Reliable measurements from fundus photographs in the presence of focusing errors.

When measuring the width of the blood column and the superimposed central light reflex of retinal vessels from fundus photographs, we find that the measurements are affected by focusing error, unless certain steps are taken to reduce its effect. Thus, we recommend using a scanning microdensitometer to extract the density profile of the blood column from the negative, and convert the density profile to an intensity profile by means of the characteristic curve of the film. In this paper we model the imaging process on a computer, and find that the half value width of the intensity profile is relatively insensitive to focus error. However, the maximum intensity of the central light reflex is still sensitive to focus error. Both these results are in agreement with experimental studies. Invest Ophthalmol Vis Sci 30:674-677,1989 We find that the error of focusing the fundus camera is important for measurements of the size of retinal structures in spite of telecentric operation of the camera. Fortunately, since the camera is telecentric, it is possible to reduce the effect of focusing error by performing the measurements according to a method previously described.' The method consists of scanning the photographic negative by means of a microdensitometer, and converting the density readings to light intensity by means of the characteristic curve of the film. The last step is of crucial importance, as will be discussed below. The current paper is.a theoretical study in which we model the effect of defocusing on a computer, and show that the "half value width" of a structure is relatively unaffected by defocusing. This theoretical result corresponds well with experimental studies of defocusing,2 thereby increasing confidence in both the theoretical modelling and the experimental method. The retinal structure of special interest to us is the blood column of retinal vessels, with its superimposed central light reflex. A reliable measurement of the diameter of the blood column is important for the accurate determination of retinal blood flow. 3 Further, changes in the dimensions of the reflex may be related to hemodynamic or structural changes due to disease or other stresses on the human body. 4 5