## 2 Materials and Methods  
We obtained ethical approval from the local institutional review board (project number: 2017-3535) and conducted this study according to the tenets of the Declaration of Helsinki. We included both healthy participants and patients with central areolar choroidal dystrophy (CACD) with a genetically confirmed p.Arg142 Trp mutation in the _PRPH2_ gene. In this specific group of patients, cones located in the central retina are primarily affected [16]. The left eye of all study participants was examined via an objective refraction measurement (Nidek Tonoref II, NIDEK Co., Ltd. Gamagori, Japan), 40 degrees color fundus photography centered on the fovea (Topcon, Topcon Corporation, Tokyo, Japan), macular 30 degrees infrared reflectance confocal scanning laser ophthalmoscopy (cSLO) imaging and spectral domain optical coherence tomography (SD-OCT; SPECTRALIS(tm) Heidelberg Engineering GmbH, Heidelberg, Germany). Our SD-OCT protocol consisted of a 25 \(\times\) 30 degrees volume of 61 horizontal B-scan lines with 128 uM inter-line distance, covering the macular area and centered on the fovea. Each line scan was obtained by averaging 11 single B-scans online, applying the Automated Real-Time Tracking (ART(tm)) mode of the Heyer(tm) software (Heidelberg Eye Explorer(tm) 6.12, Heidelberg Engineering(tm) GmbH, Heidelberg, Germany). We used the high magnification module to capture high-resolution ART(tm) images of the macular region. Five raw frames were automatically averaged by Heyer(tm) software to obtain a single high magnification module ART(tm) image. We captured images with an 8 \(\times\) 8 degrees field of view in the primary position and temporal gaze direction by adjustment of the preinstalled positions of the internal fixation lights of the high magnification module. To ensure the best focus on the photoreceptor layer, we used the spherical equivalent refractive error determined by the objective refraction measurement as a baseline setting and applied a minimal focus correction of no more than \(\pm\)0.5 diopters by manual adjustment of the focusing knob of the high magnification module. If a subject used corrective glasses or contact lenses in daily life, these were worn during imaging and baseline settings were adjusted to 0.0 diopters. Furthermore, the horizontal orientation of the high magnification module was changed by manually rotating the scanning device in accordance with the primary and temporal ocular gaze direction in such a manner that the scanning direction was always parallel to the retinal surface. The participant was encouraged to maintain a normal blinking rate while we performed the imaging. This helped to prevent tear film irregularities. To ensure reliable distance measurements, we entered corneal diameter measurements acquired from [PERSON](tm) in the Heyer(tm) software before the exam. High magnification module imaging was then performed in non-mydriatic conditions. We used a desk lamp to apply direct illumination to the non-examined eye in order to induce a consensual pupillary constriction in the examined eye, thus reducing the effect of peripheral optical aberrations [17]. To increase the signal-to-noise ratio, we computed compositions of at least five high magnification module ART(tm) frames in Heyer(tm) of a single gaze direction. The resulting compositions of each gaze direction were uploaded in the Fiji software (version 15.1 n,National Institutes of Health, Bethesda, MD, USA) [18]. We adapted pixel size to match the original scale in HEYEXT(tm). We then used the \"2D stitching\" plugin to merge the central and temporal high magnification module image to a single, continuous image [19]. The fovea was manually identified by a human grader on an OCT volume scan and marked on the accompanying 30 degrees IR image. We used the \"Big Warp\" plugin to align the corresponding IR and high magnification module images, using retinal vasculature as reference points [20].