Thanks to this unique child cohort it is possible to investigate the correlation between physical activity, myopia and retinal vascular diameters in a large group of Danish schoolchildren. We want to study the degree and reversibility of physical activity and its effect on the development of myopia and retinal blood vessel changes. However, there is still a lack of knowledge about the correlation between physical activity and retinal vascular diameters in children. Furthermore, it is still unclear whether physical activity can prevent the development of myopia; such a realization could have far-reaching consequences in form of a modified approach to the necessity for exercise and, furthermore, potentially a significant socio-economic benefit.