Author Topic: Questions about Snellen chart  (Read 5753 times)

Offline johnlink

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Questions about Snellen chart
« on: February 27, 2013, 02:07:22 PM »
1) Does it matter whether one uses a chart designed for use at 20 feet or a chart designed for use at 10 feet?

2) I have found several sources online to buy a Snellen chart, but does anyone have suggestions for where to buy?

John Link

Offline OtisBrown

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Re: Questions about Snellen chart
« Reply #1 on: February 27, 2013, 07:24:11 PM »
Hi John,

1)  You can use the 10 foot chart - just put a bright light on it.  It makes only a slight amount of difference.  I use a 20 foort chart, just to make my reading offical.  I use my own Test lenses (a -1.0 and -1/2 diopters) for the same reason. 

2)  You can obtain a Snellen chart for about $9.00.  But you can get them free on i-see.

3) Here is an Electronic Snellen - you can check yourself -  click here:


http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html

Then click on "display" several times.  The chart is set for 20 feet, so just back aways to 20 feet.  You will need someone to press "display" for your.

This is a rough-check - to get you started.  I have found it imposslbe to judge Snellen reading, from a "prescription". Let us know if this is OK!



1) Does it matter whether one uses a chart designed for use at 20 feet or a chart designed for use at 10 feet?

2) I have found several sources online to buy a Snellen chart, but does anyone have suggestions for where to buy?

John Link

Offline PROH

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Re: Questions about Snellen chart
« Reply #2 on: February 28, 2013, 01:04:26 AM »
Hi John

Technically you can use either, but I feel its better to use 20feets as this minimise errors of distance & is more standard.

You can use
http://www.i-see.org/block_letter_eye_chart.pdf


Proh

Offline aparna

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Re: Questions about Snellen chart
« Reply #3 on: April 11, 2013, 06:47:26 AM »
I have been using the snellen chart to test my son. My son was tested at -4.75 about 6 months ago. We started him with +1 and after 2 months +2 which is what he uses at home. At school he feels comfortable with +1.5 and refuses to use his +2. When we tested last week he was -4.5.
My question is to do with the snellen reading.

I tested him in a dark room, and a lighted display with this link :http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html, he can read 20/200 consistently. 20/100 is a little bit of a struggle, so I wont count that.

Outside in the shade (not bright sunlight) he will read 20/100 and can kind of make out 70ft line.

Yet in the optometrist's office he was able to read only the 400 ft line consistently. Now I am confused, is he 20/200 or 20/400 ?

Alex Fraunfeld's cm calculator said -2.5 (based on where my son said the text blurred)



 

Offline OtisBrown

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Re: Questions about Snellen chart
« Reply #4 on: April 11, 2013, 09:22:40 AM »
Deaar Arpena,

We make a tragic mistake to ASSUME that all ODs measure the same way - and that there measurements are "not biased".

In fact, if you go from OD to OD, you could well find values as different as 1, 2 and even 3 diopters.  Your eye's did not change, but the person(s) making the measurements made assumptions that were wrong.  This is why I check my own refractive status with my own Snellen and trial-lens kit.  The OD in his office, simple does not have the time to make an accurate measurement.  Here is my commentary:

+++++

I have been using the snellen chart to test my son. My son was tested at -4.75 about 6 months ago.

Otis>  These ODs BELIEVE in "maxium strength minus lens".  They "correct" for 20/20, 20/15 and even 20/10 vision.  They call this "Best Visual Acuity".  But it means for SOME people, they can read the 20/30 line - but get PRESCRIBED a -2.5 diopter lens - for full-time wear.  They are also told to wear this -2.5 ALL THE TIME.  That is the most destructive thing to do - if you are trying to avoid entry into myopia.


We started him with +1 and after 2 months +2 which is what he uses at home. At school he feels comfortable with +1.5 and refuses to use his +2. When we tested last week he was -4.5.
My question is to do with the snellen reading.

Arpena> I tested him in a dark room, and a lighted display with this link :http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html, he can read 20/200 consistently. 20/100 is a little bit of a struggle, so I wont count that.

Otis> I do all my testing with a STARARD bright light on my Snellen chart.  I suggest you do the same thing for consistency and comparison.  We do not live in a "dark room" all the time.

Arpena> Outside in the shade (not bright sunlight) he will read 20/100 and can kind of make out 70ft line.

Otis>  For a child, 20/70 is "functional". The fact that he can read through a +2.0 indicates that his vision is not as bad a -4.5 diopters.  I personally check using my own low-cost test lenses.  If you did that, I think you would find a -2.0 would "just clear" the 20/20 line for your son.


