Editor,—We were interested in the article by Myung-Kyoo Ko et al 1 regarding the visualisation of the circle of Zinn by fluorescein angiography. However, we do not agree that it is possible to truly visualise the circle of Zinn by this method. As such, we feel that the point of view taken by the authors to reach their conclusions is flawed based on the following observations.

We have been investigating the location of the circle of Zinn by measuring the depth of the circle from the retinal side of the scleral surface, along with the distance between the circle of Zinn and the optic nerve sheath. We have chosen the classic longitudinal serial sectioning technique of the optic nerve and surrounding tissues. Observation at both sides of the sectioned optic nerve, aided by serial sections, has enabled us to conclude that the circle of Zinn lies at a relatively constant depth around the optic nerve in any individual eye.

Moreover, the mean depth of the circle of Zinn was 345 μm from the scleral surface in our study, which is larger than that in the report …