Author Topic: Diopters, Lenses, and Measurement.  (Read 1620 times)

Offline OtisBrown

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Diopters, Lenses, and Measurement.
« on: January 01, 2014, 05:06:17 AM »

Subject:  Since we talk about visual acuity and diopters, I will post some short videos about self-measurement here.

Very few people know what a "diopter" is, in terms of a lens and their visual acuity.  I will post some short videos on this thread for your enjoyment and discussion.

Offline OtisBrown

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Re: Diopters, Lenses, and Measurement.
« Reply #1 on: January 01, 2014, 05:13:33 AM »
Subject: The Snellen chart - and a 1/2 and 2 diopter lenses, plus and minus.

The effect of a minus was discovered by Johann Kepler. He had spent 16 years doing intensive close work.  Eventually he found that his distant vision was "blurry", and that by holding up a -2 diopter lens - he could clear his distant vision.

http://www.youtube.com/watch?v=cKGMmpPQcCQ

The minus lens has been used that way ever since.  No one has given thought to the idea that intensive close work produces negative status for all natural eyes.  It is very hard to "resist" the use of a minus lens.  But that is how simple these lenses are.


Offline OtisBrown

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Re: Diopters, Lenses, and Measurement.
« Reply #2 on: January 01, 2014, 05:20:19 AM »
Subject: I always believe that I should know what I am doing - and do my own testing.

This is because I will believe my own OBJECTIVE measurement.  Here are what these hand-held lenses look like, and how I obtain them.  There are some persons who will insist that I am "practicing medicine on myself".  That is simply not true.  I am protecting my  distant vision, by making these critical measurements myself.  I have no desire to "fight" with an OD about the fact that I must protect my own vision - by doing this under my control.

http://www.youtube.com/watch?v=5GQ9rPTRtoM

In fact, it can be difficult to determine if a lens is "plus" or "minus".  This is of course basic physics, but you must be able to tell by doing your own checking.



Offline OtisBrown

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Re: Diopters, Lenses, and Measurement.
« Reply #3 on: January 01, 2014, 05:25:10 AM »

Subject: How can I objectively measure a plus and minus lens?

Here is a very simple way to do it.

http://www.youtube.com/watch?v=jYGnygF9eN4

The plus 1/2 diopters will form a "point" at 2 meters, or 6 feet (approximately).  The minus lens does not do that, because it diverges rays of light.

The minus lens creates an "artificial" near environment - which is how you got into nearsightedness - in the first place.

Offline OtisBrown

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Re: Diopters, Lenses, and Measurement.
« Reply #4 on: January 01, 2014, 05:35:09 AM »
Subject: Assuming you can still read the 20/40 line - how do I read with a plus lens.

Here are two videos on how to read through a plus lens, and "push print" away from you face to find the 1) Total blur point, and the 2) Pull it in until it is SLIGHTLY blurred.  The reason for doing this - it to get maximum PREVENTIVE effect from that lens - every time you use it.

http://www.youtube.com/watch?v=LXMXO-ve9eo

Here is another description - of how to use a plus "for near".

http://www.youtube.com/watch?v=jok4jAMmkNc

It should be clear that his is "plus prevention".  You must still be able to read the 20/40 line - to truly use the plus in this manner.

This is my "declaration of independence" from an OD in his office.  I know he has no interest in my distant vision, or my judgment that I must do this "pure prevention" myself.

The basic techniques of checking your own Snellen and reading though a plus, is not that difficult.  What is hard to accept, that your vision will go down by -1/2 diopter per year, if you choose to "not wear" the plus during the school year.  We always prefer the "easy way", don't we?

Offline Alex_Myopic

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Re: Diopters, Lenses, and Measurement.
« Reply #5 on: January 01, 2014, 11:33:02 AM »
Optics is quite difficult and interesting science. In 19-22 sec of http://www.youtube.com/watch?v=cKGMmpPQcCQ we see how the camera autofocuses from +0.5D to +2D filter in order to focus at 6m on the Snellen chart. Of course the myopic or even the emmetropic eye cannot focus so clear with a +2D lens at 6m as the camera in 22sec.

An interesting question is how don't we see the image reversed with very strong plus when in this video only with +1D lens over the emmetropia is reached the retinoscope produces an inverted image in the retina:
http://www.youtube.com/watch?v=kAreDffuVCQ