Technical query really.
I'm about to archive a whole series of 35mm transparencies and 6x6 negatives. I am planning of making use of my Phase 645DF, P40+ & 80mm lens coupled to a copy stand.
My question is which Extension Tube would be most applicable for this type of work. I cannot find anywhere that details focus distances, magnifications etc.
Also would the 80mm with extension be appropriate for this job?
Many thanks for your help!
Magnification is (back focus distance)/(focus distance, and the reciprocal of the focal length is equal to the sum of the reciprocals of the focus distance and the back focus distance (This is the thin lens formula)
...so the back focus distance is the focal length times (one plus the magnification).
...so for 1:1 you would need approximately 240mm minus the length of the mirror box from the focal plane to the nodal point of the lens. ...but only if you are using a "simple" lens without internal focusing.
I have recently done this algebra as I will be using a 120 macro... and for 2:1 I have ordered a long Sinar P3 (12") bellows to use in addition to the standard 6" bellows.
For greater magnifications I can use P3 to P2 conversion bellows, and two 24" (600mm) P/P2 bellows, with my set of Zeiss Luminar macro lenses, which are optimised for magnifications up to 40:1.
Filling the 645 frame with a 35mm transparency would be over-kill with a hi-res camera, but I have the option of using a 22Mpx multi-shot back that might be adequate... or does the PS de-graining function work better with higher-res files? If you use a "macro" lens that is "optimised" for 1:1 to infinity, it would not be likely to work well at over 1:1 - check the spec of the lenses.
When I used a scanner on transparencies, at 4000 pixels/inch I could see the grain, and I thought that 2000/inch might blur out the grain and produce a better result?
You will also have to think about masking, lighting, profiling...
One option is to use a projector or enlarger (with the lens removed) for lighting and masking.