Thank you all for being so helpful!
The 'A' and 'M' function on my camera lens would appear to not be working as it should. I managed to get a hold of a second Jena DDR 50mm and the noticeable difference between the two was that, if I was to click between stops on the aperture ring, I could see it opening and closing. So I'm guessing the pin/mechanism in my lens body was faulty?
Just for anyone that's curious, or if anyone comes across this with a similar problem, when shooting in 'A' mode, you can preview the depth of field by pressing the shutter release half way, without actually taking the shot.
Thank you again everyone!