Why do we use 20ft?
A patient asked me the other day, 'Why is 20ft the standard for refraction?' I had to admit had no idea. From a little research I see that the Snellen chart is designed around 20ft/6m (duh) but no reason why. Does it have to do with accommodation? Printing technology in Snellen's time? Or is it just arbitrary?
1/6M = X diopters?
1. Accommodation
2. Close enough to refracting for infinity.
3. Not close enough for Barry
The rays emanating from an object 20' away are functionally parallel when entering the eye, as they would be in actuality from an infinitely distant object - but we'd still be waiting for the infinitely-distant object's rays to arrive. Does that answer your question?
20 feet was settled upon by Snellen becasue he wanted others to condust acuity tests around Europe as well, and he determined that the longest, most commonly found room dimension that others could access would be approx. 20 feet. BTW, this is equal to 0.16D, so it does NOT represent infinity.
B
The Snellen Chart was designed in the 1860s right? And the updated LogMAR was introduced sometime in the 1980s I beleive. So even that 'new' concept is over 30 years old! Is there anything newer, more accurate, or more comprehensive as far as VA testing during a refraction? With all the work/discussions we have about POW, head tilt, lifestyle dispensing etc, are there any new ideas for the patient's time in 'The Chair' that take any of that in to account? It does seem the standard DVA/NVA calculations aren't quite cutting for most patients any more. I'd say its time for some innovation! Bring on the Holo-deck!
Patient, ".. Doctor says I have a subscription for stigmata.. Can you fill that?"
Me, "..Um.. "
Yes:Originally Posted by vcom
Try
www.dyops.org
B
Do you use in your office? Looks like it works.
I have the version on my PC. Yes...its works...and well.
B
Refraction is as much art as it is science in my humble opinion.
To prove my point I'd love to put the same patient in the same chair with the same chart and see what 3 or more refractionists say is the rx.
The time of day, what and when you've eaten I've heard, can also alter the rx. It's why making a lens to the .01D causes MD's to chuckle.
School also told us that 20' was used as optical infinity but it's nice to hear the back stories and that was before the wheel was round.
Minutes of Arc is the answer. Each portion of the big E represents 1 minute of ARC The letter portion plus the blank or white portion. So the big E has 5 minutes of arc based on principle light theory. 20/20 Vision is the ability to resolve a spatial pattern seperated by a visual angle of one minute of arc.
Last edited by PRECISIONLAB; 01-06-2014 at 06:30 PM.
The story goes... The military in 1860's wanted to screen out soldiers that had weak eyes... better to give a rifle to someone with 'good' eyes. Snellen was an old man, but his 22-year old assistant had 'good' eyes, so the line size he could see at 20 feet was considered the new 'standard' for good eyesight.
Wow, Optiboard is pretty awesome! Thanks for the great answers guys. The patient is coming by today, so I'm stoked to have an answer for him.
"Refraction is as much art as it is science in my humble opinion.
To prove my point I'd love to put the same patient in the same chair with the same chart and see what 3 or more refractionists say is the rx.... It's why making a lens to the .01D causes MD's to chuckle."
I have always said that a refraction is like a weather forecast; an educated opinion as to what will happen.
Have to agree about the .01D. When I start seeing accurate refractions done to .01D, I'll feel a greater need to obtain lenses ground to .01D. Can't wait for it to happen!
Last edited by waynegilpin; 01-08-2014 at 10:51 AM.
The story I was told and still believe to be the most accurate explanation is as follows: Snellen, was a great baseball fan and the distance from the pitchers mound to home plate is approximately 60'. There are also 3 bases, so 3 divided into 60' gives us 20'. I hope that this gives you further clarity, by the way don't bother looking up any historical references, as the parchment it was written on has long since been destroyed with the addition of the designated hitter. Hope this helps.
I've heard this a lot, but I think it's wrong, even if it seems intuitive; the fact that refractions are not terribly precise means that lenses must be produced so as to more, not less, accurately implement the Rx. If the refraction is off by something just shy of being discernible, a less-accurately-ground lens may push the total error into being discernible, while an accurately-ground one wouldn't.
That's generally true; having some inherently imprecise element in any system requires that the other elements in the system be more precise, or the system will more likely fail to meet the requirements it's intended to meet.
The 0.01D precision is not really about the paraxial region in good FF. Its about this precison allowing the execution of global optimization.
B
I like that Barry.
If 3 Doc's find 3 rx's would averaging them out give us the best rx? (Playing devil's advocate here and waiting for Shanbaum's objection!)
