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Well, it seems that the eyepiece must have too long an eye relief for the length of the eyecup.
On short eye relief eyepieces, the eyecup is long enough for you to "bury" your eye in the eyecup and be at the right distance from the eyepiece.
This is not true, however, for the Ethos. One must hold one's head a bit away from the eyecup in order to be at the right distance and not experience blackouts (which could, indeed, make using the eyepiece uncomfortable).
First, use the eyepiece with the eyecup folded UP. This will automatically force your eye a bit back from the eyepiece.
Second, approach the eyepiece from a distance and stop whenever your peripheral vision catches a glimpse of the field stop of the eyepiece. At that distance, your nose may touch the eyecup, but the rest of your eye socket will not. If you have trouble holding your head at that distance (and I presume you are sitting, since standing is not stable enough to hold the eye away from the eyepiece), you could try the Eyeguard Extender, but be aware that it will sacrifice almost half of the eyepiece's eye relief.
TeleVue must be aware of this phenomenon, since the newer Delos and Delites have eyecups that raise up to a height where people who are so inclined can truly insert their eyes into the eyecup to see the whole field of view.
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