Astigmatism + Myopia vs Astigmatism alone

Is it true that when calculating the error of an eye, it is ok to say
that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to
an astigmatism of -1.00? If not, which option (-075 + -0.25 or
-1.00)would be considered as "better vision" of the two?

 

Dear Pershing,

An astigmatic value can be converted into a spherical-equivalent.

What is done is to take 1/2 the astigmatic number and
add it to the spherical value.


Is it true that when calculating the error of an eye, it is ok to say
that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to
an astigmatism of -1.00?

Otis> No. That would be like stating that -1.00 diopter spherical
is the same as -1.00 astigmatism.

Otis> It would be more correct to say that the spherical
equivalent would be -0.75 - .25/2 or -0.87 spherical equivalent.


If not, which option (-075 + -0.25 or
-1.00)would be considered as "better vision" of the two?

Otis> The spherical part is determined first, for instance
if you read the Snellen at 20/70. A Stronger minus lens
would be used to clear the "best" line possible.
Once that is done, a cylender lens is used to
determine if your vision can be made sharper,
say to 20/15, say a -0.5 diopter lens at 80 degrees.

That is how your refractive STATE is determined with
a phoropter.

Best,

Otis

 

CORRECTION:




Is it true that when calculating the error of an eye, it is ok to say
that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to
an astigmatism of -1.00?

A prescription is stated spherical, followed by astigmatism and angle.

Thus

-0.25 diopters, -0.75 at 80 degrees (a typical prescription)

Spherical equivalent is

-0.25 - 0.75 / 2 = - 0.62 spherical equivalent.

Hope this clarifies how you convert spherical/astigmatic
into spherical equivalent.

Best,

Otis

 

If someone has +0.25 myopia and -1.00 astigmatism its spherical
equivalent is -0.25 right?

Thanlk you for your help.

 

That would be correct.

Perhaps Mike could check your calculation.

Otis

 

Good thanks, In that case I have some more small questions.

Given http://www.toronto.drdc-rddc.gc.ca/medical/visreq_e.html In
section 8 there is a standard for Pilot applicants in regard to Myopia,
Hyperopia and Astigmatism. I would like to make sure I am interpreting
the table correctly.

1. It says that for Astigmatism "Cylinder power not more than 0.75D in
either eye". If there is no sign in the number, are we talking about
max being + 0.75 and - 0.75 or there is only a limit on the positive
side (in which case -1.00 would be acceptable)?

2. For Myopia "Not more than - 0.25 spherical equivalent in either
eye". This is a reference to the actual spherical equivalent
calculation of (Cyl/2) + Sphere as described in above post or is it the
spherical equivalence of (-0.25/2) + Sphere?

3. Is the scale allowing for values between 0 and 0.25? I am trying to
see if (-0.75/2) + 0.175 = -0.25 spherical equivalence is a valid
combination.

Thanks again.

 

My current readings from yesterday are:

1. Right eye: +0.25 (Sphere), -0.25 (Cyl) * 150 , uncorrected vision
6/6 and no other problems
2. Left eye: 0 (Sphere), -1.00 (Cyl) * 180 , uncorrected vision 6/7.5
and no other problems

I was tested on another occasion about 6 months ago and my reading was
0 (Sphere), -0.75 (Cyl) in the left eye. Both times the people who
performed the tests were unaware of the standards I posted, they simply
made tests.

You say that a +0.12 (Sphere), -0.75(Cyl) would qualify me. Is there a
lot of difference between 0 and +0.12? Could this difference be
associated to measuring instruments? I mean if I try it another time
could the reading be that different? My eyes have been getting some
major pounding for the last 3 months (Comp Sc student here) so I little
bit of brake before the exam may have some influence too.

The results were not yet sent for official review so I don't know how
strict they are. I don't know if for example my good right eye could
compensate for the left eye being slightly worst. I prefer to not take
any chances and I will try to present my Opt with the standard and see
if something can be done.

Thanks for your help

 

Ok then the R eye is obviously fine. The left is the problem.
Yesterday's numbers would disqualify on both counts (over .75 cyl and
sph equiv =-.50), but the other occasion you would pass on the cyl and
barely fail on the sph. equiv.

Absolutely. .12 is well within the accuracy of the instrumentation, the
examiner, and your responses.

Could help.

The military doc may well have a bias, and I would tend to think they
are a bit on the lenient side. Your doc on the other hand if anything
is biased the other way, esp. if he/she also fits glasses. Sounds to me
like you'll make it. Avoid as much computer work and heavy reading as
you can until the official exam.

w.stacy, o.d.

 

Other than taking time of books/computer, are there any "eye exercises"
I can perform to optimize my chances? A quick google query returns some
results such as this http://www.eyesercise.com/ ... but when it looks
too good to be true it usually is.

Thank you again

 

Dear Pershing,

As suggested by some eye-docs, you could consider
using a simple reading glass for all close work for
several weeks prior to an examination. See:

www.chinamyopia.org

and

www.myopiafree.com

Best,

Otis

 

[snip]

Otis is our resident 'Net Loon. Please ignore him and you'll be just fine.

 

right on. the best thing you could do right now is get as much physical
activity as you can. Aerobics are definitely in order, and make sure
your nutrition is good. There may be no discernable effect, but as
Groucho says, it couldn't hoit.

 

Roger that. Could passing the exam early in the morning, mid day or
later at night have some positive/adverse influence? Obviously no
reading in all cases would apply. Also when you talk about nutrition,
do you have anything specific in mind? There is the suppl/Vitamin A
myth (or reality?) going around. Is there any type of food one should
avoid?

As usual, thanks again for your input.