The analysis of budget link and free space optical system performances requires the calculation of several metrics of the atmospheric turbulence-affected collected light. In this work, assuming the collected light is focused into an optical fiber or over a sensor positioned in the focal plane, we use the ABCD ray-matrix representation to calculate the impact of atmospheric turbulence on the power in the fiber or power over the sensor. Calculation of such metrics requires the knowledge of the transmitted average power that enters the receiver aperture (power in the bucket) and the long-term beam spread in the focal plane, from which the Strehl ratio can be obtained.