The decrease in size of lanthanide ion due to the increase in atomic number is known as lanthanide contraction.
The decrease in the force of attraction exerted by the nucleus on the valence electrons due to the presence of electrons in the inner shells is called screening effect or shielding effect.
Lanthanide contraction is a result of the poor screening effect of the 4f electrons. So the order of screening effect of subshells is s>p>d>f
f orbitals do undergo shielding in lanthanide element series but it is a poor shielding effect.