• Begin by zooming on the image in order to be able to select the pupil easily
(values of 400% or 800% should work)
• Select the red part of the pupil and use filter "Grayscale" or "Adjust / Color
balance" to decrease the red component. "Adjust / Gamma correction"
applied to the red layer can also give a good result. In all cases, preferably
use a circular selection shape or the lasso selection tool.
• Repeat the process on the second pupil