The length of a free ranging animal is usually estimated visualy which involves numerous judgment errors directly dependent on the observer. Three dimensional photography allows for a precise length estimation involving no direct contact with the animal.
Marine scientists have long known that the dimensions of an undersea object can be determined from:
- the displacement of the objects images on a stereopair of photographs,
- the relevant dimensions on one of the photographs,
- and the separation distance between the cameras.
Moreover the distance from the camera to the object can be calculated if one knows the distance between the second nodal point of the lens and the film plane of the cameras. This technique has been used often by marine geologists to map the topography of the sea floor.
However, measurements of biological objects are rare: e.g. determination of the 3-dimensional structure of fish schools, densities of benthic epifaunas and the measurement of lengths and 3-dimenstional positions of free-swimming schooling scalloped hammerhead sharks - Sphyrna lewini.
Even so, the use of stereophotography offers several advantages, especially in the study of large midwater animals:
- Densities of species and individuals can be studies directly;
- The spatial dispersion of individuals can be described;
- Body sizes can be measured without distorsion that often result when large animals are measured out of the water either on a flat surface or hanging from gallows.
With Stereophotography, the field biologist can determine the size, the density and spatial dispersion of individuals of a species without disturbing them, interacting with them or removing them from the study population (an especially important consideration when studying animals protected from capture, such as marine mammals and endangered species).
When a single photograph contains the image of an unrecognized object and no image of a recognisable object, it is essentially impossible to determine the scale (or size) of the unrecognised object. Stereoscopic pairs of photographs contain all the information required to obtain scale, provided the camera separation is known and the optical axes of the two cameras are parallel.