negative which worked out just great for contact printing the negatives.

[andrewch59]

I guess like most who are new to lf, it is not something you automatically think of. I have blamed my loupes or perhaps poor (clumsy) practices such as knocking the camera and the like. Now I am at least aware that this problem is more commonplace than I thought, my other cameras may now come under scrutiny. I have always been in awe of the image on my big Deardorff GG, I can see the lines in the irises of my sitters (victims) but that it is never reflected under scrutiny in the photos, it will be next on my hit list.

[consummate_fritterer]

I spoke with Bill Maxwell when I bought my Chamonix 45N-1. I knew the original Chamonix had GG alignment issues due to miscalculation of the factory Fresnel lens. I sent the camera and new focus screen to the technician Maxwell suggested, who installed it to very tight specs. I wouldn't do it any other way unless I could trust myself to do it right, which I can't.

[andrewch59]

I spoke with Bill Maxwell when I bought my Chamonix 45N-1. I knew the original Chamonix had GG alignment issues due to miscalculation of the factory Fresnel lens. I sent the camera and new focus screen to the technician Maxwell suggested, who installed it to very tight specs. I wouldn't do it any other way unless I could trust myself to do it right, which I can't.
My Shen Hao has been soft from purchase, I blamed it on the very dim GG image for a while, but eventually sent the gg and frame back to the seller, I wont mention names as I don't believe the seller of a camera is responsible for manufacturing faults. The seller in the US was excellent, no questions asked and the replacement was sent out promptly. There was no difference so I decided the dull screen must have been the problem and got the Maxwell, a very decent man to work with. Screen was much brighter but the problem remained, that's when I decided it could only be the registration of the GG. A long trial and error period but I am now confident in the camera.

[consummate_fritterer]

My Shen Hao has been soft from purchase, I blamed it on the very dim GG image for a while, but eventually sent the gg and frame back to the seller, I wont mention names as I don't believe the seller of a camera is responsible for manufacturing faults. The seller in the US was excellent, no questions asked and the replacement was sent out promptly. There was no difference so I decided the dull screen must have been the problem and got the Maxwell, a very decent man to work with. Screen was much brighter but the problem remained, that's when I decided it could only be the registration of the GG. A long trial and error period but I am now confident in the camera.

Did you send the back and new Maxwell screen to the technician Bill Maxwell recommends?

[Drew Wiley]

Greg - it has always astounded me that marketed sheet film adapters for trinocular microscopes weren't adapted for vacuum on a standard basis. It is easy to
do. Astronomers who used even roll film rather than glass plates figured that out long ago, but maybe they're fussier for measurement purposes as well as the very long exposures involved. I made a nice 8x10 vac holder, but it's inconvenient in the field, so for a long time have used adhesive holders instead,
mainly for color film since it's more likely to get enlarged beyond 20X24 than my black and white shots. But even without magnification, I can easily detect
the poor focus inherent to ordinary holders in a side-by-side light box comparison with the same 8x10 image shot using an adhesive holder too. With microscopy the photography tube is often pointed straight down, so the worst possible scenario in terms of sag. But a sad subject for me. I've been wishing for quite awhile to acquire a 70's vintage Zeiss research or medical microscope like I used in my school days. They're abundant on the used market; but I always have other equipment priorities on my limited budget. Mostly for just looking at lil' critters while I still can, and before they're feeding on me!

[andrewch59]

Did you send the back and new Maxwell screen to the technician Bill Maxwell recommends?.
No, I retired the camera for a while in frustration and used my Horseman FA, but was restricted to smaller lenses. Then got it out of cobwebs and fixed it as stated above.

[Tim V]

All of this terrifies me... Focus with my Linhof cameras has always been 100% accurate, straight out of the box. I've my tested 4x5" and Techno cameras with my 60mpx Leaf Credo digital back, and both were spot on.

I'm now in the process of acquainting myself with my new (to me) Gibellini AC-D which I've had to do some work on due to a warped rear frame (I made a new one, to a higher quality than the original,) and that has worked out well. But to be honest, I'm suspicious of the accuracy of the GG back in the sense that one corner does not lie completely flat against the surface it should, I think due to a slightly misplaced pole that presses against the spring back. Hard to explain, but I'm guessing in one corner it's about 1mm, maybe less out. I have not done a register test on the GG frame itself, and fear that might be off–It might be accurate, too!–but know that one corner is definitely in need of some careful attention.

Anyway, good thread. Thanks for the info! It's something I like about these old fashioned cameras, that the basic build and servicing of them etc is easily learnable, even to someone like myself who lacks any real woodworking / metalwork technical skills. I like the struggle...

[Tin Can]

I am partial to metal cameras

Over time I owned

Calumet C1
Linhof Kardan Color 8X10, 5X7
Horseman LX 8X10, 4X5
Rittreck 5X7
Gowlandflex 4X5
Gowland 2X3 Pocket View

Some heavy as can be, but all stable

Weight, Mass is good

I don't hike

But I do have a pickup truck with a BedRug (http://bedrug.com/) and cap

YMMV :)

[Tim V]

180587Obsessive compulsive alert...

After reading this thread I decided to test my camera for focus calibration, and I'm glad I did. It was way out, severely back focusing! After measuring the position of the GG relative to the edge of the frame with a digital depth metre, I found it was easily within tolerance (about 1/10th of a mm deeper than perfect) so got to checking how the frame itself was sitting in the camera. I found it was not seating properly on the area where the film back sits, identified the problem and fixed it. Have shot a test image and scanned at 2400dpi on an Epson V800–a very average scanner if one is looking to get the best out of 8x10", at least in my opinion. Anyway...

