You're seeing the aperture blades for the lens diaphragm. The blades are normally fully retracted which allows you to frame with lots of light in the viewfinder, and also allows the camera to meter and focus the shot. When you actaully take the exposure, the blades slide in by some amount (based on the f-stop selected) and the shutter opens. The shutter is in the camera body just in front of the sensor and internal filters (there's an internal UV and low-pass filter -- which also doubles as the piezoelectric vibration filter that shakes to remove the dust.)
Since you say this ONLY happens on your 50mm lens, check that the gold contacts that mate with the body are all clean. There are pins on the body (spring-loaded) which have to touch the contact points and it's possible that a pin might become jammed and isn't springing back out to engage contact... but if that were happening in your case then I suspect you'd see this problem with all your lenses -- not just the one.
I'm also a bit suspicious because you say the diaphram blades do retract if you completely remove the lens and then re-attach it. Old "FD" lenses had a mechanical lever that engaged the body to control the aperture blades... and misbehaving aperture blades often meant the lens wasn't completely twisted until it locked solidly. But EOS lenses don't have any mechanical interface... it's all electronic. So the fact that the blades reset if you remove the lens makes me think this is probably a dirty contact problem.
If you do choose to replace it, definitely go for the EF 50mm f/1.4 version. The build quality is MUCH better, it has the USM focusing motor (faster and quieter), and it has an 8-blade aperture which dramatically impves the look of the out-of-focus areas of your image (smoother "bokeh" quality rather than the nervous/jittery quality of the 50mm f/1.8).
Tim Campbell
5D III, 5D IV, 60Da
