The Think Tank Eyepiece completes the waterproof seal of the Hydrophobia and allows the photographer to look directly through the viewfinder. This feature provides a huge advantage over most raincovers that require you to look through a plastic shield.
 
The Think Tank Eyepieces fit particular camera bodies:

EP-NSI
Hydrophobia eyepiece (EP) for Nikon screw-in eyepiece mounts. Fits the D5, D4, D4S, D3S, D3X, D3, D810, D800E, D800, D700, D500 and others

EP-N
Hydrophobia eyepiece (EP) for most Nikon DSLR cameras such as the D600, D7000, D300s D750, D610, D7100, D5300

EP-10
Hydrophobia eyepiece fits the Canon 1Ds MII,1D MII, 50D, 60D, 60Da, 70D, and Rebel Series

EP-15
Hydrophobia eyepiece fits the Canon 6D, 5D, 5D MII, and Rebel T3/1100D

EP-20
Hydrophobia eyepiece fits the Canon 5D MarkIII, 1D MarkIII, 1D MarkIV, 1DX, 7D, 7D MarkII, Olympus E3 and E30