Author Topic: H. Snellen and normal vision, and normal refractive STATES.  (Read 727 times)

Offline OtisBrown

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H. Snellen and normal vision, and normal refractive STATES.
« on: January 29, 2015, 09:58:05 AM »
Subject: How normal (20/20) vision (emmetropia) vision was defined.

Herman Snellen developed and designed the "Snellen Chart" we use to objectively measure our visual acuity.  He found that a lot
of normal people could read the 20/20 line, on his chart, so he called this normal, vision.

It is true that some people could had slightly better visual acuity than that, but the average was that they could read
most of the 3/8 inch letters at 20 feet.

The word emmetropia, means normal, 20/20 vision - since if you define normal for an entire population
of natural eyes - they should all be able to read the 20/20 line.


If you have a problem reading the 20/40 line, on your bright Snellen, you know your visual acuity.  To
determine your own refractive STATE, you need a -1, and -1/2 diopter lens.

You simply hold up the -1/2 diopter, and see what it clears.  If it only clears the 20/25 line, then
you must measure with the -1 diopter.

If that clears the 20/20 line for you, then your refractive STATE, is -1.0 diopters.

If that still does not clear the 20/20 line, then just put the -1 and -1/2 together, making a -1.5 diopter lens

Then check again.  If it takes that double-lens to clear the 20/20 line, then you have a
refractive STATE of -1.5 diopters.

I think this definition of refractive STATE can be understood by most people who actually
check their Snellen visual acuity, and refractive STATE - personally.

I am certain that this does take a lot of "smarts", and personal resolution to be this
completely involved.  But it is a good scientific lesson in how to make
personal, objective measurements, of the refractive STATES of the natural eye.

You do yourself proud, if you master this skill.

« Last Edit: January 29, 2015, 01:01:27 PM by OtisBrown »

Offline OtisBrown

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Re: H. Snellen and normal vision, and normal refractive STATES.
« Reply #1 on: January 29, 2015, 01:00:08 PM »

Hi Hans,

Subject: I think being able to personally measure your refractive STATE, is very important for all of us.

Item:  This is time for an excellent debate about the definition of normal 20/20, and objective measurements you make yourself.

I want to make personal and objective measurements - that I can compare with others posting here.  To be consistent, I *must* measure relative to the 20/20 line.  If we all did this - we could see objective progress - by our own measurements.

Hans>  With -1 diopters, I can read further than 20/20, 20/18 to 20/15. According to Otis Brown, I would have a "positive refractive state", which is obviously not true because distant objects are still blurry.  What do you think about it? Does 20/20 mean that you are "emmetropic" (refractive state of 0.0 diopters)? I say no.

Otis> Thus 20/20 is emmetropia, when self measured.

Offline CapitalPrince

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Re: H. Snellen and normal vision, and normal refractive STATES.
« Reply #2 on: January 29, 2015, 01:39:50 PM »
an OD prescription is totally meaningless. 90% of the population will be prescribed a minus or astigmatic no matter how well you see.

usually vision below 20/40 in room lighting is considered nearsighted.

once you are below 20/20 you begin to notice a slight blurriness in the distance, and may start to affect your life. 20/20 should be considered the lowest "normal vision".

to have perfectly sharp, no blur, eseentially 20/20 night vision would probably require 20/13 in room lighting and to be able to read the 20/20 with a +0.5D.