This blend contains two nodes for working with HDRs.
One of them calculates the EV from camera settings, and the other applies a change in EV to an image
How to use
Open your blend file and use file>append, navigate to this blend, and then go into *Node tree* and select all the node groups. Add them into your file using Shift–A>Groups>[Name]
Enter the F-stop and shutter speed in seconds, the and the EV will output
Enter an EV and an image and the node will increase or decrease the EV by that amount.