I've recently bought a Zorki 4 (not the 4K, the model with the knob film wind). It's all fairly intuitive, once you've worked out that the rewind control is the collar round the shutter release. However, there's a control whose function I can't figure out. There's a ring around the shutter speed dial, which has the numbers 0, 5, 10, 15, 20, and 25 engraved on it, spaced at what seems like geometric intervals. I've tried looking for manuals on the web, and though there's plenty of information on using the camera, there's nothing on this particular control. I'd be grateful for any pointers.