Abstract Introduction Critical flicker fusion frequency (cFFF) measures visual perception, with higher values indicating better performance. It is widely used to assess the effects of breathing gases in diving and hyperbaric environments. Aim Identify confounders explaining inconsistent cFFF study results. Methods Four experiments using a manual flicker device in volunteers with healthy eyes: 1. cFFF was measured in 174 recreational divers to examine demographic effects. 2. One male diver was tested daily over 85 days. 3. In 28 divers, cFFF was measured before and after 5 min of normobaric oxygen. 4. In 24 divers, cFFF was tested with and without diving masks, plus a condition with diagonal flicker light on the mask. Results 1. Mean cFFF: 39.2 ± 5.1 Hz; decreased with age ( p < 0.001), no gender/BMI effect. 2. Mean: 43.4 ± 2.1 Hz; stable across lighting. 3. Thresholds differed by frequency order; minimal O₂ effect. 4. Masks had no effect, but diagonal light reduced cFFF ( p < 0.01). Conclusion cFFF varied by 13% across divers and 5% in a single diver. Age and observer-light geometry influenced results. Oxygen had minor impact. Findings support using age-specific norms and considering perceptual confounders in diving research.