Bizarrely i was called an hour later by my printer telling me that the focus was falling away rapidly, now this was impossible as i had shot it on f45 !! It looked more like f8/11. We eventually decided that the front lens panel was slightly out so re set it. Confidently sending off the next negatives again i got on with the job only to be told again that i was getting drop out, though one of the negs which i shot at f64 was an improvement !! I know this must be something to do with my lens but how can a lens shooting at f64 come out like f11.
I wonder if you are seeing the results of aperture diffraction limit effect.