This tool attaches to a modern light meter to use it like the classic, hard-find, and expensive Pentax Digital Spot Meter. The calculator has two models: a Zone System model and a model compatible with Nick Carver's excellent course on Precision Method of Manual metering.

How to use:

• Select your film's ISO/EI with the outer pointer
• Set your light meter to ISO 100 (regardless of your ISO/EI), and read the spot meter EVs in ⅓ stops
• Decide where to “place” the exposure (See Ansel Adam's “The Negative”, or Nick Carver's course)
• Rotate the EV scale to align with your “placement”
• For example, you read EV=15 for a middle gray. Match 15 on EV scale to either Zone V on a Zone version or 0 on the PMM version.
• Select the best aperture and shutter speed combination from bottom two rings.

How to assemble:

• Print the calculator, the iso pointer, and the adapter models
• Using duct tape, tape the bottom and middle ring so that aperture 90 matches shutter speed ¼ sec
• Tape the three rings so EV 15 matches f22 (also V for the Zone System model and 0 for the PMM model) and matches 1/60th second shutter speed
• Put super glue on the underside of the iso pointer
• Position the iso point to ISO 100, check it is centered from top, and let it cure
• Remove the duct tape

The adapter fits the Sekonic L-858D light meter's 30.5mm threads and the calculator's M42 threads. If your meter has different threads, you can buy a filter adapter to the M42 size

I used https://www.printables.com/@sbuerger to model the standard camera filter threads. His PDF is included. I add new custom threads for the Sekonic 30.5mm threads, and I added those values in a separate PDF.