“Refraction“ is the process for determining your optimal lens (glasses) prescription. When you press your face against that cold black thing, trying to decide if “one“ is better than “two,“ then your are having a refraction. Obviously it isn’t always apparent which one is better, and as it turns out, most people tend to make errors towards the minus side (so called “over-minused“). Thus it is possible, and in fact rather common, that if you are nearsighted your prescription has too much minus, or if you are far-sighted, not enough plus. The reason people tend to “over-minus“ is because of accommodation. If you were accommodating during your eye exam (and who isn’t), then you may be over-minused. One very effective way to prevent you from accommodating during the exam is to temporarily paralyze or cycloplege the muscles of accommodation using specialized eye drops. We call it cycloplegia because, well, lets face it, who would want to come in for temporary paralysis?
No, that is not what that means.
To understand what "over-minus"-ed means, you have
to read the part about " When you press your face against that cold black thing, trying to decide if “one“ is better than “two,“ then your are having a refraction."
That "cold black thing" refers to this big contraption that the optician puts in front of your face. On this contraption is a round
hole that you look through to the eye-chart on the wall --the
chart that has all these Z B Y 4 I etc... letters and alphabets
that you have to read. In the old days, on that hole that you look
through, a set of lenses are mounted. The optician will rotate
each lens into place and asks you to look through each of them
and then tell him which one gives you a better vision.
Most of the time it is difficult to determine between two choices of lenses that are giving you almost the same clarity.
According to this article, most people will tend to choose
a lense that is more negative than the one you actually need
( thus the term over-minus .)
To make a long story short, near sighted people need
lense that have negative values -100, -150, -200 etc... (
in Chinese they are called 度數﹐ 100度 etc,,,) If you are
far sighted, the lenses have positive values +100,+125, +250 etc...
What this author claims is that people who are near sighted
tend to choose the more negative ones (e.g., if they
are actually -150, and the next on on that rotating lense
set up is -175, they would say that the -175 is better.)
For the far sighted, instead of +175, they will say the +150
is better.
I do not know how to express that in Chinese, but I am
sure with all these information you will have no trouble
in working that out on your own.
--
For people who still remember their 高中物理光學。
The度數 is 100 x (1/f) with f in units of meters.
E.g. f=-0.5, 1/f= -2 --> -200 度
myopia是近视的意思，
pseudo-，not real, false.
人的眼球调节能力容易引起假性近视，青少年的调节能力尤强。
这种调节能力还会导致你所得的度数往往深于实际度数。
所以，我们小时候配镜子最好散瞳（Cycloplegic Refraction）。
散瞳后验光度数和散光轴的位置能检查得更准确一些。
“over-minus”可不可以就简单译成“度数过高（深）”，或“验光不准”。
验光--Refraction
散瞳--Cycloplegic Refraction
自动调节能力--Accommodation
u为物距即物到透镜（眼睛）的距离，v为像距，就所成像到透镜的距离。
P.S.
如果从前面平行进入我们眼球的光线集焦点于视网膜前方,这种情况就成了近视眼.
反之,若眼前平行进入我们眼球的光线,集焦点于眼球后方,这就成了远视眼了.上述这些
情形都叫屈光不正,都需要用眼镜来矫正,才能看清眼前的一切景物.
屈光不正可以分下述几类:
一.近视:进入眼球之平行光,集焦点于视网膜之前方.
二.远视:平行光焦点于视网膜后方.
三.散光(乱视):因角膜或水晶体等之折射面不成球形,而系成椭圆形,则相交
成90度的两个经线的折射度便不同,因而可造成:
(1).单性或复性近视性散光.
(2).单性或复性远视性散光.
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http://www.lasikeye.com.tw/light/light1.htm
屈光不正是指眼在不使用调节时，平行光线通过眼的屈光作用后，不能在视网膜上结成清晰的物像，而在视网膜前或后方成像。它包括远视、近视及散光。
一、近视（myopia）
近视眼是指眼在不使用调节时，平行光线通过眼的屈光系统屈折后，焦点落在视网膜之前的一种屈光状态。所以近视眼不能看清远方的目标。若将目标逐渐向眼移近、发出的光线对眼呈一定程度的散开，形成焦点就向后移，当目标物移近至眼前的某一点。此点离眼的位置愈近，近视眼的程度愈深。
原因：近视发生的原因大多为眼球前后轴过长（称为轴性近视），其次为眼的屈光力较强（称为屈率性近视）。近视多发生在青少年时期，遗传因素有一定影响，但其发生和发展，与灯光照明不足，阅读姿势不当，近距离工作较久等有密切关系。
类别：大部分近视眼发生在青少年，在发育生长阶段度数逐年加深，到发育成熟以后即不发展或发展缓慢。其近视度数很少超过6D，眼底不发生退行性变化，视力可以配镜矫正，称为单纯性近视。另一种近视发生较早（在5～10岁之间即可发生），且进展很快，25岁以后继续发展，近视度数可达15D以上，常伴有眼底改变，视力不易矫正，称为变性近视。此外，习惯上常将3D以下近视称为轻度近视，3～6D者称为中度近视，6D以上者称为高度近视。
二、远视（hyperopia）
远视是指眼在不使用调节时，平行光线通过眼的屈光系统屈折后，焦点落在视网膜之后的一种屈光状态。因而要看清远距离目标时，远视眼需使用调节以增加屈光力，而要看清近目标则需使用更多的调节。当调节力不能满足这种需要时，即可出现近视力甚至远视力障碍。
原因：常见的原因是眼球前后轴较短（称为轴性远视），其次是眼的屈光力较弱（称为屈率性远视）。远视也可以认为是眼球发育不全，在儿童时一般常为远视，以后随年龄增长而程度减低。
三、散光（astigmatism）
散光眼是指眼球的不同经线，甚至在同一经线上，具有不同屈光力的一种屈光状态。因此，散光眼不能将外界射入眼内的光线焦合在一个焦点上。
散光眼分规则与不规则两类，一般屈光学上所说的散光眼都是指前者
五、假性近视(pseudomyopia)
由于调节紧张，甚至痉挛造成的功能性近视，称假性近视。学龄儿童或青少年，眼的调节力较强，如果用近视力的机会较多，加上不注意眼的卫生，过度使用调节，常常导致调节紧张与痉挛，视远物不清，表现为近视状态。
假性近视可出现在原为正视眼 远视眼甚至近视眼者，因而常把正视眼、甚至远视眼误认为近视，而使原来近视者表现出度数更高的近视。
假性近视经过休息或应用睫状肌麻痹剂，使睫状肌放松或麻痹，从而得以缓解或消除。如果得不到适当处理，仍长期使用近距离目力，能促使眼球前后轴拉长，形成真的轴性近视。因此，在假性近视阶段，应加强预防措施，严格注意用眼卫生，适当减少阅读和近距离工作使之消除更多的调节，如不缓解，则应用睫状肌麻痹剂或雾视法以解除其调节痉挛，短时间停止看书，并可作眼部按摩、远眺、户外活动等，切勿在未经扩瞳验光的情况下，急于配戴近视眼镜，以致迫使付出更多的调节，形成程度很深的近视。
此外，高度远视及高度散光的人，视远、近物都不清楚，为了得到较大的影象，勉强提高视物能力，常把目标物拿得很近。这种形似高度近视状态者，习惯上也称为假性近视，但与上述功能性近视是完全不同的。
