Difference between 5" letters at 20' and 10" letters at 40'?

Is there a difference to the eye between 5" letters at 20', 10"
letters at 40', 20" letters at 80', etc.?



--

 

Mike , do you mean that it is same when a person uses 20/10 line at 10
feet for 20/20 and 20/20 line at 20 feet? What could be the peripheral
and atmospheric cues? Lighting?

John

 

Dear LurfysMa,

Subject: Eye-chart letter size, and resolution
of the eye.

Re: How this test was set up by Dr. Snellen


Re: Is there a difference to the eye between 5" letters at 20', 10"
letters at 40', 20" letters at 80', etc.?

The intention of this "Standard" was to
have eye's checked at 20 feet (or 6 meters).

It was judged that 6 meters was far enough to be considered "optical
infinity" for the eye.

The human eye ( under good circumstances)
can "resolve" to 1 minute-of-angle.

Snellen made the letters 5 minute-of-angle in size, which makes the
letters 0.9 cm at 6 meters.

Keeping the angle constant, then this (20/20) letter would be 1.8 cm at
12 meters.

Thus you are measureing the resolving power of the human eye by this
method.

For eas of use, some charts can be designed for 3 meters. (Small
office.) Then, you could say 10/10 -- to be technically correct.

Best,

Otis
Engineer



--

 

That was my thinking, too, but it doesn't seem to work that way.

I have been trying on contacts the past few weeks. I have been using
reading glasses for 20 years, but never had a problem with distance. I
swear I used to be able to read license plates a block away (I wish I
had taken good measurements).

I went in for an annual eye exam and the doctor told me I was
borderline for passing a DMV eye test. I had noticed that distance
vision was not as good, but I guess I hadn't realized how much it had
degraded.

Anyway, I went to see about contacts. The optician used the standard
eye chart to determine which lenses were good for me. It seems to me
that lenses that make the bottom line on the eye chart clear do not
make it easer to read street signs and license plates at much longer
distances.

I've read here about focal length. Is that a factor?

We tried monovision and that didn't work at all for me. I might not
have given it a long enough trial, but I couldn't see well enough to
drive safely. Close up vision was great.

I now have distance-only contacts. They seem to help the most at
intermediate range, but not much at long distance.

The optician said that far-sighted people are the hardest to fit.
They are used to being able to see gnats on mountain tops and they
aren't happy with anything less.

Any suggestions? I need to go in next week for another fitting.

--

 

Dear Robert,

Thanks for your input.

The word "emmetropia" has several meaings, including refractive
states running from -1/2 diotper to +3/4 diopter -- unless you
specify that "emmetropia" is a refractive state of EXACTLY
zero.

The remark of the 10 foot (3 meter) is just the simple
fact that some doctors use is for simple convenience.

The "offical" chart remains at 20 feet, and refractive
status to be measured with a trial-lens kit (or
phoropter).

Best,

Otis

 

!! I've never thought of that. I always assumed the moon
looked larger low in the sky due a magnification effect of
the atmosphere. Low in the horizon you are seeing it though
more atmosphere than overhead. I'll have to measure the
image next time.

-Quick

 

Dear LurfysaMa,

Just for the fun of it you might enjoy checking your DMV-Snellen at:


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

Just type 1 inch for line-length, and 10 feet for distance
and see what you can read.

The DMV generallly requires 20/40 or better in most states.

If the "contacts" bother you, and you pass the DMV, you could
go back to using (plus) reading lenses.

Best,

Otis
(Engineer)

 

Just for the fun of it you might enjoy checking your DMV-Snellen at:

Otis, For some reason (OK, the design) they have a horizontal split in the
screen that make the screen difficult to scroll.

Don W

 

Ummm, getting back to earth... If I have a 1 mm segment
in my contact lens, what is the size of the image that would
be covered at 30"?

thanks,
-Quick

 

Dear Don,

Subject: Snellen Chart for Your Monitor.

The "Snellen" does not "scroll". Just click on "Display" to get
a new set of random letters.

You can change the size of the letters by clicking
on "Larger" and "Smaller".

Easy!

Best,

Otis

 

Using simple geometry and some fairly crude assumptions/estimations I
get about 30 mm or 12 inches. That of course would be best case
scenario, and assumes perfect (infinitely thin) ledges/lines. Your
mileage will vary...

w.stacy, o.d.

 

Perfect! That should just about cover the viewable area of my
20" CRT... Although I guess you don't really look at that much
at one time? Seems that if I look at a spot or word of text anything
outside of about a 4" diameter circle is sort of "peripheral"
to what you are looking at and you can't really tell if it's in focus
or not. You have to shift your gaze past that.

