5x7 holder depth to septum

[peter brooks]

This query might be better filed under the 'you're just being paranoid' section but...

All the specs I have seen for the distance from the outer face of the film holder to the septum (for 5x7) have said 0.228", +/-0.010" (so between 0.218" and 0.238").

However when I measure a Fidelity Deluxe holder (for instance) I get more like 0.204".

My calipers are good quality digital ones but admittedly it is tricky to measure accurately with the pointy depth thing.

Can anyone confirm the correct distance? This is to make a removeable ground glass 'frame' that will be interchangeable between several 5x7 cameras.

I'd also be interested to know how critical the accuracy is with regard to focus and the end result (I guess the quoted tolerance is the guide here), and where does film thickness come into it?

Thanks!

[Sal Santamaura]

...when I measure a Fidelity Deluxe holder (for instance) I get more like 0.204"...

I have several dozen 5x7 holders, a mix of Lisco and Fidelity Deluxe, all purchased new. Their depths to septums range from 0.212" to 0.236" when measured using a flat-ground piece of bar stock and the protruding pin on a digital caliper. However, those extremes are the two outliers; all others fall within +/- 0.006" of the 0.228" standard. Yours sounds like it's damaged or one that the factory should have rejected.

[John Layton]

The potential issue with simply adding film thickness to the film holder flange plane to septum surface depth is that films often have a tendency to rest very slightly above the surface of the septum plane - and to do so unevenly from the central portion of a given sheet and moving out towards the edges. This can be furthermore complicated by a host of other variables, both environmentally film holder related.

I actually did a bit of research here while designing the L-45A camera - gaining access to a climate controlled (and somewhat climate controllable) lab which was equipped with a very sophisticated measuring table featuring a multi axis, computer controlled, ruby-tipped measuring probe.

With the above setup, and using a variety of holders loaded with a variety of films, we tweaked temp. and humidity in the attempt to duplicate a variety of environmental conditions, being very careful to minimize the effects of moisture and heat emanating from our hands on the results. Measurements were taken starting on-axis, then moving outwards towards the films edges. We also took a few measurements while the film holders were tilted a various angles, always starting with a septum surface reference measurement prior to carefully loading the holders (and giving them time to stabilize after handling the films).

At any rate…it was by averaging out the above, with added bias towards what we’d construed as a hypothetical “overall” average of shooting conditions, that we’d arrived at a “standard” from which was then calculated the ground glass focussing surface flange to bottom flange of ground glass frame depth…which was then recomputed based on the formula of the combination focussing surface/fresnel sheets from Bill Maxwell, which are used in these cameras.

The premise of the above research is that I’d always suspected that a focussing system (particularly those for LF) could not be designed reliably based on holder flange to septum surface depth alone…nor could this be done by simply factoring in a films average thickness.

Furthermore, I’d never quite embraced the old standby assumption that one can simply “stop down the lens a bit more” to correct for variables encountered in the field - especially given that the above described research and development scenario was aimed at arriving at but a single standard measurement…which itself becomes suspect during more extreme environmental variations encountered in a host of uncontrolled conditions in the field, which can mean that, yes, a bit of stopping down might be required during such conditions…but if the actual “correct” standard had been used as a design basis from the get-go, then at least the user could at least be know that they’d had the best chance of minimizing the deleterious effects resulting from these variables.

And what is my “standard measurement?”…I don’t remember this offhand as the research was completed almost twenty years ago…and my notes, engineering drawings, and C+C cd’s are buried and will take a bit of time to access - and to be honest I really don’t feel like digging for this stuff right now. (sorry!)

But to be a bit more useful - you can actually repeat the same research scenario which I’ve outlined above at home…using a straight edge steel ruler (or machinists straight edge) with its narrow edge spanning across the shorter dimension of a given film holder flange…and by then setting a caliper on the top edge of the ruler, in such a way that allows for the caliper “probe” (skinny) end to move downward to delicately make contact with a film’s surface (be sure to subtract the width of the ruler!). You can do this at various locations over the film, and take an average of this, being careful while doing so to keep your (warm, moist) fingers as far from the film’s surface as possible. Wearing surgical gloves will help to mitigate moisture issues.

At any rate…I’ve posted the above under the assumption that you are building a camera - but if not…you can at least follow along and do a few measurements yourself as I’ve outlined - so that you might at least be able to better evaluate your collection of film holders. Good luck!

[Sal Santamaura]

Given all that research, John, and since you as well as Bill Maxwell are still around and kicking, how about finally putting your L-45A into production? I'd be one definite customer.

[Jim Jones]

. . . All the specs I have seen for the distance from the outer face of the film holder to the septum (for 5x7) have said 0.228", +/-0.010" (so between 0.218" and 0.238"). . . . Can anyone confirm the correct distance? This is to make a removeable ground glass 'frame' that will be interchangeable between several 5x7 cameras. . . .

Your information is correct according to a fairly recent copy of ANSI/PIMA IT3.108.1998. The publication also gives the "T" dimension as 5.79mm, +/_0.254mm

[Tin Can]

Measurement error can ruin your week

I worked 40 years in a test lab constantly measuring similar dimensions

We replaced our 'certified straight' edges at least once a year, we had 50

Starrett 380-12 12-Inch Steel Straight Edge is a good start

Then a depth gauge Starrett 449AZ-3R Vernier Depth Gauge

The straight edge needs to be checked before and after every usage, no nicks allowed, no FIXING nicks

To check a straight edge you need a reference https://www.mcmaster.com/granite-tables/

BUT I also tested the tools

The straight edge is a MUST

I have compared Starrett and offshore plastic gauges like this

https://www.amazon.com/s?k=depth+gau...f=nb_sb_noss_1

We also tested the tool users

Most could not make a good measurement ever

We had a dedicated, stable room with a very fancy machine the could digitally copy anything to data

Common practice to copy, perfectly any part

Spies also

no cameras anywhere at that time

THEN we did real life

[Bernice Loui]

Previously discussed here:
https://www.largeformatphotography.i...r-film-holders

The idea-belief "stopping down" the lens lots is a focus cure-all is not a good idea or belief at all. It is possible this idea-belief came out of a time when camera fit holder seating area to ground glass tolerances/precision/accuracy were poor and marginal at best. Since then, this specific area of the modern high quality metal view camera changed this lots. Question of these tolerances become more significant once large full aperture view camera lenses are used at their full aperture.

Beyond tolerances of film holders, there is the often neglected reality of sheet film in film holder flatness, sheet film sag, film "popping" and more. Larger the sheet of film, larger this problem. Of why larger is not always better..



Bernice

[peter brooks]

Thanks all for your input, fascinating as always. It's almost a case of the more you know, the more worried you could be... Those ULF guys must have the potential for real issues with film flatness.

I will have to check more holders, if they are consistently different to the expected (lower, it would seem) it could be that my calipers are out.

Which leads on to - how can an amateur check the accuracy of a measuring tool? Are there common (=not expensive) items that are a known and consistent size? I'm in the UK so US coins etc. are of little use.

Yes, the ground glass frame is for a camera. I previously built a simple mdf box camera (for a 90mm Nikon) with a removeable ground glass frame, and I want to build on the concept for another camera (a birch ply box with bellows and a Sinar shutter - for large and heavy barrel lenses).

[John Layton]

Actually...you could check the relative depths of your film holder flanges to septums without a specific measuring tool - but just use a straightedge as described above with a vertical component (like a popsicle stick or sharp pencil) as the "probe," clamped to the edge of this straightedge so that the probe barely touches the septum surface (with no film loaded). If you do this with all of your holders you'll likely find a majority of them that are either very close or exact to each other, with a good chance that these will perform better overall than the "not so consistent" ones.

While this procedure would ignore the above mentioned possible deleterious effects of film swelling/buckling, etc. - at least you will have evaluated your holders for consistency of depth. But maybe don't throw out the "not so good" holders until you've had a chance to test out the "good" ones for focus.

But if you are building a camera from scratch...then by all means, design the focussing back to the spec. of the good holders - but maybe first try the "pencil and straightedge" test again with film inserted to see if there is a close to "standard deviation" which you can use instead as a basis for your final focussing back specs.

[Bernice Loui]

Odds of precise/accurate/consistent registration from film holder seating area to GG should be very good if complete Sinar modules are used for this DIY camera. This means using the Sinar 5x7 rear frame with GG frame and film holder module. Using the standard Sinar front frame would allow ease of using the Sinar shutter between the shutter to bellows to rear of the DIY camera. Know the Sinar shutter is designed to be used behind the lens to bellows, it is NOT designed or built to support the weight of a BIG lens.


Bernice

Yes, the ground glass frame is for a camera. I previously built a simple mdf box camera (for a 90mm Nikon) with a removeable ground glass frame, and I want to build on the concept for another camera (a birch ply box with bellows and a Sinar shutter - for large and heavy barrel lenses).