Focusing The Fundus Camera: A Clinical Approach

Ophthalmic photography is a system of information transferral. In fundus photography, a visual description of the three-dimensional retinal and choroidal tissues is transferred to film. This twodimensional representation is used by the physician to make judgments concerning the health and treatment of the patient's retina. An accurate representation of the retinal and choroidal tissues is the goal of the ophthalmic photographer. To this end, a sharp, well focused fundus camera image is essential.This paper describes the fundus camera's aerial image focusing system and its correct use. The well focused fundus photograph is defined and proper focusing technique is linked with retinal anatomy and pathology. Focusing obstacles are discussed. Suggestions for achieving proper focus in difficult clinical situations are provided. Special techniques for optimizing sharpness in stereo images are noted. The Fundus Camera's Focusing System In their most basic form, optical systems require light, a subject, a lens, and a receiving plane. Light reflects off of the subject, is refracted by the lens, and is projected onto a receiving plane as an image. Clinically focusing the fundus camera consists of adjusting the relationship between the subject and lens so that the subject lies within the lens' depth of field and the receiving plane lies within the depth of focus of the image. In fundus photography, the patient's retina becomes the subject, the optics of the fundus camera and the patient's eye replace the simple lens, and film becomes the receiving plane (Fig. 1). Most fundus cameras utilize a single lens reflex viewing system. Your ability to maximize focusing skills will be enhanced if you are familiar with the single lens reflex (SLR) design and the fundus camera's aerial image viewing system. THE SLR VIEWING SYSTEM The objective lens of the fundus camera transmits light for both viewing the subject and exposing the film inside the SLR camera body (Fig. 2). The hinged mirror is positioned differently for viewing and taking the picture (Fig. 3 a,b). The image is seen on the focusing screen in the viewfinder when the mirror is down. The hinged mirror then flips up and out of the way to allow film exposure. Figure 1: Α sharp fundus photograph is created when a specific retinal layer is the subject, the fundus camera optics serve as a lens, and the fil m plane coincides with the plane of sharp focus. Figure 2: Α simplification of the fundus camera's SLR viewing system. Standard components of a single lens reflex camera system are identified. Most modern fundus cameras use an SLR system for viewing the fundus image. 8 Journal of Ophthalmic Photography Vol. 14, No. 1 September 1992 Figure 3: Before a photograph is taken, the photographer previews the scene. Light from the lens enters the camera body, reflects off of the hinged, 45° mirror and up into the focusing screen. The photographer views the image on the focusing screen through the viewfinder (A). When the photographer chooses to expose an image, the shutter release is depressed. This initiates a sequence of events (these events may vary according to the specific camera and flash utilized) which include the mirror flipping up and out of the way, momentarily darkening the viewfinder. This allows the same light which has just been viewed to expose the film (B). An SLR system is basically a "what-you-see-is-what-you-get" system. The distance between the focusing screen and the lens system is equal to the distance between the film plane and the lens system (C). When the image appears sharp on a single lens reflex camera's focusing screen, it will also appear sharp on the film. In a single lens reflex system, the focusing screen helps the photographer judge image sharpness before exposing the film. A standard focusing screen is usually constructed of ground glass and is always located the same distance from the objective lens as the film plane (Fig. 3 c). When an object appears sharp on the focusing screen, it will also be sharply imaged at the film plane and therefore will be in sharp focus on the film. The photographer focuses by adjusting the objective lens to achieve the sharpest image on the textured focusing screen. Unfortunately, even when an SLR incorporates the finest ground glass manufactured, the intensity of the light decreases as it passes through the focusing screen. A ground glass focusing system will always dim the view of the object being photographed. This darkening of the focusing screen is of little importance to most photographers. However, in fundus photography, a number of restrictions preclude the use of a ground glass focusing screen: 1. Relatively small amounts of light must be used when viewing the eye. The possibility of retinal damage is increased as the quantity and duration of illumination increases.' In addition, many patients are sensitive to bright lights, making examination with high intensity light difficult. 2. High magnifications (for example 2.5x in a 30° fundus image) are used to photograph the retina. Α fixed amount of light is reflected off of the subject. This light energy (from a small portion of the subject) is distributed over a physically greater area on the film when the image is magnified. This results in a lower level of light available for imaging any particular portion of the subject. 3. The grainy structure of a ground glass focusing screen breaks up the fine detail of the fundus image. The large size of the grains in a ground glass focusing screen makes it difficult to focus on small detail, including fine blood vessels. Fundus cameras replace the ground glass of a standard 35mm SLR camera system with a clear glass focusing screen containing etched lines (Fig. 4). The clear glass maximizes the light entering the photographer's eye. The etched black lines (called the focusing reticle) allow the photographer to focus on the plane of the focusing screen and to therefore achieve sharp focus at the film plane. Because the image in the viewfinder is not `captured' by the ground glass, but rather floats in the air space both in front of and behind the clear glass focusing Figure 4: Various designs of focusing plane reticles are illustrated. The reticle for your specific camera may vary according to manufacturer and date. Patrick J. Same Focusing The Fundus Camera: A Clinical Approach 9 screen, it is termed an aerial image. Learning to focus this aerial image using the reticle is essential to obtaining sharp images with a fundus camera. VIEWING THE AERIAL IMAGE An opthalmic photographer looking through the fundus camera's viewfinder with his or her best corrected vision, sees one of four possible views. The image inside the viewfinder may be totally out of focus, without distinct reticle lines or sharp areas of retina. The image alone may be in focus, only the black reticle lines may be in focus, or both the image and the black reticle lines may be in focus. Only in the last case will a sharp image be recorded on film. If both the focusing reticle and the fundus image are completely out of focus, then one of two conditions may exist. The first is that the photographer's eyes may not be corrected for best visual acuity. A routine eye exam is recommended at the beginning of, and at regular intervals throughout the fundus photographer's career. The second is that the fundus camera's reticle and focus adjustment may be improperly set. If this is the case, the user is referred to the section `Obtaining a sharp fundus photograph' below. Of course, a combination of these circumstances may occur. If the retina is in focus, but the reticle is not (Fig. 5 a), then the photographer has achieved a clear image focused in front of the focusing screen. This situation occurs when the photographer does not concentrate on seeing the reticle but instead focuses his or her eyes (instead of the fundus camera) in an effort to see sharp retinal detail. The photographer's eye accommodates and focuses above the focusing screen (Fig. 5 b). In this instance, the lens system to aerial image distance is greater than the distance between the lens system and the film plane. The distance between the lens system and the aerial image seen by the photographer must be identical to the distance between the lens system and the film plane in order to produce a sharp fundus photograph. Any discrepancy between these distances results in unsharp photographs, and even though the photographer sees a clear image in the viewfinder, the resulting photograph will be blurry (Fig. 5 c). A correct setting of the camera's eyepiece (see below) and a conscious effort to be aware of both the reticle and subject throughout the photographic session will result in sharper photographs. The black reticle may be in focus for the photographer and the subject out of focus, as when first looking through the viewfinder after aligning a patient (Fig. 5 d). In other words, the photographer's eye is focused on the proper plane, but the lens system of the camera has not yet been adjusted to achieve maximum sharpness (Fig. 5 e). If a picture is taken, it will be just as unsharp as seen by the photographer (Fig. 5 f). Only when the photographer adjusts the camera to obtain a critically sharp image, while retaining sharpness in the black lines of the focusing reticle (Fig. 5 g), will the resulting photograph be sharp. A sharp, well defined reticle indicates that the photographer's eye is focused to the correct distance (that is, on the focusing screen). A sharp image of the patient's eye indicates that the lens system has been adjusted properly (Fig. 5 h). Both the retina and the reticle must be well defined in order to generate a sharp fundus photograph (Fig. 5 i).