We thank Dr Bloom for his letter and the opportunity to amplify the explanation we published concerning the definition of an MPS disc area. The MPS disc area and all previous MPS measurements are based on the assumptions that the optic disc is equal to 1500 μm and that the magnification of the photographs on a 35-mm negative is 3.0. These assumptions were made to derive a standard unit for measuring an area because the exact size of each optic disc differs and the magnification of cameras differs, not only by individual camera but also by refractive properties of the eye.1 Because of these variations, we have been careful to use the term MPS disc areas rather than the unmodified term disc areas. These standard areas have been published, and templates with these standard areas to use on 35-mm negative angiograms derived from a 30° fundus camera
Bressler NM, Alexander J, Hawkins BS, Maguire MG, Fine SL. Subfoveal Neovascular Lesions in Age-Related Macular Degeneration-Reply. Arch Ophthalmol. 1993;111(7):901. doi:10.1001/archopht.1993.01090070018005
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