Sometimes you cannot simply pull a tree branch out of the way to illuminate a gravestone that is difficult to read. Many times a mirror can throw some light on a gravestone, creating shadows in the carved letters.
Do I carry a mirror in my cemetery kit? Nope. But I do carry a roll of aluminum foil.
See how nicely a bit of foil can be used to reflect light onto a gravestone?
This gravestone was not impossible to read without the light enhancement, but I wanted to see every mark on it: It is the gravestone of my 4g grandfather.