Lady of the Lawn

"I thought the background would be a lot less sharp shooting the lens wide-open."

That film format, with that lens, at that f-stop, I would have thought so too!

You are probably not far off focusing at infinity.
I have found that trying to throw the background out of focus is a lot harder than you would imagine.
It is easy to get into a situation where the annoying background is not sharp but it is far from the indistinguishable blur that you would hope.
Martin