One of the characteristics of durations in closed environments like undersea habitats is the limited visual range of only a few meters inside the habitat and appr. 30 – 40m outside depending on the current water visibility. This may not represent a problem for short-term stays, but might be of certain importance during long-term durations, when Aquanauts are not able to focus their sight on more distant objects.
A related and well documented similar problem occurs during long computer works which could lead to myopia: “Evaluating 40 international studies, Australian scientists found that myopia is growing worldwide. One in three Europeans is now short-sighted. One reason for this is probably a changed lifestyle with increased screen usage. This is confirmed in particular by the investigation by Professor Dr. Frank Schaeffel from Tübingen, who was already able to show in 1999 that intensified close-up work promotes the growth of the eye by projecting the image perceived by the eye behind the retina, thus rendering blurred. In order to produce a sharp image on the retina, the latter reacts with the transmission of signals which lead to an increased length growth of the eye. Preventive measures are […] regular breaks from the screen with a view into the distance, as well as stays outdoors […].” (Source: pharmazeutische-zeitung.de; german; 2007).
On the contrary the vision problems referred to as visual impairment intracranial pressure (VIIP) that has been reported in nearly two-thirds of space explorers after long periods spent aboard the International Space Station (ISS) are not due to extended short distance sights, but an effect of the zero gravity in space. (Source: BBC). Besides astronauts always have the alternative to look out the window where they are able to enjoy views to very far distances.
One proposal describes views into artificial distances generated by Virtual Reality devices that could be used on a daily bases for periods of time yet to be determined. This proposal could be combined with gym equipment like an exercise bike.
(Source of entry image: By SucreRouge (Own work) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons)