What is Blur
There are many sources of failure to recognize characters, blur is the only one addressed by EM. Blur is when the edges of the thing you are looking at is not well defined. Print may be too small to see without being blurry. A normal healthy eye can experience blur by looking through a lens not designed for it.
Myopic vs Hyperopic Blur
Myopic blur is the type of blur normally experienced by people with uncompensated myopia. Hyperopic blur is the type normally experienced by people with uncompensated hyperopia. In order to induce blur, use a lens with the opposite diopter sign as the lens normally used for blur correction, so a plus lens will give myopic blur to an emmetrope.
Studies show that extreme hyperopic blur conditions induce myopia in growing animals, and myopic blur in growing myopic animals can partially reverse this.
To format properly later: Proteomic analysis of chick retina during early recovery from lens‑induced myopia - Yun Yun Zhou et al. - May 3, 2018 https://www.spandidos-publications.com/mmr/18/1/59?text=fulltext
Diffusion such as frosted glass, causes myopia just as strong myopic lenses do in growing animals. This is also called form deprivation.
Text appears smeared in a direction. This is the defining symptom of astigmatism
Defocus as a stimulus for eye length changes
The eye needs stimulus to make corrective changes. Studies have shown that inducing myopic defocus is protective against myopia progression and can induce axial lengthening and shortening.
EM Blur Adaption
"Blur Adaption" unfortunately has two meanings. The meaning commonly meant by EM forum members is encountering so much blur that your eye no longer tried to accommodate the blur. This is something like learned helplessness at the biological level.
Clinical Blur Adaption
When a doctor uses the term "Blur Adaption" they mean that a person has adapted to perform visual tasks despite their blur. This is almost the opposite of EM blur adaption.
https://www.sciencedirect.com/science/article/pii/S0042698906005621 - Conceptual model of human blur perception