Prost Leica L39 M39 Mount Lens to Sony NEX = 10.8 mm
Working part of the camera SONY 18 mm
Rod ring 16.4 mm from the set of macro rings M39
10.8 mm+18 mm+16.4 mm=45.2 mm
Helios with Leica L39 M39 Mount Lens to Sony NEX and screwed in a 16.4 mm ring will focus on infinity on the mark on the lens.
Rings extendible М39 https://fotocccp.ru/kolca-udlinitelnye-m39
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