How to add edge which follows geometry I have created a rough head model and added a subdivision surface modifier then set the mesh to follow geometry with the 'adjust edit cage to modifier'. See below:

I want to add a couple of edges back but when I click two vertices and press F, I get the following result:

I don't know why this 'pinching' is occurring but is there a way I can add the missing edges so that they are smooth and follow the geometry?
F creates an edge between the 2 vertices but it won't cut the face, it will just create an edge over the face, what you need to use is J, which will cut the face. You can also use the knife tool: K to activate, C to cut perpendicularly to the view, Z to cut through.