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.