Journal ArticleDOI
Improvements on Littmann's method of determining the size of retinal features by fundus photography.
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TLDR
Three new methods of determining Littmann's factor q become available: simply reducing the axial length by a constant 1 · 82 mm; constructing a personalized schematic eye, given additional data; and ray tracing through this eye to extend calculations to peripheral retinal areas.Abstract:
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.read more
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References
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Journal ArticleDOI
Determination of the real size of an object on the fundus of the living eye
TL;DR: A method is described which allows the real diameter of an object on the fundus to be determined by means of a system of curves which are the result of trigonometrical calculations and which give an approximate value.
Book
Clinical Visual Optics
TL;DR: The eye's optical system visual acuity and contrast sensitivity spherical ametropia astigmatism subjective refraction accommodation and near vision ocular motility and binocularVision convergence anomalies of binocular vision - heterophoria and heterotropia stereopsis and the stereoscope.
Journal ArticleDOI
Determining the true size of an object on the fundus of the living eye
TL;DR: The result was represented by curves which enable factors used in calculating the true diameter to be determined, but instead of the curves a quadratic equation is used to produce the same factors.
Journal ArticleDOI
A method of determining the equivalent powers of the eye and its crystalline lens without resort to phakometry
TL;DR: If the positions of the principal points of the crystalline lens are conjectured, its equivalent power and that of the eye can be calculated as described from ocular dioptrics.