Hi Smudger, thanks for clarifying your response, apologies for suggesting your premise was false as you weren't answering my question but the OP's.
Originally Posted by Smudger
I'm certainly not suggesting the magnifying lens remains out of focus at all ! And if it is out of focus, by all means fix it. In my case I have a bigger problem to fix first (the discrepancy btwn WLF focus and film plane focus). My magnifying lens may or may not be optimised but to my eyes it is good enough for now while I solve the main problem.
My measurement results are becoming more and more confusing. Last night I repeated the testing inside to discover that focussing on objects at 3m and 15m did not give the discrepancy (but focussing at objects 60cm away did). Whereas a couple of days earlier when testing outside (on different objects), the discrepancy existed at 3m and infinity (I didn't test 60cm outside). In all cases when the discrepancy exists it is approx. the same magnitude and direction.
Something must be changing between my measurements but I can't work out what. Each time I do a set of measurements I repeat the focus at least 3-4 times with near identical results to ensure it isn't just a simple repeatability issue (e.g. caused by the limits of the system or my ability).
I'm attaching some pics to show
- The tracing paper screen I set up behind a perspex/plexiglass pressure plate (held in place with blu-tack). Paper and plate were all correctly dimensioned so they rest on the appropriate set of silver rails.
- Two pen marks on the focus knob showing an example of the two focal positions equating to a linear displacement of about 0.6mm. The mark at 11:58 o'clock is when the WLF screen is in focus, the one at 12:00 o'clock is when the film plane is in focus.