explain pretty well about WHY, not only how and what.

I always verify I read letters correctly. I use test lenses to establish my refractive state. All values are objective and repeatable. Refractive status, +0.5 Diopters. Snellen, 20/20.

In the living eye, accommodation does not stop or relax. It is continuously adjusting based on micro blur detected at the surface of the retina.

Use an electronic Snellen, but keep the 50% judgment.