There are different neutral density (ND) filters on the market which enable completely new possibilities of photography. I really like "big stoppers" like this one*. They can be fixed right in front of a lens and add a lot of aperture steps. This allows long exposure shots on sunny days to create, for example, a soft water surface or blurred clouds.
During my first tries, I noticed some purple / pink / orange and sometimes green horizontal lines on the images. They appeared every time and the problem could be reproduced. Here is an example:
I first thought a little light would pass between the lens and the filter onto the sensor. But I was pretty sure everything is 100% tight. At home, I tried and googled a lot and found the root cause of the problem: the camera viewfinder!
When I was taking the photo above, sunlight came in from the viewfinder onto the sensor. I made some experiments to validate it by aggressively bringing light into the camera with a simple flashlight pointing to the viewfinder.
And yeeep, this results in a broken image again:
Okay, great, problem identified! But how to solve this? There are some small plastic clips on the market that are called eyepiece caps* (just search for the correct one for your camera model) . Add them onto the camera viewport and everything should be fine.
The same shot, on the same day, with the same settings and a flashlight pointing to the viewfinder with the eyepiece cap on it solved it.