 An object of size formula_33 at redshift formula_1 that appears to have angular size formula_30 has the angular diameter distance of formula_36.This is commonly used to observe so called standard rulers, for example in the context of baryon acoustic oscillations.If the intrinsic luminosity formula_37 of a distant object is known, we can calculate its luminosity distance by measuring the flux formula_38 and determine formula_39, which turns out to be equivalent to the expression above for formula_40.