  Aims. We attempt to determine the optical depth of spiral galaxy disks by a
statistical study of new Tully-Fisher data from the ongoing KLUN+ survey, and
to clarify the difference between the true and apparent behavior of optical
depth. Methods. By utilizing so-called normalized distances, a subsample of the
data is identified to be as free from selection effects as possible. For these
galaxies, a set of apparent quantities are calculated for face-on positions
using the Tully-Fisher diameter and magnitude relations. These values are
compared with direct observations to determine the mean value of the parameter
C describing the optical depth. Results. The present study suggests that spiral
galaxy disks are relatively optically thin tauB = 0.1, at least in the
outermost regions, while they appear in general to be optically thick tauB > 1
when the apparent magnitude and average surface brightness are studied
statistically.
