infield rule

Question
If an infielder (has to) turns his back to home plate to catch a fly ball, is that considered 'ordinary effort', to call the 'Infield fly, batter is out'

Answer
"Ordinary effort" really comes down to a judgement call and there is no set of factors that specifically determine it.  As a rule of thumb, I personally do not make the call until the I see the player who should catch the ball stopped at a point to make the catch.  Whether that happens at the exact spot he started or 10 steps into the outfield really doesn't matter.  That's how I make the call, but it is always left to the umpire's descretion.  I hope this answers your question.