What is lanthanide contraction? What are its consequences ?

In Lanthanides as atomic number increases, size gradually decreases from left to right is known as Lanthanide contraction. This is due to poor shielding effect of 4f- orbital.
Consequences :
i) 4d and 5d series elements have almost similar size, because of lanthanide contraction.
ii) They are not separated by the crystallization method because of lanthanide contraction.
iii) Zirconium (Zr) and Holfnium (Hf) have almost equal size, because of Lanthanide contraction.