Surely this has been quantified/characterized?

(should I have started a new thread? or is this still
on topic for this one?)
thanks,
-Quick

about 30 mm or 12 inches. That of course would be best case scenario, and
assumes perfect (infinitely thin) ledges/lines. Your mileage will vary...

w.stacy, o.d.

Quick wrote:
Ummm, getting back to earth... If I have a 1 mm segment
in my contact lens, what is the size of the image that would
be covered at 30"?

thanks,
-Quick

Robert Martellaro wrote:

On Wed, 17 Aug 2005 13:52:19 GMT, "Mike Tyner"



"Peripheral" cues are all those other surrounding
objects, relative motion, relative size - for instance,
why the moon looks bigger when it's low in the sky.

Although this explanation has merit, the primary effect
is a physiological one.
As one tilts his head back to observe the moon at the
zenith, the lens of the
eye is flattened by gravity and its magnifying power is
decreased. (Thanks to
the folks at sci.astro)



Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical

~~~~~~~~~~~~~~~~~~
"An expert is a person who has made all the mistakes that
can be made in a very narrow field." - Niels Bohr

 

You could do that at any time (like I just did). I don't think trifocal
contact lenses have been studied much. Good luck with them. Sounds like
you're on the right track. Let us know how it works out, since you are
probably a pioneer in this area.

w.stacy, o.d.

 

doh... all those business terms come to mind. "opportunity",
"interesting problem". I was starting to get clues though.
"maybe only at handful have fitted those in the US" and
when Fused Kontacts told me they didn't have a fitting kit
for the trifocals but they did have very good consultants
that would work directly with my OD to design the lenses.

I'm looking forward to it. Seems there is a large hole with
bifocals. One can go with multifocal simultaneous vision
solutions which I've come to gather all have "compromises"
that are significant. Sacrifice off of both ends or significantly
off of one end or the other or resort to monovision. The flare,
"soft" vision and other anomolies. I was a bit surprised this
hasn't all been sorted out some time ago.

I'm a slightly hyperopic (~+1.00) presbyope (~+2.00) who
sits in front of a computer all day. I've always been very
picky about my vision. I guess I could be in the worst case
section of the fitting text book. The whole idea is to do
without spectacles. I mean, that's what contacts are for right?
I can do without correction just fine for distance although
I do miss the sharpness of being able to see the tree
line on a mountain ridge a few miles away. I want to get
rid of the readers. If I have to insert and remove contacts
depending on what I'm doing then we're back to readers.
So my goal is perfect distance, perfect reading, and perfect
computer (~30"). I'm willing to compromise at about 5 feet
which is the distance between my nose and the golf ball
on the tee since I can't golf like I used to anyway (and I've
accepted that ). Besides, once you're lined up you should
be able to swing with your eyes closed. I think I can deal
with adapting to moving my head more than my eyes most
of the time.

So it seems that all the stuff is aimed at broad range with
"compromise" everywhere or 2 focal points, distance
and some point closer. I would have thought 3 focal points
to be the minimum requirement and more would have
been done with trifocals already?

-Quick

 

As you probably know, spectacle trifocals are quite common and
appreciated by many folks who have zero or close to zero accommodative
amplitude, and for those 3 focal lengths is necessary/useful.

The problem with trifocals that when either (or both) seg line infringes
on the pupillary area, there is a certain amount of light scatter and/or
double refraction at and around the line(s).

Our theoretical clear zone of 12 inches with the 1 mm intermediate seg
on the TS Tri would be well overlapped by interference from both
distance and near zones, since the pupil size is probably on the order
of 3 mm. Hopefully the "main" clear image through the intermediate seg
would overshadow the other "focuses" to the extent that you won't have
appreciable degradation of your vision. YMWV

At least with the TS bi you can get the pupil fully covered by the
distance area, with no interference from the seg when viewing distance
objects, and vice-versa with near. What's probably kept me from trying
the TS tri is that I'm pretty much guaranteed *some* interference in
that mid seg.

But however much interference there is from the lines themselves and
from the other focal areas, I'm willing to bet that it will still give
better vision than you'd get with ANY of the aspheric (smeared optics)
alternatives, and maybe, just maybe it won't even be noticeable. That's
why I'm interested in your input, as I always like to hedge my bets.
Again, you'll be the first I ever encountered who has tried them, and
I've been around a while (have fit a dozen or so TS Bis over the years).

w.stacy, o.d.