Improvements on Littmann's method of determining the size of retinal features by fundus photography
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Littmann's formula relating the size of a retinal feature to its measured image size on a telecentric fundus camera film is widely used. It requires only the corneal radius, ametropia, and Littmann's factor q obtained from nomograms or tables. These procedures are here computerized for practitioners' convenience. Basic optical principles are discussed, showing q to be a constant fraction of the theoretical ocular dimension k′, the distance from the eye's second principal point to the retina. If the eye's axial length is known, three new methods of determining q become available: (a) simply reducing the axial length by a constant 1 · 82 mm; (b) constructing a personalized schematic eye, given additional data; (c) ray tracing through this eye to extend calculations to peripheral retinal areas. Results of all these evaluations for 12 subjects of known ocular dimensions are presented for comparison. Method (a), the simplest, is arguably the most reliable. It shows good agreement with Littmann's supplementary procedure when the eye's axial length is known.
KeywordsPublic Health Retina Additional Data Photography Axial Length
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- 1.Bennett AG (1988) A method of determining the equivalent powers of the eye and its crystalline lens without resort to phakometry. Ophthalmic Physiol Opt 8:53–59Google Scholar
- 2.Bennett AG, Rabbetts RB (1990) Clinical visual optics, 2nd edn. Butterworth-Heinemann, OxfordGoogle Scholar
- 3.Littmann H (1982) Zur Bestimmung der wahren Größe eines Objekts auf dem Hintergrund des lebenden Auges. Klin Mbl Augenheilk 180:286–289Google Scholar
- 4.Littmann H (1988) Zur Bestimmung der wahren Größe eines Objekts auf dem Hintergrund eines lebenden Auges. Klin Mbl Augenheilk 192:66–67Google Scholar
- 5.Rudnicka AR, Steele CF, Crabb DP, Edgar DF (1992) Repeatability, reproducibility and intersession variability of the Allergan Humphrey ultrasonic biometer. Acta Ophthalmol 70:327–334Google Scholar
- 6.Sorsby A, Benjamin B, Davey JB, Sheridan M, Tanner JM (1957) Emmetropia and its aberrations. Special report series of the Medical Research Council, no. 293. HMSO, LondonGoogle Scholar