Testing ground glass register

[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]

before, after
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.