The nautilus eye is a cup lined with photoreceptors and open to the sea through a small aperture, typically a fraction of a millimetre across. Light enters through that hole and projects an image directly onto the retina. There is no cornea and no lens, and the interior is filled with seawater rather than a refracting medium.
The trade-off is unavoidable. A narrow aperture sharpens the image but admits little light; a wider one brightens it but blurs it. The nautilus lives in dim deep water and sits at the dim, coarse end of that compromise. Behavioural work indicates it relies heavily on chemoreception, using rhinophores to track the scent of carrion, with vision playing a supporting role.
The eye is often called primitive or an evolutionary relic. That framing is misleading: nautilus lineages have been evolving for as long as any other, and the pinhole eye is a working design for a slow scavenger, not an unfinished version of a camera eye. Nautiluses are also threatened by shell collection, and the genus is listed on CITES Appendix II.