What is Blur
There are many sources of failure to recognize characters, but blur is the only one addressed by EM. Blur is when the edges of the thing you are looking at are 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. A weak negative lens will give myopic blur to a myope.
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
Peripheral blur is the blurring of the part of the image outside your center of focus. This happens normally when something to the side is at a different distance than the thing you're looking at. While wearing glasses this becomes more complex because the outside of your lenses are often stronger than the center of focus, and produce hyperopic blur (which may be a contributor to lens induced myopia).
Defocus as a stimulus for accommodation
The lens in your eye is adjustable to try to compensate for moderate amounts of blur. As your eye experiences Defocus, the short term response is to move the lens in the direction needed to correct for that blur.
Defocus as a stimulus for eye length changes
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
- Zhou YY, Chun RKM, Wang JC, Zuo B, Li KK, Lam TC; et al. (2018). "Proteomic analysis of chick retina during early recovery from lens‑induced myopia". Mol Med Rep. 18 (1): 59–66. doi:10.3892/mmr.2018.8954. PMC 6059693. PMID 29749514.CS1 maint: multiple names: authors list (link)
- Tarutta, Elena (2016). "Long -term effects of optical defocus on eye growth and refractogenesis" (PDF). Pomeranian J Life Sci.
- Zhu, Xiaoying; McBrien, Neville A.; Smith, Earl L.; Troilo, David; Wallman, Josh (2013-04-01). "Eyes in Various Species Can Shorten to Compensate for Myopic Defocus". Investigative Ophthalmology & Visual Science. 54 (4): 2634–2644. doi:10.1167/iovs.12-10514. ISSN 1552-5783.