A log of photoelectric absorption properties. The log measures the photoelectric absorption factor, P_{e}, which is defined as (Z/10) ^{3.6} where Z is the average atomic number of the formation. P_{e} is unitless, but since it is proportional to the photoelectric cross section per electron, it is sometimes quoted in barns/electron. Since fluids have very low atomic numbers, they have very little influence, so that P_{e} is a measure of the rockmatrix properties. Sandstones have low P_{e}, while dolomites and limestones have high P_{e}. Clays, heavy minerals and iron-bearing minerals have high P_{e}. Thus, the log is very useful for determining mineralogy. In interpretation, PEF is normally converted to the simpler volumetric cross section, U in barns/cm^{3}, by taking the product of PEF and density.
The log is recorded as part of the density measurement. The depth of investigation is of the order of one inch, which is normally in the flushed zone. PEF can be affected by heavy minerals such as barite in the mudcake or mud filtrate. PEF logs were introduced in the late 1970s.