You lose almost no light. You are changing the area of the sensor the part of the scene is spread over. Its a triviality as a result of us expressing the F nu

You lose almost no light. You are changing the area of the sensor the part of the scene is spread over. Its a triviality as a result of us expressing the F number as the ratio of the focal length and the aperture size. The effective SNR of the sensor is increased due to more sensor being used for a given part of the scene, so the lower light level does not increase noise comparatively.