[d_tr]

> Is the back of an eye a sufficient observer? :)

It definitely is. The retina "measures" photon positions, which is why you see images. Observation is just interaction, no need to consider whether A can observe B. If they interact, some kind of observation takes place.

[HPsquared]

When the photon passes through the lens and the watery liquid part, its path is bent by refraction.

Does that process also count as an "interaction", or is that different to what happens when the photon hits the retina?

[d_tr]

It does count as interaction. The systems are different.