In response to Carl Neilson's well documented steps for replacing the C330f's viewing screen assembly's foam, I re-foamed mine. Something clearly needed to be done since sharp focusing below f5.6 wasn't possible, however it didn't solve the discrepancy between the viewing focus and the image on a focus screen I constructed for the film plane. Before I detail my problems, I'll add some advice for others changing their viewing screen assembly foam.

I was able to predict before changing the foam that it wouldn't have made a difference by dabbing some slow drying ink onto the 3 posts in the C330f's body then putting the screen assembly back onto the camera, removing it and seeing 3 ink dots transferred to the underside of the screen. This was despite my foam being in poor condition.

For others reading this thread, the two problems causing my discrepancy were as follows:

  1. The top of the lens plate/board was able to rock back and forth by about 1mm because the stiff retaining wire didn't press on it when latched as it should. I solved that problem by inserting a small piece of sheet metal (LxWxH about 2mm x 3mm x 1mm) to take up the slack.
  2. I had previously swapped the front of the viewing and taking lens with each other. This was because I had a small amount of fungus on the taking lens. After considering this might have been the problem I noticed that both lens bodies had different thickness shims between them and the lens plate, also the filter thread of the viewing lens was about 0.5mm further forward than that on the taking lens. After I swapped them back again the focus was identical (to within limits of repeatability).

The amount the focus was out was a distance of 0.6mm linear travel of the focusing rails and about 2.2mm circumferential rotation of the focus knob.