Arpena> Yet in the optometrist's office he was able to read only the 400 ft line consistently.

Otis> I, personally, have no "trust" in any OD measurement.  TRUST what you objectively measure.  That is what "counts" in this effort.


Arpena> Now I am confused, is he 20/200 or 20/400 ?

Otis> Stop the confusion.  The OD is making very poor measurements.  Trust  what YOU measure.  That is what truly counts.  If you continue to wear the plus, (and I hope your child does), for the next six months, your child might be able to read the 20/60 line on your own home Snellen. That is the only way you can measure his success.






I have been using the snellen chart to test my son. My son was tested at -4.75 about 6 months ago. We started him with +1 and after 2 months +2 which is what he uses at home. At school he feels comfortable with +1.5 and refuses to use his +2. When we tested last week he was -4.5.
My question is to do with the snellen reading.

I tested him in a dark room, and a lighted display with this link :http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html, he can read 20/200 consistently. 20/100 is a little bit of a struggle, so I wont count that.

Outside in the shade (not bright sunlight) he will read 20/100 and can kind of make out 70ft line.

Yet in the optometrist's office he was able to read only the 400 ft line consistently. Now I am confused, is he 20/200 or 20/400 ?

Alex Fraunfeld's cm calculator said -2.5 (based on where my son said the text blurred)



 

Offline johnlink

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Re: Questions about Snellen chart
« Reply #5 on: April 11, 2013, 01:58:02 PM »
I have been using the snellen chart to test my son. My son was tested at -4.75 about 6 months ago. We started him with +1 and after 2 months +2 which is what he uses at home. At school he feels comfortable with +1.5 and refuses to use his +2. When we tested last week he was -4.5.
My question is to do with the snellen reading.

I tested him in a dark room, and a lighted display with this link :http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html, he can read 20/200 consistently. 20/100 is a little bit of a struggle, so I wont count that.

Outside in the shade (not bright sunlight) he will read 20/100 and can kind of make out 70ft line.

Yet in the optometrist's office he was able to read only the 400 ft line consistently. Now I am confused, is he 20/200 or 20/400 ?

Alex Fraunfeld's cm calculator said -2.5 (based on where my son said the text blurred)

I have three questions for you:

1) At what distance does your son use the +2 at home?

2) At what distance does he use the +1.5 at school?

3) What is the greatest distance at which your son can read books with his naked eyes?

The 20-foot measurements exhibit wide variation based on the conditions in which they are made. I suspect that the assessment of -2.5 based on the Frauenfeld calculator is the most reliable measurement of all.

Offline OtisBrown

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Re: Questions about Snellen chart
« Reply #6 on: April 11, 2013, 03:34:38 PM »
Hi Aparna,

While the Electronic Snellen is good for a quick general check - I strongly suggest you down-load this chart.

Put it at 20 feet, and shine a bright light on it.  (There is considerable variation in what you read - if you don't do that.)  Obviously we must have reasonable goals - that we must meet.  I drive a car - so I must exceed the 20/40 line.  With a child it is different - he does not have to exceed the 20/40 line. But reason tells us that he must eventually read and pass the 20/60 line.  Here is the chart - just down-load it.

http://www.i-see.org/block_letter_eye_chart.pdf

You should allow your child to get "comfortable" with glancing and reading the chart.  Don't pressure him to do so.  Just have him understand that it is important in his life.

This will take time, provided he can continue to wear the +1.5 to +2.0 for all close work.  If he is persistent, then I would expect he could begin to read the 20/60 line in about six months.  This is very difficult in terms of understanding and effort.


I have been using the snellen chart to test my son. My son was tested at -4.75 about 6 months ago. We started him with +1 and after 2 months +2 which is what he uses at home. At school he feels comfortable with +1.5 and refuses to use his +2. When we tested last week he was -4.5.
My question is to do with the snellen reading.

I tested him in a dark room, and a lighted display with this link :http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html, he can read 20/200 consistently. 20/100 is a little bit of a struggle, so I wont count that.

Outside in the shade (not bright sunlight) he will read 20/100 and can kind of make out 70ft line.

Yet in the optometrist's office he was able to read only the 400 ft line consistently. Now I am confused, is he 20/200 or 20/400 ?

Alex Fraunfeld's cm calculator said -2.5 (based on where my son said the text blurred)



 

Offline aparna

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Re: Questions about Snellen chart
« Reply #7 on: April 12, 2013, 05:45:08 AM »
We have been telling him to put his chin on his hand and read...seems to be about 8-9 inches...I believe that he will be able to push the book further as his eyes get better at reading with the +2.
I have been using the downloaded chart for testing him in the shade.

