Based on Ref. 6, a summary of the relationships of depth,
pressure, and density gf sea water for standard conditions of
0°C temperature and 35 /oo salinity for 12 depths from O to
7,000 meters are tabulated in Table Be
Of practical interest to the fields of underwater photography
and photogrammetry is to estimate the refractive index of sea
water for a given depth that is within tolerable limits to
preclude excessive resolution degradation owing to an out of
focus condition. For example, assume an underwater camera is
in focus at a given sea water depth. If the camera is lowered
to a greater depth, the focal surface will translate from the
film surface owing to the increase in the refractive index of
the sea water. It is necessary, therefore, to determine the
permissible magnitude of the focal surface translation from the
film surface to conform with a limiting resolution that is
specified for the out of focus condition. The translation of
the focal surface from the film surface is equal to the back
focal distance of the focal surface minus the back focal dis-
tance of the film surface. This subject matter is covered in
greater detail in subsequent paragraphs.
Private communications (Ref. 7) with Jackson on the development
of an equation to determine pressure variation as a function of
water depth are gratefully appreciated. Much data on the sub-
ject is now available and with a relatively small further effort,
a refined presentation is available. However, in the interest
of making an initial presentation that is relatively simple and
approximate, Eq. 16 is derived on the premise that the arithmetic
mean of the water densities at the surface and at depth is the
effective density of the water column.
. 2:0395P Ya, 16
PtP 2.
water depth in meters
water pressure in decibars
water density at surface in g/ml(old)
water density at depth Z in g/ml(old).
Utilizing the sea water pressure and density data in Table 8,
the sea water depths computed by Eq. 16 are listed in Table 9
with their respective errors.
