Fitting new showerhead to odd-sized (old?) elbow

I need to install a handheld shower head in my bathtub, but the measurement of the elbow seems to be 11/16 of an inch. Since all new shower heads are 1/2 inch, is there some sort of adapter that will solve my problem?

 

1/2" pipe is 1/2" on the inside (nominally), so the outside is much bigger (around 11/16"). I'm pretty sure you're just measuring wrong and it will screw right in. As far as I know, NPT has been the standard for as long as America has had plumbing. There is nothing else it could be.