That was done strictly for the benefit of both Spielberg and Ford, the daily cost in a set can be upwards of a million dollars. Right after Douglas said "f5.6", his assistant would be right behind him checking to make sure. If you were Douglas wouldn't you love Spielberg and Ford telling fantastic stories about you. Not to mention the moment a producer would get a whiff of a DP doing something like this(and believe me, they will be told) would inmediately make it to the set and quietly pull him aside.