  Focal ratio degradation measurements of optical fibers for the PMAS integral
field instrument have been performed using the Photometric Testbench of the
Potsdam Astrophysical Institute (AIP). An effort has been made to obtain
sufficiently large statistical samples of individual fiber measurements. The
optimization of the measurement process has made it possible to study
quantitatively the effects of different coupling methods (air--glass vs.\
immersion) and of imperfections like defocus, lateral displacement, and angular
misalignment. Furthermore, the effects of mechanical stress on focal ratio
degradation have been investigated, confirming the presumed cause of modal
noise which is known to limit the maximum signal-to-noise one can achieve with
fiber-coupled echelle spectrographs.
