A diaphragm is used to increase a lens' effective diameter. What effect does this have on the lens' f-number?

Answer 1

The focal length of the lens divided by the effective diameter of the lens give the f-number. Therefore, double the effective diameter by opening the diaphragm a certain amount and you halve the f-number. By the way, halving the f-number increases the amount of light passing through the lens by a factor of 2^2 = 4, because the area of the lens is proportional to the square of its diameter.