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On Designing Underwater Camera Lenses

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On Designing Underwater Camera Lenses FRONTlSP1ECE. System 5: The.Nikkor 21 mm. J/4 u.nderwater objective lens using concentric phencal surfaces for the aIr-water window. ZE:"Jl \\1 AKIMOTO* Nippon Kogaku, K. K. (Nikon) Tokyo, Japan On Designing Underwater Camera Lenses It is not advantageous to attempt to design a lens for use both in water and air. (Abstract on next page) INTRODUCTION wideangle lens, or by employing an illuminat­ ing device. Actually this is the method cur­ N NDERWATER PHOTOGRAPHY we en­ rently in use. Here we shall consider some I counter various difficulties. A camera to be points on the optical system inside the camera used in the water requires a different con­ for underwater photography. struction from one to be used in the air. Aside For any camera, and whatever optical sys­ from the mechanical considerations such as tem is employed, i.e., taking photographs one water-tightness, there are a lot of additional by one, or consecu ti vely for teleyision or ?ptical problems, such as absorption, scatter­ movies; the conditions under which the cam­ Ing and transparency of water. This may be era is used in the water will always be the solved, to some extent, by bringing the cam­ same. Therefore, in this article we shall not era closer to the underwater subject using a touch on the types of cameras for underwater. I t is obvious that the difference between * Presented at the Annual Convention of the American Society of Photogrammetry, Washing­ the underwater photography and air photog­ ton, D. c., March 1967 as one of several papers on raphy lenses is based upon the fact that the underwater photography, all contained in this subject being photographed is either in the issue. Edited by Mr. Stuart Held, Special Optics water or in the air. However, if both the lens Sales Manager, EPOI, Photo Technical Products Div., 623 Stewart Ave., Garden City, N. Y. 11533 and sensitive material could be immersed in 925 926 PHOTOGRAMMETRIC ENGINEERING water, the problems ,,'ould be simplified. Although several studies have been pub­ Thus, the refracti"e indices both in the sub­ lished on the methods to eliminate the deter­ ject and in the image area become the same ioration of the image due to water in the sub­ and we need only to consider the refractive ject area, most of them have been made only index of air as replaced by that of water. Us­ on the basis of some fundamental and ap­ ually, a lens immersed in the water has a proximate formula. There are actually only a weaker refracting power than that in the air. few exacting optical systems developed ex­ and shows different aberrations. However, no clusively for underwater use. substantial change will take place in the func­ It is a well known fact that in designing a tion of the lens. Therefore, any underwater lens of excellent image characteristics; it is lens to be used this way needs only to be cor­ necessary to carry ou t the so-called "optical rected the same way as lens for air photog­ ray tracing" method. In other words, the raphy. Several years ago, a trial com bi nation elemental arrangement of a lens system is not of an ordinary lens and light sensiti,'e mate­ sufficient. In the last few years equipment for rial was made by the Konan Camera Institute swim mi ng, picture taki ng, illu mi nation, etc, A RSTRACT: .11any types of lenses have been designed for photographic use under standard atmospheric conditions, but only very few lenses have been buill exclu­ sively for underwater photograph)'. In many cases those that have been designed for standard use are applicable to underwater photography without any altera­ tion. The Nippon firm is the manufacturers of the lenses for our underwater camera NIKONOS. These lenses have remarkable differences in characteristics from atmospheric photographic lenses. In designing such lenses, the d~fferences to be considered were theoretically examined and all the conceivable types of underwater lenses were systematically class~fied, Several examples of the under­ water lenses are in current manufacture, and some interesting types of lenses can be expected in the future. for underwater photography, where neither has undergone a marked developmen t, water-proofing nor water pressure was consid­ whereas the lens for underwater photography ered. The construction was extremely simple. has been neglected. The institute realized that even though the We have closely studied the problems of film is immersed in seawater for several days, developing proper underwater lenses. Below the film is usable enough for making a picture we shall discuss some problems