How is use of f128 possible/practical?

[Eric Woodbury]

Just guessing, but it is probably just a matter of that lens in that shutter and that's what you get. Not that anyone wanted f/128, but the shutter could go that far, why not mark it?

[Robert McClure]

So, could it be said, therefore, that unusually tiny apertures (like f128) had true practical value only when it came to graphic arts applications?

Further, could it be said that most large format lens manufacturers (arbitrarily) decide to not go past, say, f90 because they feel the "image degradation" will generally be unacceptable to most users (even when contact printing the negatives)?

One local used photo equipment dealer had once told me that the idea of re-working a lens to allow stopping to f128, for example, even though it was said that Weston did it, was utterly foolish.

"The manufacturer has already determined in testing that diffraction would be far to severe at those tiny apertures!" he chided. If recall feeling like a real putz for even considering the idea.

I later observed that my 355 G-Claron, though having markings on its Copal 3 aperture ring only to f64, can easily allow stopping down to probably f360. I guess I will have to be the one to decide how much diffraction I can live with!!!

Thanks everyone for your great and informative input. Very, very interesting and stimulating!

Robert

[Struan Gray]

The cross-ruled screen acts as an array of pinholes, so each halftone dot is a pinhole image of the aperture. For optimum results you need to space the screen the right (small) distance from the graphics arts film.

Everything I know about this I picked up from posts by Jean-David Beyer and Richard Knoppow in the usenet group rec.photo.equipment.large-format. Credit where credit is due, and more detail, here: http://tinyurl.com/qdbq3.

[GPS]

So, could it be said, therefore, that unusually tiny apertures (like f128) had true practical value only when it came to graphic arts applications?
Robert

That's all what there is to it, I'm afraid - except if you have the same purpose with another lens.

[Jim Galli]

Robert, I have an ancient Wollensak f10 process lens of 19" (protar 8 elements 2 groups type) that easily covers the 12X20. I have stopped it down to f90 1/2 often but that seems to be a practical limit for me. Beyond that and I consider even the contact prints too soft from diffraction.

[Capocheny]

Maybe the lens designers thought as I do that diffraction isn't a practical problem with 4x5 film until the enlargement factor gets up to at least 20x24, maybe larger.

Brian,

So, I've been worrying about this for nothing all this time!

Since 16x20 is the largest I normally enlarge to... this is even less of an issue!

Thanks for this great bit of information!

[BTW... you missed a fun workshop! Next years is slated for Sequim/Port Angeles... in your neck of the woods I believe. ]

Cheers

[Robert McClure]

Pretty cool to learn from you guys. I will content myself with the "arbitrary" aesthetic of "tack sharp" images. That said, some of Jim Galli's images were pretty awsome.

Robert

[Emmanuel BIGLER]

Thank you Struan for the reference to the historical crossed-grid technique to make printing plates from a halftone image. Fascinating ! And certainly very difficult to properly adjust.

[Roc Chan]

The subject of small F stops on Graphic Art Lens came up several times over the years.To controll contrast of graphic art photo sensitive materials,a flash,non subject(white light) exposure is use ot "fog the film".This exposure is use to boost one zone to the next without affecting the adjecent zones.This Pre,Post flash exposure are small fraction of the subject exposure,thus small F stops are use.In later Agfa Repromaster ,the flash exposure occures in the bellow between the lens and the film.

[Michael Graves]

Aside from all of the other practical considerations mentioned by various people; isn't it possible that f128 came about for the simple reason that the manufacturer used the same diaphragm assembly for a 12" lens as they did for an f64 24" lens?