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Abstract

Background
Imaging of the retina is accompanied by distortions so that positions in fundus photographs cannot be linearly translated to positions in the retinal fundus. The purpose of this study was to describe an algorithm and evaluate its reproducibility in identifying points on the retina from their representation on serial fundus photographs.

Methods
A mathematical formalism was derived to describe how the location of the fovea and the optic disc in fundus photographs, together with the centre-nodal point distance (d) and the curvature radius (r) of the eye, can be used to calculate spatial coordinates on the retina that correspond to given locations in the photograph. The effect of manual markings of the fovea and the optic disc as reference points was evaluated, and the formalism was tested on 105 photographs recorded with different image modalities and centring from 10 different patients.

Results
Retinal locations could be identified in serial fundus photographs with a reproducibility of at least 50 μm when the centre‐nodal point distance and the curvature radius were set to those in Gullstrand's standard eye. A main source of the variability was the manual identification of the fovea and optic disc used as reference points.

Conclusions
The reproducibility in the identification of locations on the retina from serial fundus photographs depends on the accuracy in the definition of the fovea and optic disc used as reference points. This affects the potential for describing locations, distances, areas and changes in retinal lesions on serial photographs over time.

Publication Type:	Article
Additional Information:	© 2025 The Author(s). Acta Ophthalmologica published by John Wiley & Sons Ltd on behalf of Acta Ophthalmologica Scandinavica Foundation. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
Publisher Keywords:	fundus photography, image processing, optical distortion, retinopathy grading
Subjects:	R Medicine > RE Ophthalmology
Departments:	School of Health & Medical Sciences School of Health & Medical Sciences > Department of Optometry & Visual Science
SWORD Depositor:	Symplectic Administrator

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