the post process is very different from 4.7 to 4.9. 4.9 is very grainy. I’m on a macbook pro 15in el capitan with an AMD Radeon R9 M370X 2048 MB
We made some major changes to our Post Processing settings and functionality within the editor between 4.7 and 4.9, and we are aware of this visual impact it has on certain cards and machines. We are working to have this issue (UE-21214) resolved and fixed in an upcoming version major release or hotfix.
This has to do with Temporal Anti-Aliasing, and if you turn this off you should see the grainy effect go away. There are known issues we are also still investigating and working on that deal with the AMD R9 cards.