Australian Journal ofOphthalmology. (1981),9. pp. 3 15-319

WIDE ANGLE ATTACHMENT FOR FUNDUS CAMERAS

IJ TURNER Dip Phot Tech

I FAVILLA FRACO FRACS

WR BARRY PhD

Abstract A simple and inexpensive method for increasing the angle of view of existing fundus cameras used for fluorescein angiography is described. The addition of a concavo-convex positive lens extends the usefulness of the camera and contributes to the efficiency of diagnosis.

Keywords: Fundus camera, fluorescein angiography, fundus photography.

INTRODUCTION No other single technique of retinal observation has developed to the extent of sodium fluorescein fundus angiography. The rapidly-expanding literature relating to this technique, described originally in 1960, testifies to its value in Ophthalmology. The development of the fundus camera has progressed in two directions, by

(i) increasing the photographed field. The first camera covered less than 20°, while there are now cameras available with fields of view up to 148'. (ii) improving the illumitiation of the fundus by reducing reflections and scatter, and by increasing the contrast of the retinal structures.

However, to cover the full range of possible fields of view, several cameras are required. The simple, low-cost, removable attachment described significantly increases the field of view of an existing camera, and, although by no means covering the full range available, allows much fuller utilisation of the camera, leading to savings in examination and photographic preparation time.

METHOD The wide-angle attachment (see Figure 1) consists of a removable simple concavo-convex lens mounted in front of the fundus camera objective. Increasing the lens power increases the field of view; however, there is a limit to the degree of angular coverage as reflections from the corneal surface and the edge of the iris reduce the photographic quality. Also, since the supple- mentary lens introduces new surface/air interfaces, further reflections can be produced.

These aberrations (mainly the image of the annular mirror inside the fundus camera reflected at the front or rear surfaces of the supplementary lens) make wide-angle colour photography difficult (see Figure 2). However, when taking wide-angle fluorescein angiograms, these reflections are virtually eliminated (if good quality interference filters are used), since they originate from the blue excitation beam and are removed by the yellow-green barrier filter before reaching the image plane.

It is necessary, when the supplementary lens is

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WIDE ANGLE ATTACHMENT FOR FUNDUS CAMERAS 315

Figure f o r Wide angle lens fitted to Kowa RC-WF3

fitted, to move the camera closer to the patient, since the illumination beam converses more rapidly, and to refocus slightly. In general, this causes no problem. We routinely use + 3 and + 7 dioptre lenses to produce approximately 20% and 5033 increase in angle of view on an Olympus GRC-IIS camera, and 15% and 30% on a Kowa RC-WF3 camera. Figures 3 and 4 show some Figure 20: Normal red-free view with Kowa camera examples. Figure 5 shows, very diagramatically, the decreased working distance and wider angle of the extra lens, the image of a given section X of the view produced by the supplementary lens. With retina produces a smaller image X, at the

eyepiece/film plane of the fundus camera, which Ib; Wide lens fitted to OIYmPus results in a larger area of retina being visible, GRC-IIS

Figure Zb: + 3 dioptre lens fitted to Kowa: note reflections at centre

3 16 AUSTRALIAN JOURNAL OF OPHTHALMOLOGY

Figure 30: Olyrnpus GRC-IIS fluorescein angio- grams: Inner circle: normal (28" view)

Outer circle: + 3 dioptre lens Black border: + 7 dioptre lens.

If we define: X, = image height with extra lens, X, = image height without extra lens, f,, = focal length of objective f, = focal length of extra lens, and s = separation of two lenses

then XJX, = (fs + fo), approximately. Figure 3b: Normal view

WIDE ANGLE ATTACHMENT FOR FUNDUS CAMERAS

Figure 3ct + 3 dioptre lens

Thus for the widest angle of view (i.e. minimum X,/Xo, s and f, should be as small as possible, consistent with minimisation of undesirable reflections and other aberrations.

Method of use When the supplementary lens is fitted to the camera, reflections become evident; however, these do not record on the film during angiography. A

Figure 3d: + 7 dioptre lens

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Figure 40; Kowa RC-W F3 fluorescein angiograms: Inner circle: normal (45") view Outer circle: + 3 dioptre lens Black border: + 7 dioptre lens

soft flare will appear at the perimeter of the image in the eyepiece, indicating that the camera is too far away. Move the camera closer until the flare is reduced to a fine sharp ring, then check focus. There is little tolerance with reflections from the iris, but with practice the technique can be mastered. No exposure alteration is required.

Figure 4b: Normal view

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Figure 4c: + 3 dioptre lens

DISCUSSION The wide-angle attachment does not replace the wide-angle fundus cameras available, but it does increase the versatility of existing cameras by allowing a larger area of retina to be photographed for diagnosis, thus saving both film and time, since fewer photographs are required. Although a small loss of contrast results from the slightly scattered illumination beam, there is negligible loss of resolution.

Figwe 4d. i 7 dioptre lens

AUSTRALIAN J O U R N A L OF O P H T H A L M O L O G Y

The supplementary lenses may be obtained from any optical manufacturer, after specifying the diameter and power required. Alternatively, ordinary photographic close-up lenses may be quite successfully used, and are available in a variety of diameters and powers.

/- \ image of X fundus camera

objective X s - f s * s xo k*'o -_ -

I SUPP lens I /

c5-

1

lXs ACKNOWLEDGMENTS The authors wish to thank Sally Williams for her valuable assistance in the preparation of this paper, Sam Camilleri for his previous work on the technique, and OPSM for supplying the lenses.

References Figitre 5: Relative image sizes with and uithout wide angle lens. See text for explanations of symbols.

I . De Laey JJ. ed. International Symposium on Fluorescein Angiographv. The Hasue: Dr W. Junk bv. 1976.

2. Pomerantzeff 0. W e b 6 R. Delori F. Imaoe formation in fundus cameras. Investig Ophthalm 1978: 18:630-637.

3. Pomerantzef 0. Ovignon J. Design of a wide-angle ophthalmoscope. Arch Ophthal 1971: 86:420-424.

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