This is a (used) soldering sponge, images are 5mm across or so.
Shot with a Ihagee Exakta Varex IIb, using a Pentacon 200mm lens, and a roughly 600mm long extension made out of cardboard.
You would think you’d need to line up the lens and camera super accurately, but you really don’t. Have lens and camera roughly pointing into the same direction, and put a light-tight tunnel between them, and tada – super close focus. Exposure compensation is also really easy to calculate – 600mm extension, divided by 200mm lens is 3 (f-stops). So I basically set the lens to f/22 (as an example) but exposed for f/64 – and that’s not a super accurate way to calculate this but it worked. Shot on Ilford Delta 100, scanned negatives.