Two examples of dilation lag (Horner's syndrome). In the first example, the right pupil dilates much faster than the left pupil when the light is turned out. In the second example, the left pupil dilates much faster than the right pupil when the light is turned out.
Video credit: Digre KB, Jacobson D, Balhorn R. Dilation Lag. Video. [Neuro-Ophthalmology Virtual Education Library: NOVEL Web Site]. 2005. Available online. Accessed 02-27-2019.