When held at its correct working distance from the eye, the condensing lens produces a magnified image of the fundus approximately the same distance in front of the lens. The field size and magnification are dictated by the design of the lens and the image magnification can be further adjusted by the slit-lamp magnification control (Figures 1a and 1b).
Some clinical examinations can be carried out on undilated eyes but imaging through a small pupil is extremely difficult. In addition, observing the image binocularly can mask some reflection and artefacts and therefore it is recommended that imaging should be attempted monocularly through dilated pupils whenever possible. Attention should also be paid to the condition of the condensing lens as scratches, fingerprints or other marks will significantly reduce the image quality.
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