Smartphone fundus photography: Optics revisited

There is a large amount of literature on the use of smartphones for fundus photography, which are a good tool for documentation, teaching, and teleconsultation. 1,2 We review some of the optics involved and suggest how to use these to get to most of a smartphone's camera.Visibility of the fundus through a condensing lens (+20D) is based on Gullstrand's principle, which states that the illumination and visualization axes must fall within the pupil simultaneously, not intersecting anywhere other than the retina, for the latter to be visible. 3 Theoretically, the closer the 2 axes are, the better the image should be.To test this hypothesis, we used 6 different smartphones for comparison.The video was captured with flash 'on' for subjects with adequate mydriasis (at least 6 mm).During attempts to focus a retinal image, edge artefacts (iris reflections and shadows) are often seen and represent partial/ complete fallout of the illumination/visualization axes from the projected pupil.Still images from the videos with minimal 'noise' were used for comparison.The phone camera, with minimal 'noise' and capturing a uniform, wider, artefact-free fundus field on the condensing lens, was considered better for fundus photo-graphy.The photograph's clarity was not considered, as it depended on the camera's pixel resolution.Fundus images from 6 smartphones are in Fig. 1a-f, arranged in order of increasing artefacts.The distance of the source of flash from the primary camera for the corresponding smartphones were 1.9 cm, 1.9 cm, 2.7 cm, 2.5 cm, 2.5 cm, and 4.3 cm, respectively (Fig. 2a-f).Devices with a shorter distance between the flash and camera proved superior for fundus photography.Only a small area of the fundus photo could be captured with a smartphone with a large distance between the flash and the primary camera and it was impossible to capture the temporal fundus and disc simultaneously due to shadow artefacts (Fig. 1f).A subject with nuclear sclerosis was deliberately chosen to assess the utility of smartphone fundus photography for a cataractous lens used in Fig. 1 a-e while for the sixth camera (Fig. 1f), a patient with a relatively clear lens was used to minimize confounding due to nuclear sclerosis.Peripheral retinal visualization with smartphones involves a small manoeuvre (Fig. 3a-c).If the inferior peripheral retina has to be examined, the eye should be in dorsum-duction so that the