What is the function of the iris diaphragm of the microscope

What is the function of the iris diaphragm of the microscope? If the diameter of the field of view is 5.6 mm at 40x, what is the diameter at 80x?

Iris Diaphragm

If the diameter of the field of view is 5.6 mm at 40x .what is the diametr at 80 x

80 x /40x =2

=5.6 divided by 2 =2.8 mm –answer