I don't know what you mean by "what it's about" (except in the context of the Hokey Pokey). There's a substantial contribution to accuracy in not having to round curves to those in the set of available laps, especially with high index materials, whether you're optimizing the surface for POW (or to eliminate marginal astigmatism, or power error) or not. Of course, those optimizations aren't really available with traditional surfacing anyway (unless the modifications are so significant as to cause a different lap to be used, and then only in a very coarse way - as coarse as the increments of the laps). If you're saying the optimizations are more important to you than the increased accuracy - OK. My dispute is with the assertion that because "the Rx may be wrong anyway", you can't benefit from the greater accuracy of FF processing. I say you can, for the reasons I stated.
Agreed!
B
When do you use this and why? I showed to my doctors and they are all for anything that helps, looks cool and can differentiate us from the rest of the world.
I could mount a monitor on the wall and show the chart to see the VA with current glasses and help get some exams for our doctors.
Thanks as always for your help.
Ha! You would need an infinitely large waiting room.
One work-around is to load your phoropter with lenses that have a refractive index of n=0, resulting in infinitely fast light.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
The Snellen Test as a "Pious Fraud"
Dear Pogu and Vcom,
The major advantage of the Snellen test in 1862 is that it provided a simple benchmark test for vision researchers that was easy to reproduce and adequate for 1862 standards and vision needs.
The convenience of the test in 1862 was far more important than its scientific validity. The range of 20/200 to 20/10 images could be displayed on one side of an 8.5 inch by 14 inch piece of paper. It was also profitable for Snellen as the copyright owner.
The difficulty 150 years later is that scientifically the test is a inaccurate as well as inadequate for today’s computer literate population.
The Snellen concept of "acuity" was based upon the ability to discern the smallest gap between two lines using the irregular gaps in letters. Instead, the Snellen test is more of a measure of cognition than acuity. And even the culturally neutral ("academic") Landolt test is flawed by relying on Snellen's assumptions. The actual Minimum Angle of Resolution for the eye is closer to 0.5 arc minutes than 1.0 arc minutes. However, the use of a smaller 0.5 arc minute gap would have resulted in the letters being about 1/3 larger and the Snellen test (and Landolt test) having to be printed on two pages thus increasing the cost. The thinner letters would also have resulted in the smaller 20/10 letters smudging and blurring due to 1862 printing techniques. Snellen's use of 1.0 arc minutes for the Minimum Angle of Resolution was akin to using 5.0 as the value of Pi.
See http://www.dyop.org/professional.htm for information on the 21st century refraction test with the specific test at http://www.dyop.org/documents/DyopRefraction.html
Regards Allan@Dyop.org
Additional comment to Pogu and Vcom and anyone else:
The Snellen/Landolt concept of LogMAR is a lie resulting from the excess thickness of the Snellen/Landolt images rather than the decrease in acuity. The 10% stroke width a Dyop image has an almost linear Best Visual Acuity (BVA) ratio with 4 diopters of blur having a Dyop BVA of about 20/90. The Snellen 20/320 BVA ratio has more to do with the excess Snellen stroke width and the fuzziness of projected image tests than the actual acuity of the patients. The 0.5 diopters in difference between the projected and computerized Snellen tests (see links below) indicates just how arbitrary the Snellen test actually is.
http://www.dyop.org/documents/2013_AAO_Harris.pdf
http://www.dyop.org/documents/2013_A...a-Summmary.pdf
Wow! great response!!Dear Pogu and Vcom,
The major advantage of the Snellen test in 1862 is that it provided a simple benchmark test for vision researchers that was easy to reproduce and adequate for 1862 standards and vision needs.
The convenience of the test in 1862 was far more important than its scientific validity. The range of 20/200 to 20/10 images could be displayed on one side of an 8.5 inch by 14 inch piece of paper. It was also profitable for Snellen as the copyright owner.
The difficulty 150 years later is that scientifically the test is a inaccurate as well as inadequate for today’s computer literate population.
The Snellen concept of "acuity" was based upon the ability to discern the smallest gap between two lines using the irregular gaps in letters. Instead, the Snellen test is more of a measure of cognition than acuity. And even the culturally neutral ("academic") Landolt test is flawed by relying on Snellen's assumptions. The actual Minimum Angle of Resolution for the eye is closer to 0.5 arc minutes than 1.0 arc minutes. However, the use of a smaller 0.5 arc minute gap would have resulted in the letters being about 1/3 larger and the Snellen test (and Landolt test) having to be printed on two pages thus increasing the cost. The thinner letters would also have resulted in the smaller 20/10 letters smudging and blurring due to 1862 printing techniques. Snellen's use of 1.0 arc minutes for the Minimum Angle of Resolution was akin to using 5.0 as the value of Pi.
See http://www.dyop.org/professional.htm for information on the 21st century refraction test with the specific test at http://www.dyop.org/documents/DyopRefraction.html
Regards Allan@Dyop.org
Reputation acknowledgement on the way!!!
Keep posting!
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