----------
Aparna> Outside in the shade (not bright sunlight) he will read 20/100 and can kind of make out 70ft line.

Otis>  For a child, 20/70 is "functional". The fact that he can read through a +2.0 indicates that his vision is not as bad a -4.5 diopters.  I personally check using my own low-cost test lenses.  If you did that, I think you would find a -2.0 would "just clear" the 20/20 line for your son.
-----------
Does this mean that his test in the shade is valid ? The optometrist was of the opinion that testing in daylight causes a pin hole effect that will obviously enable him to see better. Hence its a not a true estimate of his vision.

Thoughts ?


Offline johnlink

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Re: Questions about Snellen chart
« Reply #8 on: April 12, 2013, 06:05:45 AM »
We have been telling him to put his chin on his hand and read...seems to be about 8-9 inches...I believe that he will be able to push the book further as his eyes get better at reading with the +2.

I'm not sure which of my three questions you meant to answer. Would you answer them all so that we could have a more complete understanding of your son's vision?

1) At what distance does your son use the +2 at home?

2) At what distance does he use the +1.5 at school?

3) What is the greatest distance at which your son can read books with his naked eyes?

Offline OtisBrown

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Re: Questions about Snellen chart
« Reply #9 on: April 12, 2013, 06:10:11 AM »
Dear Aparna,
I believe in doing my own measurements - as much as possible.  I do a lot of "home checking", of my own Snellen.  No OD, in an office, can do that type of consistent checking, and I need an excellent, consistent well-illuminated Snellen - to get results I TRUST.  If I put this Snellen in a "dark corner" - I think the result would 1) Not be consistent, and 2) Not be accurate, repeatable or valid - in any sense of the word.  I make my Snellen as close to the DMV test as possible - so when I go for their test (at 20/40) I am certain to pass that test.  For any child, reading the 20/40 line is very reasonable - if you use that consistent Snellen.  That is why I recommend that you do it in this manner.  If you child does get to the 20/60 range, after wearing the plus for six months, then I truly "do not care" what a "prescription" might be.  I will show you how to "prescribe" - for a "learning process" - if you wish.  As an adult, I can go "all out" to make a commitment to achieve results like this.  For a child - it is indeed very difficult.  But you do need a consistent Snellen so your child knows what he is doing.  Even he will know when he reads the 20/60 line.  Don't make it difficult for him.  (From a range of -4.5 diopters - that would be a profound success - so give it the time that it requires.)

Assuming that he continues to wear the PREVENTIVE +2 or +1.5, and he does begin to read the 20/60 line, that will be a major success.  You can function with 20/60 line in our society.  I truly hate wearing any lens for "distance" - if I can avoid doing so.  That is why it is YOUR objective measurements that counts for everything.  I know that this work will tragically set up "conflict" wit your OD - and I regret that "situation" - because I have been through it many times.

For now, get that Snellen, and get your child to continue to do this for the next six months - and aim for that objective goal.  If you "get there", objectively, then we can consider the next steps.

Otis



We have been telling him to put his chin on his hand and read...seems to be about 8-9 inches...I believe that he will be able to push the book further as his eyes get better at reading with the +2.
I have been using the downloaded chart for testing him in the shade.

----------
Aparna> Outside in the shade (not bright sunlight) he will read 20/100 and can kind of make out 70ft line.

Otis>  For a child, 20/70 is "functional". The fact that he can read through a +2.0 indicates that his vision is not as bad a -4.5 diopters.  I personally check using my own low-cost test lenses.  If you did that, I think you would find a -2.0 would "just clear" the 20/20 line for your son.
-----------
Does this mean that his test in the shade is valid ? The optometrist was of the opinion that testing in daylight causes a pin hole effect that will obviously enable him to see better. Hence its a not a true estimate of his vision.

Thoughts ?



Offline peterg

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Re: Questions about Snellen chart
« Reply #10 on: April 13, 2013, 06:41:05 AM »
Does this mean that his test in the shade is valid ? The optometrist was of the opinion that testing in daylight causes a pin hole effect that will obviously enable him to see better. Hence its a not a true estimate of his vision.

I am of the opinion that the optometrist is probably correct. However, this is a natural situation, our eyes will obviously always adjust to various depths of vision changes based on lighting conditions.

Vision lens prescribing (in my opinion) is figuring which "basic" lens under which test conditions (i.e. lighting) to subscribe at.   And given the current standard is one lens for all situations, you will indeed get a lens that over corrects in bright daylight and likely gives maximum accuity in the dark. 

