I had assumed you were describing what you see through the eyepieces. But now I'm thinking you were describing what you see coming straight out of the illuminator. Through the eyepieces, a field diaphragm will affect flare but not brightness. On the other hand, it can definitely change the total amount of light coming out of the illuminator.It adjusts from almost black to bright enough to make you see spots all day.
None better. In fact there is an old story about a college instructor teaching his students about these controls. He carefully explained the standard advice about closing the aperture to 70%. Then he asked each student to adjust his or her own scope and report what the best setting was. Answers ranged from 10 to 100. "Exactly," said the instructor. "The best setting depends on what you want to see. The standard advice is just a guideline."I generally have been opening it till things look good. What better way to do it?