A plane light wave with wavelength = 0.54m goes through a thin converging lens with focal length

Question:

A plane light wave with wavelength λ = 0.54μm goes through a thin converging lens with focal length f = 50 cm and an aperture stop fixed immediately after the lens, and reaches a screen placed at a distance b = 75 cm from the aperture stop. At what aperture radii has the centre of the diffraction pattern on the screen the maximum illuminance?