Which is more dangerous eyesight, a minus 5 diopter Rx or a minus 7.5?
It depends on what you mean by "dangerous." Obviously the higher degree of myopic (nearsighted) Rx with a -7.5 D. correction is going to be worse uncorrected at distance than the -5 D. eye. But the difference is negligible from the point of view of how well you could function in the real world without glasses or contact lens correction. It is a little like asking if it is more dangerous to have 5 miles or 7.5 miles to swim to get to shore in a boating accident: either one is about the same in terms of how likely you are to survive without help.
In general, the higher the myopic glasses correction necessary to provide good distance vision, the larger the diameter of the eyeball. Anatomically (with rare exceptions) ▲ Myopia = ▲eyeball size. The retina gets "stretched" as the eyeball grows, sometimes causing weak areas, or even tears, to develop in the periphery. It is these weak areas that can sometimes break open and become the source of potentially serious issues such as a retinal detachment. A -5 eye and a -7 eye are not much differently at risk, but both are significantly more at risk of retinal problems than a more normal, non-myopic eye.
These are rare, though, so no cause for alarm. Just know in advance the signs and symptoms of a retinal tear or detachment if you are very myopic.
_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic