An ophthalmoscope is an instrument for viewing the retina and associated tissues—the ocular fundus. It consists of 3 essential elements: a source of illumination, a method of reflecting the light into the eye, and an optical means of correcting an unsharp image of the fundus. Imaging of the fundus is carried out either by direct or indirect ophthalmoscopy.
Direct ophthalmoscope
With direct ophthalmoscopy, the examiner uses the optics of the patient’s eye as a simple magnifier to look at the retina. If both the examiner and patient have emmetropic vision and are not accommodating, rays of light coming from a point on the patient’s retina exit parallel, with zero vergence, and continue through the empty peephole of the direct ophthalmoscope. These parallel rays of light are then focused onto the examiner’s retina. Thus, the examiner’s retina becomes conjugate to the patient’s retina when a direct ophthalmoscope is used.
When the examiner looks through the peephole of a direct ophthalmoscope, with no lenses in place, just past the edge of (or aperture in) a mirror that reflects light into the patient’s eye, almost coaxial to the examiner’s view, an upright, virtual, magnified retinal image is seen. The optics of the emmetropic normal eye are approximately +60 D, so using the formula for a simple magnifier, the magnification is 60/4, or 15× (Fig 8-16). This means that the patient’s retina appears 15 times larger than if the retina were removed from the eye and held at 25 cm. Only a small field of view is seen with the direct ophthalmoscope (about 7°) because, even when being as close to the patient as possible, the peripheral rays that are coming from the peripheral part of the patient’s retina cannot be captured, as they do not enter the examiner’s pupil (Fig 8-17).
If the patient or examiner has an uncorrected spherical refractive error, a series of auxiliary lenses is available to dial into the path of the direct ophthalmoscope for compensation. If the patient’s eye is myopic, a minus lens is dialed in, to overcome the extra plus power “error lens” inside the patient’s eye. Those 2 lenses create a Galilean telescope effect, increasing magnification and decreasing the field of view. Similarly, the retina of a hyperopic eye will be magnified less than 15× because of the reverse Galilean telescope created by the minus power error lens inside the patient’s eye and the plus lens of the direct ophthalmoscope.
Indirect ophthalmoscope
In indirect ophthalmoscopy, an ophthalmic “condensing” lens is used to increase the field of view, by capturing the peripheral rays (that are lost in direct ophthalmoscopy) and bringing them into the examiner’s pupil (Fig 8-18). Thus, a much wider field of view is seen with the indirect ophthalmoscope (eg, about 25° with an ordinary 20 D condensing lens).
Assuming that the patient’s eye has normal vision, rays of light coming from a point on the patient’s retina leave the eye with zero vergence and are gathered and focused by the condensing lens into what is called an intermediate aerial image; that is, an image of the patient’s retina in space. In case of a 20 D condensing lens, this image is located one-twentieth of a meter closer to the examiner, who therefore sees an optically real, inverted image of the patient’s retina that appears to be 5 cm closer to the examiner’s eye than the 20 D lens. With the examiner looking at that aerial image, it will be focused on the examiner’s retina. Thus, in indirect ophthalmoscopy, the patient’s retina, the aerial image, and the examiner’s retina are all conjugate to each other.
The most important conjugate planes in indirect ophthalmoscopy, however, are the cornea and the faceplate of the indirect ophthalmoscope (Fig 8-19). The main purpose of the condensing lens, other than the purpose of forming the aerial image, is to make the faceplate of the indirect ophthalmoscope conjugate to the patient’s cornea, so that the bright illumination light passes at a different place through the cornea, offset from where the examiner’s pupils are looking, to avoid reflections back from the cornea into the examiner’s eyes. This is very important because the cornea reflects about 2% of the light, but the observed retinal image is only 0.1% of the light. Thus, the retina cannot be seen if any light from the cornea is reflected back into the observation pathway. Therefore, in indirect ophthalmoscopy, the light pathway is separated from the observation pathway by imaging the faceplate on the cornea with the condensing lens, so that the aerial image of the retina can be seen. The images of the observer’s pupils in the plane of the cornea are very small circles—about 10% of the observer’s pupil’s diameters—and thus form virtual pinholes (the drawing in Figure 8-19 shows these images much larger than they are in practice). These tiny entrance pupils limit the light available for the observer to view the fundus, but also allow very clear images to be appreciated even in the presence of imperfect ocular media, such as cataracts or vitreous debris. This is a mixed blessing—it allows for better views of the fundus than can be obtained in many cases, for example, with the direct ophthalmoscope, but prevents the observer from appreciating the visual impairments caused by the media imperfections.
The binocular eyepieces in the indirect ophthalmoscope, via mirrors and/or prisms, reduce the interpupillary distance from about 60 mm to 15 mm, to fit the images of examiner’s pupils along with the light source within the patient’s pupil, allowing for binocular viewing. (If the patient’s pupil is small, the illuminating and observation pathways can be brought closer by varying the positions of mirrors or prisms in the eyepieces). This causes a reduction of the examiner’s stereoscopic vision by 60/15, or 4×, which fortunately is compensated for by the axial magnification of the aerial image.
To appreciate this, let’s look at the transverse magnification of the aerial image, which turns out to be the power of the eye divided by the power of the condensing lens (see Fig 8-20), that is 60/20, or 3×, for an emmetropic eye and a 20 D condensing lens. The aerial image is thus wider than the actual object on the retina. Recall that the axial magnification is the square of the transverse (lateral or linear) magnification, that is, in our case 9×. The image that is observed is thus greatly distorted in depth, which helps make up for the loss in stereoacuity due to the reduced interpupillary distance. The eyepieces reduce depth fourfold, so the overall axial magnification is 9/4, or 2.25×. Thus, things are observed 3 times wider and 2.25 times increased in depth. Other choices of condensing lens power result in different ratios of transverse and lateral magnification.
However, the threefold transverse magnification of the aerial image is not the overall transverse magnification of indirect ophthalmoscopy. The overall transverse magnification depends upon the distance from which the aerial image is observed. From about 40 cm, from where it is usually observed, the overall transverse magnification is about 3×25/40, or 1.87, with the 20 D condensing lens (Fig 8-21), much less compared to a direct ophthalmoscope, which provided 15× magnification (see Fig 8-16). In summary, small details are observed with the direct ophthalmoscope that cannot be seen with the indirect ophthalmoscope.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.