I had a 50 summicron that had separation around the edges of the front element. I bought some of the flat black paint used to paint the interiors of film chambers and painted over the bad areas on the lens that were separated; these were only two spots that intruded about 1/8" to 3/16" circumferentially. My f2 lens was now (rough estimate) a f2.3 lens. I never could see any difference in the quality of my prints/negs and used the lens for almost an additional 10 years. I can't recall what finally happened to it.
Originally Posted by TheFlyingCamera
From 2.8 to 16 I am sure it was completely out of the image frame. However, if the separation is in the middle (unlikely) you won't be able to salvage the lens; it will have to be heated, separated, cleaned and re-cemented.
Certainly was ugly looking at the front element but did not appear to be of any major consequence.