origin of "say cheese"

does anyone out there know where the term “say cheese” for a photograph came from?
checking linguistics and phraseology sites yeilded the question asked several times,but no answer.
yeah, I know why they say it but who said it first?

It’s cited by the OED as early as 1930. I think it was the invention of photographers, as was the earlier “Watch the Dickie Bird(1901).”