Maller et al.. It's important in many other ways, but here is an interesting excerpt on why TF relation is different:
However, with the notable exception of the Tully-Fisher relation these distributions and relations are traditionally measured in terms of the observed properties of galaxies. That is, the measurements used are K-corrected and corrected for foreground dust extinction, but no correction is attempted to compensate for the viewing angle from which the galaxies are observed. In contrast, the Tully-Fisher relation is not a relationship between a galaxy's observed luminosity and rotation velocity, but a relation between a galaxy luminosity and rotation velocity corrected for inclination. The inclination correction attempts to recover the intrinsic properties of a galaxy and not properties that are measured because of the particular angle from which the galaxy is viewed. For a long time there was great controversy over whether spiral galaxies are optically thin or optically thick (e.g., Holmberg 1958; Disney et al. 1989), but in the early 1990s it was established that galaxies become redder and fainter as they are inclined (e.g., Burstein et al. 1991).
However, with the notable exception of the Tully-Fisher relation these distributions and relations are traditionally measured in terms of the observed properties of galaxies. That is, the measurements used are K-corrected and corrected for foreground dust extinction, but no correction is attempted to compensate for the viewing angle from which the galaxies are observed. In contrast, the Tully-Fisher relation is not a relationship between a galaxy's observed luminosity and rotation velocity, but a relation between a galaxy luminosity and rotation velocity corrected for inclination. The inclination correction attempts to recover the intrinsic properties of a galaxy and not properties that are measured because of the particular angle from which the galaxy is viewed. For a long time there was great controversy over whether spiral galaxies are optically thin or optically thick (e.g., Holmberg 1958; Disney et al. 1989), but in the early 1990s it was established that galaxies become redder and fainter as they are inclined (e.g., Burstein et al. 1991).
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