and suggest after it is washed with fresh water and devel­ different types of lenses for underwater oped. However. this is an exceptional case, photography, based on our experience in de­ In general, the underwater camera is sup­ signing underwater lens systems which will plied with a piece of window glass or the like perform as satisfactorily as the lens systems for water-tightness and is designed so as to for air photography. produce an image in the air contained within the camera. Then, the refraction of light takes PROllLEM 0:-< UNDERWATER LENSES place at the boundary surface between the Looking at a subject in the water from water and the air, thus giving rise to various abo"e through the air, it appears as if the su b­ problems. Therefore in designing a lens sys­ ject were closer to the surface of the water tem the difference of refractive index between than it actually is, If we observe it slantwise the subject and image area should be con­ to the surface of water, the edge of the subject sidered. As an example, the oil-immersion ob­ in the water is accompanied by color fringes. jecti,'e lens for a microscope, Until now, for This is attribu ted to the fact that the ligh t most underwater lenses, owing to many coming from the subject undergoes refraction difficulties encountered in taking a picture, no as well as color dispersion at the boundary great importance has been placed on the im­ surface between water and air. To such a age characteristics. I n most cases, in under­ phenomenon, the law of refraction is applied, water photography, the lens designed for air whereby the refracting angle Oa as against the photography is applied without any altera­ normal angle is always larger than the inci­ tion, dence angle Ow in the water. ON DESIGNING UNDERWATER CAMERA LENSES 927 Furthermore, this refracting angle, \'arying with the wavelength (color) of light, brings about color dispersion (Figure 1). _7lYI __ If the picture of a subject in water is taken, using a photographic lens which is well cor­ rected for use in the air, the subject \\·ill be re­ produced as a photograph. the same as seen by the eye. If a distorted and colored image of the sub­ ject as seen by the eye were to be taken as real, nothing would be said about the under­ water lens. Problems arise though in the fact that the image of the subject the eye sees is not the same as the actual image of the sub­ FIG. 2. The law of refraction. ject. The first problem is that the Rat surface appears as a cun'ed one and distorted in its shape. I n order that the subject)' and its im­ seem to approach the water surface and be age y' are in an analogous relation to each more distorted. This, if taken photographi­ other, the dimensional ratio is to be constant cally, will show a different form from the between them. This means that it is necessary actual one on the figure, caused by the lens to fulfill the following equation (Figure 2): aberrations, the so-called distortion and the cun'ature of the image plane. y' tan 0' -- = c --- = Conslant The second problem is the occurrence of y tan 0 color fringes on the edge of the image of the subject caused by the di persion of light. In However, according to the law of refraction, color photography the fringes result in colored as (sin O'/sin 0) is equal to and constant, nw smudges which appear more markedly on the (tan 0'/tan 0) is not constan t, bu t will be edge of the image. As the image locates far­ different depending on the value of O. ther from the center of the picture, a red­ For this reason, the larger the incidence colored smudge on the inside and a blue­ angle 0, the more distorted the image. For one colored smudge on the outside \\·ill appear. [n who stands on the edge of a swimming pool black-and-white photography, the outline of and obsen'es the Rat bottom of the pool, the the image will be blurred to an extent of the farther the subject, the closer the subject will \\"id th of the color fri nges. From the view point of lens design this occurrence corre­ sponds to the chromatic aberrations which D produce the same result as using a lens with its color aberrations not corrected, which is an unfavorable condition causing a lowering of the resolution in the photograph. The actual subject has no such color fringe. The third problem is concerned wi th the fact that even using a wide-angle lens, the unden\'ater picture area cO\'ered by this lens is narrowed to a noticeable degree, and the angular field of the lens is restricted. This \\·ill be easily understood if \\'e consider the re­ fracting angle in the air which becomes larger than the incidence angle in the water accord­ -_ - _-_wo!f:t- ing to the law of refraction.
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