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.
10mm, 17mm, 31mm Extension Tube Set with 50mm Lens at Infinity Focus
If the camera has through the lens metering, then the meter will give the correct reading through the tubes and lens combination. If you’re using a hand held meter, then a calculation is needed.
Rather than bore you with the calculations,
with 50mm lens at the “infinity” setting, you get the following results in the form:
Tube or combination of tubes, time multiplication factor, number of additional f-stops required.
10mm, 1.44x, 0.53f
17mm, 1.80x, 0.84f
31mm, 2.62x, 1.39f
(10mm + 17mm), 2.37x, 1.24f
(10mm + 31mm), 3.31x, 1.73f
(17mm + 31mm), 3.84x, 1.94f
(10mm + 17mm + 31mm), 4.67x, 2.22f
If you focus the lens closer, the exposure requirements increase slightly, but not by much for a 50/1.8 normal lens with its relatively short maximum extension.