MPP VII Critical Focus

Discussion in 'Large Format' started by hoody, Nov 30, 2020.

  1. Camera: MPP VII
    Lenses: Plaubel Anticomar 15cm f/2.9 & Schneider Xenar 135mm f/4.7
    Film holders: MPP (older style wooden holders) & Fidelity Elite
    Loupes: Hoodman, Silvestri 6x & Peak 8x
    Film: Ilford Multigrade RC Paper

    So I can't get critical sharp focus wide open, what I perceive on the ground glass as sharp and in-focus is coming out a couple inches behind where I focused and it seems to get slightly worse the further the subject is from the camera. Tried it with multiple loupes and two different lenses and get the same result being consistently out of focus by a few inches.

    I set up a tape measure at a 45 degree angle and took some test shots using both lenses and both types of film holder which consistently showed me where the camera was actually focused to, then re-focused the camera to the in-focus area which resulted in having to retract the lens approx. 2mm towards the camera.

    I assumed it might be to do with the focusing screen as it's about the same thickness so I turned it round and took another test shot which was even further away, so the screen was the correct way round to begin with, smooth side out, rough side facing the lens.

    My best guess is the screen needs to be approx. 2mm further into the camera for the film plane to match the image on the GG, which unfortunately isn't possible on this model.

    Any ideas would be greatly appreciated.

    Chris
     
  2. Thanks John I'll take a look. I've done some more testing and I think I have it backwards, to get the correct film plane and GG register the GG needs to come backwards a couple mil. I'm going to shim it out and see with trial and error since I haven't got my vernier with me.
     
  3. Early MPPs had a non-standard 'register' - the distance between the camera back-plate and the GG/filmholder depth.

    That distance should be between 4.75 and 4.8mm to be compatible with a modern plastic DDS. These have a depth of 5mm, with ~ 0.25mm allowed for the thickness and sloppy fit of a sheet of film.

    Unfortunately, MPP saw fit to ignore this standard on early Micro-Technical cameras. Also with their wooden filmholders (which are 5.5mm deep I believe). However, the Mk7 with Graflock back, and later models, should definitely adhere to the 4.8mm standard. Most Mk6s do as well.

    MPP never changed their wooden holders though, making them pretty useless on anything other than very early MPP technical and press cameras. They're as useful as chocolate fireguards these days!

    Grafmatic multi-sheet backs can also have a non-standard register, and I've seen warped Fidelity holders where the closure flap sticks out and prevents the holder from sitting flat against the camera.

    Toyo holders are the only ones I'd recommend buying. Especially used.

    Then there's always the possibility that the back has been swapped for one from an earlier camera. Unlikely if it's fitted with an 'International' or Graflock back.
    Well, that's going to be thicker than any film and throw another bunch of spanners into the works!

    What's wrong with actual film?
     
  4. I might just have to give it a go with some FP4 before I try shimming the GG, could just be the thickness throwing it off.

    As you can just about see in the pics none apart from the house (that was stopped down) is really in-focus, at least not where I focused and that was focused to the nearest corner, but the chimney is actually in sharper focus.

    Obviously nothing wrong with film but I do like the look of the paper negs, especially with the old lens I have, and I'm a cheapskate
    ;).

    MPP-3.jpg MPP-4.jpg MPP-2.jpg MPP.jpg
     
  5. A back-focus indicates that the image plane rests closer to the lens than the GG screen.

    You can find out exactly how much by multiplying the subject error by the magnification on film.

    So, say your tape measure inches (square on, not at 45 degrees) come out as 1/8th inch on film with a back focus of 1/2"; that's a 1/8th magnification and tells you that there's a (1/2x1/8) = 1/16" difference between film-plane and GG surface - which should always be facing the lens BTW.

    If you post the exact parameters - lens FL, subject distance and back-focus amount (preferably all in millimetres or metres), then the exact error can be worked out.

    But first check the basic stuff.
    Are you loading the darkslide correctly? With the film or paper slid behind the thin guides? And not sitting on top of them.

    Paper cut to exactly 5" x 4" will not fit in a modern double-darkslide. It'll buckle as you push it under the retainer slot. '5x4' film is actually closer to 3+7/8" by 4+7/8" or 100mm x 125mm.

    It's also not uncommon to find card, paper or fibre-board shims underneath a ground-glass. Sometimes these get misguidedly removed or lost if the GG is replaced or removed for any reason.
     

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