Attached is a resultant 100% crop of that scan. I can tell that it's about 6-8mm front focused, as the gridded lines resting on the horizontal surface of the table are spaced 10mm apart, maybe 1mm less when taking into account the thickness of the lines. The full resolution scan of the entire negative, printed at 300dpi, would measure 80" on the long edge, and this crop would translate to being a 6" wide area out of the near centre of that print. The lens used is a very average condition 12" Goerz Dagor AM Opt. Co., and the object pictured is the rear of a plastic CD case standing at 90 degrees to the tabletop. I've taped a laser printed grid of fine lines to the rear of the case to provide sharp lines to focus on. The focus distance was approximately 1.75m from the film plane.

I'm thinking that this result tells me that the limiting factor is my eyesight and, to a good degree the resolution of the ground glass, as it has very coarse grain. using an 8x loop over my 4x loop only made the focusing experience more frustrating as it offered no greater clarity of exact point of focus.

I wonder, considering the fine tolerances talked bout here–as little as 0.016"–Would others here find this result acceptable?

[Drew Wiley]

I suspect you have several variables to still iron out and not just one, though it seems you're making progress. But you're the ultimate arbiter of what is
"good enough". At then end, it all comes down to whether you're satisfied with your resultant prints or not, if that is in fact your end game. Incidentally,
Dagors aren't typically great at close range, as nice as they can be for other applications.

[Bernice Loui]

*Precision metal cameras only, Sinar or similar. Check and verify camera alignment of front to rear standards are accurate and precise at the reference zero alignment position.

*No sheet film larger than 8x10, larger the sheet film greater the sag and problem with flatness due to temperature changes and more.

*No fresnel lens, these have caused more grief than what any improvement in perceived GG brightness can offer. Using a GOOD focusing lope really works, IMO WAY better than a fresnel, specially on wide angle lenses due to the ray angles involved.

*Verify GG to Film Holder dimensions using a depth micrometer, and do this with proper precision to better than 0.005" across the entire film holder and camera film back area.. This is why wood cameras are not preferred, IMO.

*Film holders of known precision and free of light leaks, yes film holders have tolerances and DO develop light leaks at the flap and dark slide slot (really shot film holder). Film holders warp, get out of shape and do the things materials do with time, temperature and wear.

*Tripod with mass and dampening, anything less can cause vibration related problems during exposure specially when working outdoors.

*Tripod head that is proven stable, those scrawny things are not gonna work. Sinar Pan-Tilt is one example of good.

*Above items becomes very significant when working with LF lenses with larger apertures of f8 and larger.

*Use a proper cable release to reduce vibration and movement transmitted to the camera during exposure.


Linhof made a portable vacuum film holder decades ago to address the problem of film flatness, not a common item and not well known item. In the graphic arts process camera world, vacuum film backs are pretty much standard method to maintain film flatness of those BIG sheets of film.


Bernice

[andrewch59]

180604before, after180605
The difference, three thicknesses of thin greaseproof paper
These are 8x10 sheets at 100% magnification after being scanned at 3200dpi on an Epson v800 scanner

[andrewch59]

I had better add that these were taken with a Fujinon 250mm f6.7 wide open

[Tim V]

With my first test I was severely back focusing, so after my small modification that didn’t change the gg holder at all, just how it sits within the back of the camera, it’s a night and day difference. Much like your results. I think in real world shooting it’ll be perfect, as my gg is so grainy that it’s hard to differentiate such small details under the loop and after measuring the gg position with a depth micrometer, it’s more likely user error than anything else.

Going through this process has been good though as I’ve learnt a lot about camera design and will look to modifying my camera with more logical parts in the future.

[andrewch59]

With my first test I was severely back focusing, so after my small modification that didn’t change the gg holder at all, just how it sits within the back of the camera, it’s a night and day difference. Much like your results. I think in real world shooting it’ll be perfect, as my gg is so grainy that it’s hard to differentiate such small details under the loop and after measuring the gg position with a depth micrometer, it’s more likely user error than anything else.

Going through this process has been good though as I’ve learnt a lot about camera design and will look to modifying my camera with more logical parts in the future.
Tim the hardest part of the whole process for me was finding a loupe I could trust, I have the end of an old microscope that has focus adjustment. Now I have removed two variables, loupe and registry and feel a lot more confident in image capture.

[Tim V]

Yeah, finding a good loop is a mission in itself! With my Linhof Techno, I have the newest bright screen which has super fine grain and exceptionally bright. I use a 12x loop from Linhof Studio, UK., and even with 80mpx capture and a 90mm lens wide open (f5.6) on a relatively tiny sensor (at least compared to sheet film) I can nail focus almost 100% of the time. For 8x10", I compared my Rodenstock 4x, Schneider 3x and 8x, a Mamiya 8x and Silvestri 10x. I found that going higher than 4x actually made focusing harder, as the Gibellini screen is so coarse it does more to confuse the situation than make things any clearer. The reason I'm a little obsessed with making sure things are really accurate is simple one of dollars and cents. 8x10" is an expensive game, and I don't want to be second guessing myself in the field or find myself disappointed with the results after putting in the extra effort with the format.

Anyway, for all intents and purposes, at least until I can afford a new ground glass–I'll get one from Steve Hopf–I think my camera is about as accurate as I can get it for now. Even so, I'm going to remeasure the GG frame depth with a larger digital micrometer as the other one I used for my initial check was a bit fidly. I think it'll be fine though.

[andrewch59]

The glass I have been using I made myself, it is not that hard and a damned sight cheaper, and a knowledge the next one is just a sanding away.