I was shooting some portraits outdoors last month, and knew from experience that I need about f/8 for close-ups to have enough depth of field to be satisfying to my eye, while at the same time isolating the subjects from the background. My Summitar just works so well at f/5.6 or 8.
Had I opened up more, all the way to f/2, the background becomes too busy and distracting again, while focusing on the fly will be far more difficult.
I'm sure just getting a portrait of a fly is difficult enough...
I do use a digital device in my photographic pursuits when necessary.
When someone rags on me for using film, I use a middle digit, upraised.