**1.1 AO flood illumination imaging**

AO flood illumination imaging was obtained by coupling a wave front sensor and a deformable mirror to a high-magnification fundus camera, thus providing some of the first organized studies of retinal photoreceptors [14]. This is the principle of the AO rtx1 camera (Imagine Eyes, Orsay, France), which has been extensively used in the study of retinal photoreceptors [15–17] and vasculature [18, 19]. Compared with the other two devices, its axial resolution is smaller (~300 μm) [10]. However, an important advantage of this type of imaging is the speed at which the entire retinal image is acquired (a few milliseconds). Thus, with a CCD camera, 40 retinal images are taken in 4 s, which are to be further processed by specialized software in order to deliver a final image [20]. One single image is minimally influenced by eye movement, and the system is capable of providing very high frame rates with high sensitivity [21].
