The Film plane isn't where it should be!
I will prevail. Man this is harder than I first thought.
Originally Posted by clayne
I put a roll of film through and shot a focus test sheet at 45°, and much to my amazement it was still out consistently by an amount between 1/2 to 1 depth of field distance for all distances tried.
I think I know what the problems are.
1. The first and biggest problem is that the emulsion plane is at least 0.2mm behind the plane of the inside film rails where I have been placing the Perspex with Scotch tape on it to focus. There is too much of a gap in the film transit area ! Using a CoC of 0.0474 my Depth of Focus at f2.8 varies between 0.266mm and 0.352mm when the object is either at infinity or 30cm from the lens plane. So close ups are therefore better. with such a small DoFocus, all it takes is for the film to move back from the rails to the plate by 0.133mm and the focus point on the object will be out by 1/2 a Depth of Field ! That is huge with such critical focussing requirements. Why didn't Mamiya or othe camera mfg fornthat matter put this disclaimer in their user manual?!?
2. The technique I had been using to compare the viewing focus to the film plane focus was to temporarily attach a sharp pin on the body atop the focusing knob and draw a fine mark on the focusing knob below the pin. I would refocus and see how far the mark had moved relative to the pin. I discovered this approach is flawed, and I should have realised this earlier. There is a small amount of mechanical hysteresis and other non repeatabilities which mean that even when focussing using the same view option (either viewer or film plane) that the mark didn't always return to the same spot!!! I have now changed methods and must use my ultra short term memory to try and see if the alternate view is in focus or not by turning the knob ever so slightly one way and then the other to see if it was in or out of focus. Harder but much more repeatable ! (focussing would be made easier if I had a split screen focus, but I only have a clear round circle in the middle of the viewing screen.)
Any ideas on how I should solve the problem of the really wide film transit area (it is at least 0.3mm I reckon)?
One idea is to place some contact sheet (adhesive on one side) on the pressure plate to force the film to press against the inside film rails. Another idea is to use some blank + developed film as a focus plane with a clear glass or Perspex block replacing the pressure plate, then I would need to readjust the height of my viewing screen up by about 0.2mm again.