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Redshift is not a shift, it is defined as a fractional change in wavelength.Nevertheless, it is a fairly common misconception that Delta-z c represents avelocity where Delta-z is the redshift separation between two galaxies. Whenevaluating large changes in a quantity, it is often more useful to considerlogarithmic differences. Defining zeta = ln lambda_obs - ln lambda_em resultsin a more accurate approximation for line-of-sight velocity and, moreimportantly, this means that the cosmological and peculiar velocity termsbecome additive: Delta-zeta c can represent a velocity at any cosmologicaldistance. Logarithmic shift zeta, or equivalently ln(1+z), should arguably beused for photometric redshift evaluation. For a comparative non-acceleratinguniverse, used in cosmology, comoving distance is proportional to zeta. Thismeans that galaxy population distributions in zeta, rather than z, are close tobeing evenly distributed in comoving distance, and they have a more aestheticspacing when considering galaxy evolution. Some pedagogic notes on thesequantities are presented.

Reinventing the slide rule for redshifts: the case for logarithmic wavelength shift