Underwater vision
Underwater, objects are less visible because of lower levels of natural illumination caused by rapid attenuation of light with distance passed through the water.
The visual acuity of the air-optimised eye is severely adversely affected by the difference in refractive index between air and water when immersed in direct contact.
[1] Stereoscopic acuity, the ability to judge relative distances of different objects, is considerably reduced underwater, and this is affected by the field of vision.
A narrow field of vision caused by a small viewport in a helmet results in greatly reduced stereoacuity, and associated loss of hand-eye coordination.
[4] Instruments are available for field estimates of visibility from the surface, which can inform the dive team on probable complications.
{{expand section}|daylight, lighy emitted by organisma}, artificial lighting}} Water has a significantly different refractive index to air, and this affects the focusing of the eye.
[citation needed] The crystalline lenses of fishes' eyes are extremely convex, almost spherical, and their refractive indices are the highest of all the animals.
Double-dome masks restore natural sized underwater vision and field of view, with certain limitations.
Framed lenses are available for wear in some helmets and full-face masks, but they can be difficult to defog if there is no fresh, dry, gas flow over them.
The frame may be mounted to the helmet or mask, or worn on the head in the usual way, but they cannot be adjusted during a dive if they move out of position.
Diving masks can be fitted with lenses for divers needing optical correction to improve vision.
Corrective lenses are ground flat on one side and optically cemented to the inside face of the mask lens.
Scuba divers with interest in underwater photography may notice presbyopic changes while diving before they recognize the symptoms in their normal routines due to the near focus in low light conditions.
[11] The Moken people of South-East Asia are able to focus underwater to pick up tiny shellfish and other food items.
[14] This is due to the contraction of the pupil, instead of the usual dilation (mydriasis) that is undergone when a normal, untrained eye, accustomed to viewing in air, is submerged.
[4] The theoretical black body visibility of pure water based on the values for the optical properties of water for light of 550 nm has been estimated at 74 m.[16] For the case of a relatively large object, sufficiently illuminated by daylight, the horizontal visibility of the object is a function of the photopic beam attenuation coefficient (spectral sensitivity of the eye).
It has been shown that the function 4.8 divided by the photopic beam attenuation coefficient, as derived by Davies-Colley, gives a value for visibility with an average error of less than 10% for a large range of typical coastal and inland water conditions and viewing conditions, and the beam attenuation coefficients for a single wavelength band at about 530 nm peak is a suitable proxy for the full visible spectrum for many practical purposes with some small adjustments.
In very clear water visibility may extend as far as about 80m,[18] and a record Secchi depth of 79 m has been reported from a coastal polynya of the Eastern Weddell Sea, Antarctica.
Depending on manufacturer, nephelometers measure scattered light in the range between about 90° to 165° off the axis of the beam, and usually use infra-red light with a wavelength of around 660 nm because this wavelength is rapidly absorbed by water, so there is very little contamination of the source due to ambient daylight except near to the surface.
(some mask-integrated head-up displays may be legible) Low visibility is defined by NOAA for operational purposes as: "When visual contact with the dive buddy can no longer be maintained.
"[21] DAN-Southern Africa suggest that limited visibility is when a "buddy cannot be discerned at a distance greater than 3 metres.
