Shimming up septums?

[Bart B]

In checking the fourteen 4x5 Riteway film holders I've got, some of 'em have septum depths from the flats as much as .201". The 40 year old film that came with my SSG had black .004" thick 4x5" paper between each sheet. After sliding one of the black paper sheets in the septum of the holders with too much/little depth for me, the new depths came up to .196" to .197". Sheet film that's .008" thick slides on top of the paper sheets, but with a small bit of resistance atop the black paper that rides on the small raised ribs just below the septums lips that hold the film.

Seems to me like a decent way to salvage film holders whose septum depth is a bit deep (for me, that's greater than .199"; 2 thousandths deeper than the .197" spec. 12 of the holders now have depth to the effective septums of .195" to .198". That's a .003" spread that's well inside the .014" spread around .197" allowed by ANSI specs. 2 holders have one septum depth of .193" and .194"; these won't be used for critical sharpness. I think any depth less than .194" will begin to show image sharpness problems that increase with larger apertures and longer focal lengths.

And with the film held tighter in these shimmed-up septums, it'll stay in place better and not shift around 1/16th inch or more. There's enough clearance in most of the septums to allow gravity to pull the loaded film around as the SSG's orientation's changed.

Comments and suggestions welcomed.

Bart

[Jack Dahlgren]

A few quick passes against some 80 grit sandpaper stuck to a sheet of glass could bring down 2/1000 pretty easily. Last thing I want when loading film is having to fiddle with an extra sheet of paper that can get wrinkled or mis-aligned. It may also make the film bulge in the middle...

While you are at it. Check your ground glass and make sure it matches. It is as likely to be out of spec.

[Bart B]

A few quick passes against some 80 grit sandpaper stuck to a sheet of glass could bring down 2/1000 pretty easily. Last thing I want when loading film is having to fiddle with an extra sheet of paper that can get wrinkled or mis-aligned. It may also make the film bulge in the middle....Are you referring to bringing down the flat part of the septum? Or something else?


While you are at it. Check your ground glass and make sure it matches. It is as likely to be out of spec.The ground glass behind the Ektalite field lens is well positioned to get sharp images when the rangefinder's focused at infinity, 100, 50, 25, and so on down to 4 feet. With the ground glass .189" back from the flats on the Graflock back (where .008" thick film is), the same focus vs rangefinder is good.

[BetterSense]

I think he's suggesting that you sand down the holder where it rides against the camera back. Doing so accomplishes the same thing as shimming the film up higher.

[erie patsellis]

Personally, if you're stressing over a few thousandths (still within the ANSI spec, incidentally) then you probably don't want to think about how much the film will bow in the holder due to temperature and humidity changes.

Are you considering adhesive, a la Sinar? Or a vacuum back?

What kind of work are you doing that requires such obsessiveness to holding to nominal specs? Have you calculated how much of a focus/DOF shift you will have being 4 thousandths of an inch in front of the nominal film plane?

Are you checking using a depth gauge at several points over the septum or just at the ends?

Heck, if you really don't want any holders that are more than .002" out, I'd gladly take all you care to get rid of.

[Bart B]

Personally, if you're stressing over a few thousandths (still within the ANSI spec, incidentally) then you probably don't want to think about how much the film will bow in the holder due to temperature and humidity changes.Yes, I do want to think about how much the film bows. I'll add some approximations to the depth to septum to get the total spread the film may be from specs.


Are you considering adhesive, a la Sinar? Or a vacuum back?.No.


What kind of work are you doing that requires such obsessiveness to holding to nominal specs?All sorts of work. Includes using a circle of confusion (COC) of .001" so large prints will have good resolution in the details. That's a bit smaller than the often used COC standard of 1/30th of a millimeter; 0.0013". Another standard's the image diagonol divided by 5000, which for 4x5 film's 0.0012".



Have you calculated how much of a focus/DOF shift you will have being 4 thousandths of an inch in front of the nominal film plane?.Yes. With the film .0045" in front or back of the nominal film plane at an aperture of f/4.5, the depth of focus shifts .001". And the finite point used to focus at is now the size of the circle of confusion (.0045" divided by f/4.5 equals .001")


Are you checking using a depth gauge at several points over the septum or just at the ends?.Yes. I used an 8 x1 x 1/2 inch thick aluminum bar with a dial indicator through the middle of its wide part. The bar was placed on the flats with the dial indicator plunger vertical on the septum. This let me measure septum depth at most any place on the septum; I used a 3 x 4 matrix spaced an inch apart evenly around the septum. Then averaged the 12 readings, then calculated average.


Heck, if you really don't want any holders that are more than .002" out, I'd gladly take all you care to get rid of.For a price, I just might do that for the ones that are more than .003" away from spec. But will probably keep them all and mark the worst side as DNU (Do Not Use) and use the dark slide as a spare.

[erie patsellis]

I'm curious, what lenses do you own that are capable of resolving that level of detail at f4.5? Most I know of and use don't come into their own until f11 to f16. With the exception of some of the newer digital lenses, whose image circles are useless on 4x5, that is.

[Bart B]

I'm curious, what lenses do you own that are capable of resolving that level of detail at f4.5? I don't know. But after reading this:

http://www.largeformatphotography.info/holders.html

then doing some optical/lens spreadsheets and focus tests with the SSG's Graflex 1000 Rodenstock Optar f/4.5 136.4mm lens wide open at infinity targets for several .001" increment shifts of the lens, I'll make shure the emulsion's within .002" or .003 of where it should be. Granted, film can bend from atmospheric condition, but I'd just as soon have it at a mean starting point across all my film holders. If the holder's septums put the septum .003" closer than spec and the film's bulged the same amount in the same direction, it's now .006" away from spec. That's visible to me using a 15X loupe on the ground glass.

[erie patsellis]

Once again, not trying to be contentious whatsoever, but for *most* LF users, who shoot at f11 to f16 (or smaller) and don't enlarge to 15x, which would equate to 60"x75" prints, it more of a Quixotic exercise at best.

[Bart B]

The ingenious Hidalgo Don Quixote's a close friend of mine. He's told me several times he has never been found guilty of tilting at windmills. But he did mention that if I use only the smaller 2" square center of my 4x5 film holders, I'll need a narrower range of film positioning tolerance else a 10X enlargement of it will not be close to the sharpness a scalpel or his sword has.

The above aside, I know what you're talking about. I just want tolerances a bit closer than most folks and not have to use Linhof stuff.