lanthanide contraction across the period.... help

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I cant understand lanthanide contraction....as we go along the period if the number of electrons increase in the f-subshell, then the shielding effect should be more pronounced..instead they say that the shielding effect is reduced and hence the nucleus can attract the outer shelll electrons more strongly and ultimately the size decreases...but WHY the addition of the electrons in f-subshell across the period reduce the shielding effect??