5x4 Film and glass plate holders: same offset?

[barnacle]

As the title; I'm using modern Fidelity film holders in a camera designed for wooden glass plate holders (Micro Precision Press) - is anyone aware whether the offset from the bedding plane to the image plane is the standard 4.80mm in both cases? I seem to have a slight softness - though it could equally well be the seventy year old uncoated lens...

Thanks,

Neil

[IanG]

If it's an early MicroPress then it has a non standard register 0.2"/5.08mm. MPP made film holders for this camera as well as plate holders. I have a box of MPP holders some quite early so I might be able to help you out.

Ian

[Jim Jones]

See if http://mppusers.com/registers.htm helps.

[C. D. Keth]

Your question relies very heavily on precisely the camera you are using. Any kind of worldwide standard for these things is relatively new.

[IanG]

Your question relies very heavily on precisely the camera you are using. Any kind of worldwide standard for these things is relatively new.

The camera is post WWII after the modern standard was adopted but not initially by MPP.

Ian

[barnacle]

Hmm. That would explain things. Thanks, folks.

I do have a few of the wooden holders, but only for plates. They have some home-made metal film holders in them but I have no idea what the offset might be. Time to see if I can locate a dial gauge...

Neil

[barnacle]

Hmm. Spent half an hour comparing the new plastic carriers and the old wooden carriers, but with some puzzlement; there is a pair of springs in the back of the wooden carrier that would push the plate forwards, but no apparent lip against which it might push as a reference. Odd.

So I took the ground glass carrier off the MPP and measured the clearance to that. Turns out I have dozens of measuring thingies, but no suitable depth gauge, but I managed to locate at least a comparison tool. I can't yet get an accurate measurement, but it seems that the glass is perhaps a quarter millimetre further back than the Fidelity carriers (ten thou or so). There are four shallow pillars on the alloy frame which provide the depth reference (quite worn, after seventy years!) which I suppose I could machine down, after careful measurement.

On the other hand, how 'out of focus' do things get at one five hundredth of the focal length, or so?

[Jim Jones]

The use of a thin metal film septum in plate holders is perhaps a poor substitute for a proper film holder. It can introduce slight focusing errors. If your MPP is a later model that accepts standard film holders, using such holders seems logical . Even your camera initially had a register measurement of 5.08mm, it may have been altered to conform to the present standard.

Film holders aren't really precise, either. The current specifications for the register (often called T distance) is 5mm +/- 0.178mm. The slot in which the film rides is 0.3mm while film is typically about 0.15mm thick, and occasionally up to 0.25mm thick. Thus, even if the film is perfectly flat, its position can vary by perhaps 0.193mm and still be within standard tolerances. A shift of this amount from perfect focus can certainly affect critical sharpness in the image with large lens apertures. Errors in film holders and in ground glass position further compound the problem. I try to stop down to f/22 or even f/32, and don't worry about these possible errors.

[barnacle]

Yes, and I'm an f22 sort of person as a rule.

http://www.dnbprojects.co.uk/Wells/inri.jpeg

Here's a pixel-pixel image from the centre of the Wells Cathedral shot I posted a few days ago. The image was scanned at 1200 dpi and the vertical strokes in the INRI are three pixels wide and showing three different gray-levels, so that's probably 400-600lpi minimum resolution. That's only 24 l/mm though, about half what the lens should be capable of (particularly with a blue-sensitive film).
I'll scan at a higher resolution and see what comes out.

Neil

[barnacle]

And after a bit more scanning, this time at 6400 dpi:

http://www.dnbprojects.co.uk/Wells/inri2.png

No post-processing, no compression, and plenty of gradation through the pixels that make up the letters. But no significant improvement in sharpness.

Neil

[Jim Jones]

The 6400 dpi scan looks better to me. This may be due different exposure and to JPEG compression in the smaller scan. Many lenses will resolve over 50lp/mm on film in rigorous tests. Achieving this in everyday photography is more difficult, in part due to the variations in film holders as discussed above. The detail of the Wells Cathedral is sharper than too much of my 4x5 photography.

[barnacle]

Cheers Jim. I guess I should stop worrying about it then :)

Neil