This is why I believe wearing glasses or contacts full time leads to changes in our eyesight and vision system as a whole. Essentially, to be fairer we would need a magic lens that auto-corrects its stregth depending on the individuals needs at any one time.

By the way, reading glasses are tested in a similar fashion.  You are perscribed based on what amount of detail the optometrist believes you need to see at what distance. Again, this leads to using a lens in front of the eye in all situations, including those for which you don't need the lens (for example in bright daylight).

In both situations, it is an outcome (perscription) is influenced by the test conditions.

For a child in the classroom, my maximum recommendation would be a perscription that allows the child to see (modestly clear) regular blackboard writing from a middle distance inside the classroom under "regular" daylight  conditions (i.e. not too bright).   I suspect a correction to 20/25 on a snellon at 20 feet under average daylight would approximate.   Possibly 20/30.  But I'd have to test with my child in their classroom to see if it correlates.

Offline aparna

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Re: Questions about Snellen chart
« Reply #11 on: April 15, 2013, 05:59:12 AM »
My son measured between 16-17 inches with the naked eye and can read about 10 inches with his  +2. He refused to cooperate so couldn't test with +1.75. My son is 6 years old, likes reading books and gets pretty involved in them. We noticed that he leans forward once he starts reading. Hence the general rule that his chin has to rest on his hand when he reads. I think it requires some amount of self awareness to actively maintain the required distance. I know I cant expect a 6 year old to do this, so we are trying what we can.


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peterg> In both situations, it is an outcome (perscription) is influenced by the test conditions.
-----------
Agree. In the classroom, he seems to be able to manage with his current -2.5 prescription when needed.  Given that he can see 20/100 clearly in the shade, we didnt buy the new prescription.

Offline johnlink

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Re: Questions about Snellen chart
« Reply #12 on: April 15, 2013, 06:21:51 AM »
My son measured between 16-17 inches with the naked eye and can read about 10 inches with his  +2. He refused to cooperate so couldn't test with +1.75. My son is 6 years old, likes reading books and gets pretty involved in them. We noticed that he leans forward once he starts reading. Hence the general rule that his chin has to rest on his hand when he reads. I think it requires some amount of self awareness to actively maintain the required distance. I know I cant expect a 6 year old to do this, so we are trying what we can.

Your son being able to read with his naked eyes at 16 to 17 inches implies myopia of no more than 2.50 or 2.25 diopters. Your son being able to read with +2.0 lenses at 10 inches implies myopia of no more than 2.00 diopters. So his current prescription of -2.50 might be a little too strong. Perhaps, as Otis has suggested, he might be able to clear the 20/20 line with -2.00.

With the amount of myopia your son currently has I see no reason for him to use a plus lens when reading, provided he keep the reading material at least 16 inches away.

Offline OtisBrown

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Re: Questions about Snellen chart
« Reply #13 on: April 15, 2013, 06:50:43 AM »
Hi Aparna and John,

I am not an "excessive critic" of anyone.  But I try to understand HOW and WHY our refractive STATE changes from "plus" to "minus", when we are very young.  I can only "watch" this - and say almost nothing.

http://myopiafree.i-see.org/Kid10D.jpeg

I advocate prevention for people (pilots) who still have 20/40 to 20/60 vision (about -1.0 to -1.5 diopters) and can be expected to understand their personal goal-in-life.  The tragedy of a child (and I admit it) is that they can not be given "instructions", nor expected to follow the instructions.  I that sense, no one is to "blame" for 1) Entry into  negative status for the natural eye and 2)  The minus lens.

John, you are correct.  The child is between -2.0 and -2.5 diopters, not the -4.75 diopters "prescribed" for "Best Visual Acuity".  I know we all give advice here - but, for a child, I hold out for eventually reading the 20/60 line on a bright Snellen.  That would be a real challenge for an adult.  If the child reaches 20/60 on her HOME SNELLEN, then  the child can avoid wearing any minus (just keep it with her).  That is what I would pray for.  If she gets there, then continued wearing of the plus could get her to 20/40 range.  If you can drive a car with 20/40, there is no reason to wear any minus for a child.

Offline aparna

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Re: Questions about Snellen chart
« Reply #14 on: April 15, 2013, 06:58:33 AM »
Thanks John and Otis. It helps clarify where we stand now.
John, we asked him to wear atleast the +1.5 in school as he forgets to put his chin on his hand sometimes.
Just trying what we can to prevent it from getting worse.