Measurement of choroidal and retinal thickness in ETDRS fields. 1A. Demonstrates the 1, 3 and 6 mm diameter ETDRS circles superimposed on the pseudocolor infrared image. The horisontal line shows the cross section shown on 1B, where choroidal thickness was measured manually with the inbuilt caliper in the center of each ETDRS field. 1C shows the automatically calculated average thickness and volume of the retina in the field.