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Overview
Macro rings or tubes are essentially mount-specific spacers that place the lens farther from the film plane and so achieve closer focus than you get with the lens in the normal position. For some mounts, movable bellows are available that give the same effect but infinitely variable and even closer. Kind of the opposite of a tele-extender, though with one similar effect — an extreme shortening of the depth of field.
The macro tubes for Kiev 60/Pentacon 6 are part of the full boxed kit of a Kiev 60 if you can get one. I had to buy mine separately. You can use them individually or stack them to get even closer, one important thing to note is that even though there is no glass, there is still a decrease in the amount of light falling on the film (according to the inverse square law), so you have to increase exposure using filter factors. This is where that metered prism (finally) comes in handy.