M42 extension tube questions
Just got myself a set of Pentacon M42 extension tubes. There are 3 tubes, measuring about 10mm, 17mm and 31mm. I plan to use it with one of my M42 bodies mounted on a Fujinon 50/1.8 lens. The tubes didn't come with a manual or chart, and this is my 1st time trying out extension tubes, so I have some questions:
a) How do I know how much exposure compensation I need to apply? Does the compensation required differ with which tubes I use, e.g when I only use the 10mm tube and when I use all 3 tubes together? I'm guessing it does..but by how much?
b) The tubes I have doesn't have an M42 'pin', in which case, how does it stop down the lens then? I tried it just now using the Fujinon lens, and beyond f5.6 or so, the lens won't stop down anymore, no matter what fstop I set it to. When used wide open at f1.8, it functions fine. Will I have to use it wide open all the time, then? I noticed that there are other types of M42 tubes that have a regular M42 pin.
Thanks in advance, would appreciate any feedback/help.