Redshift distribution of the galaxies in the CANDELS/3D-HST fields for the colour selection for clusters at 1.2 ≲ z ≲ 1.7, employing the R20 photometric redshift catalogues. The selected source galaxies (solid lines) are mostly at redshift z ≳ 1.7. Removed galaxies (dashed lines) are mostly at redshifts z ≲ 1.7. We additionally display the average lensing efficiency curve as a function of redshift (grey dash-dotted line) at the median lens redshift of the clusters at z_l = 1.4.