If you see this in a Nikon F prism viewfinder - you have no real chance of repair

[forest bagger]

If you see such a cloudy spot through the eyepiece

and the cloudy spot is visible twice from the ground glass side

you have no real chance of removing it.

It is the decomposing foam strip that has attacked the black paint of the prism.

You would have to remove the prism, remove all the paint, clean and repaint the prism, and replace any deteriorated foam strips when reassembling the viewfinder.

 

[Dan Daniel]

Viewfinders are highly overrated. Learn to look at the scene without a viewfinder, and use the viewfinder to confirm edges, focus, maybe timing if that is part of the image. Smudges, desilvering, dust... disappear in actual usage. If you are looking at the desilvering, you aren't using the camera as it is intended... (semi-joking, quite serious, but also completely off-base. desilvered prism, whatever the cause, are painful to see)

 

[therepairman]

If you see such a cloudy spot through the eyepiece

View attachment 397726



and the cloudy spot is visible twice from the ground glass side

View attachment 397727



you have no real chance of removing it.

It is the decomposing foam strip that has attacked the black paint of the prism.


View attachment 397728



You would have to remove the prism, remove all the paint, clean and repaint the prism, and replace any deteriorated foam strips when reassembling the viewfinder.

 

[therepairman]

Forest. If the damage is superficial, meaning that you can touch it with bare hands, it IS repairable.
If it's inside the prism, that's still repairable via disassembling the prism itself. I do NOT recommend doing it, for few reasons: the prism was glued at the time of manufacturing; by de-gluing you run a fair amount of risk to brake it; and last, if all went good to that point, re-gluing is a process where calibration and special tools are needed.

 

[Besk]

Viewfinders are highly overrated. Learn to look at the scene without a viewfinder, and use the viewfinder to confirm edges, focus, maybe timing if that is part of the image. Smudges, desilvering, dust... disappear in actual usage. If you are looking at the desilvering, you aren't using the camera as it is intended... (semi-joking, quite serious, but also completely off-base. desilvered prism, whatever the cause, are painful to see)

Viewfinders (particularly SLR viewfinders) help with composition also.