1Wear builds up
Every shot, the shutter’s curtains are flung across the sensor in about 3 ms and slammed into a stop, and a spring or small motor re-cocks them. Over hundreds of thousands of shots, the latches, cams and springs wear. This camera is past its rated 300 000 shots.
2The second curtain slows
The tired spring and worn latch release the second curtain late and push it more weakly, so it slows down as it crosses. The exposure slit is then wider at the end of its travel than at the start, so one side of each picture gets more light than the other. It shows most at fast shutter speeds, where the slit is narrow.
3A blade bends and rubs
The blades are only about 0.1 mm thick. One gets bent, and it drags against its neighbour and the frame. The friction slows the curtain further, and the bright band in the pictures gets worse. The rubbing also sheds tiny particles onto the sensor.
4The curtain stalls
The bent blade jams partway across, so the curtain stops before the end of its travel. Part of every frame stays covered and comes out black. The optical sensor that watches for the curtain never sees it arrive.
5How it is caught and prevented
- Curtain sensors: small LED-and-photodiode pairs check that each curtain arrives on time. If one doesn’t, the camera shows a shutter error and stops firing, so it doesn’t wreck the sensor.
- Rated life and shot counters: makers test shutters to a rated count (here 300 000), and the shot count is stored in each photo’s data, so you can see wear coming.
- Electronic first curtain removes one curtain’s movement from each shot, cutting wear and vibration.
- A fully electronic shutter has no moving parts at all; new global-shutter sensors also avoid the bending it causes with fast motion.
- The fix is a replacement shutter unit, a routine repair-centre job.