12 set of spacer rings in black. These rings slip over the thread of an adapter/extension tube, and are primarily used for fine tuning backfocus for astrophotography, but they can be used for other purposes as well.

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• Each set comes with 12 individual spacer rings, with 4 different thicknesses. 3 of each are included: 0.4/0.6/0.8/1.0 mm.
• The width of the rings is 3.95 mm, and the inner diameter is 0.5mm larger than the specified size for good clearance on the threads (they will not get stuck and are easy to insert). 

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This item is made out of PETG plastic and can handle up to 70C (158F) temperatures without deformation.