Out-of-position ground glass plane

As posted in a thread in which someone was trying to decide whether to purchase an MPP, I measured the ground glass plane offset and discovered it to be ~5.10mm. The back of the film on my Fidelity holders is @4.80mm. The film is 0.15mm thick (150 microns).

From this I deduce that the MPP focus plane is 0.45mm behind - further from the lens - than it needs to be. This is not good news, and explains some soft pictures over the years... I calculated the actual object distance with that extra offset for a range of distances correctly focussed on the ground glass:

1_____2______3______4_____5_____
20000 135.917 135.467 39124 19124
18000 136.020 135.570 32100 14100
16000 136.149 135.699 26217 10217
14000 136.314 135.864 21217 7217
12000 136.536 136.086 16916 4916
10000 136.847 136.397 13176 3176
8000 137.317 136.867 9895 1895
6000 138.107 137.657 6993 993
4000 139.715 139.265 4407 407
2000 144.772 144.322 2090 90
1000 156.069 155.619 1019 19

(haven't worked out how to properly format a table)

The first column is the object distance, in mm; the second is the image position with a notional 135mm lens on the MPP ground glass; the third the position of the film plane; the fourth the actual object distance based on the film plane; and the last the difference between the intended focus and the actual focus point.

It's not too bad at close distances - the offset is a smaller proportion of the image-lens distance - but it's over half a meter out at four meters, and if focussing at 20 meters, the actual focus is at almost twice that.

I can see two obvious ways to fix this: either an 0.45mm shim between the fidelity holder and the reference plane, or make a new ground glass carrier with the correct offset; as far as I can see it should be 4.65mm from the reference plane.

Because the shim would have to move out when the ground glass is in use, the simplest approach there would be to add one to each side of the Fidelity, but I think it might be easier to have a word with the guy with the CNC mill at work...

Neil

Out-of-position ground glass plane - MPP