  Both the detection of galaxies and the derivation of the luminosity function
depend upon isophotal magnitudes, implicitly in the first case, and explicitly
in the latter. However, unlike perfect point sources, the fraction of a
galaxy's light contained within the limiting isophote is a function of
redshift, due to the combined effects of the point spread function and
cosmological dimming. This redshift variation in the measured isophotal
luminosity can strongly affect the derived luminosity function. Using
simulations which include the effects of seeing upon both disk and elliptical
galaxies, we explore the size of the systematic biases which can result from
ignoring the redshift variation in the fraction of detected light. We show that
the biases lead to underestimates in the normalization of the luminosity
function, as well as changes in shape. The size of the bias depends upon
redshift, and thus can mimic galaxy evolution. Surprisingly, these biases can
be extremely large without affecting <V/V_max>. However, these biases can be
detected in the full distribution of V/V_max, and in fact may have already been
detected in recent surveys. Because the systematic biases result from the
redshift variation in the fraction of lost light, the biases are not
significant when the fraction of lost light is always small over the entire
survey volume, for all galaxy types; however, modern surveys usually violate
these conditions. We show that the expected biases are not necessarily
eliminated when using aperture magnitudes, FOCAS "total" magnitudes, or Kron
magnitudes, but may be reduced if Petrosian magnitudes are used. These
considerations may also apply to samples of clusters selected in X-rays.
