I got an email the other day that got me thinking. A guy simply asked “How far do I have to stop down a lens to get maximum performance. I’ve heard two stops from wide open. I’ve heard down to f/8. Which is correct?”
I asked him which lens he was referring to, and was he talking about the center point, corners, or overall. He didn’t realize that it mattered. He thought all lenses were the same and had this idea that eventually there was an aperture where the lens was maximally sharp and the corners were as sharp as the center. At this point, I realized there was no way I could tell him everything he needed to know in an email and I decided to write a blog post about it.