When I looked into the viewfinder of the third Canon FX camera and saw the same rectangular dirty spots on either side of the viewfinder image, I got suspicious. This mustn't be accidental, I thought to myself. So, I took off the top cover of one of them to investigate.
The prism is covered with a semi-transparent plastic hood. An inverted V-shaped metal plate secures it to the socket below. There is a strip of foam between the metal plate and the plastic hood to provide padding. But note, that the plastic hood is slightly smaller than the prism.
This is looking into the prism through its bottom after it's been removed. The green arrows point to the problem areas. These look like fuzzy, dirty spots in the viewfinder. But, as we will find out, this is not dirt...
Here we go. On the area not covered by the plastic, the deteriorating foam attacked the coating on the prism (black paint and a thin layer of silver or aluminium to provide reflection). It must be some vicious chemical, because to some extent it also went through the plastic and left its mark on the covered areas.
Now.. it makes one wonder, what were the engineers smoking when they designed this?
The same problem is present on the Pellix and TL. But, the FP, FT, and later models are OK.
This is an FT. Notice the larger plastic hood, which now covers the entire prism. It's good to see that Canon learned from past mistakes.