Glasses for Computer use.

Hi,
I'm blowing in to ask if anyone can advise me. A year ago I got my
first pair of reading glasses. I haven't been using them for computer
work but lately I've had trouble focusing on the screen and I've been
getting headaches. My reading glasses are too strong though and I
don't like using them on the computer, I've tried them a few times.

The screen is 24" (.61 m) from my eyes. My prescription is .125 both
eyes, nothing else. The optician said it was age typical presbyobia.
I don't want to pay another visit to the optician's just yet - next
time I go, and if I need new glasses I will get them for computer use
and get OTC readers for reading as I spend more hours doing the
former. But until then should I get .100 or .75? OTC readers for
computer work? And how do you work it out? I understand .100 means
lengthening the focal point one metre.

Graminator

 

I assume your reading Rx is +1.25 diopters since +.125 is too small of a
power bother with glasses, and most lenses only come in increments of .25.

Optometrists, like all doctors, take pride in their sloppy and illegible
handwriting (without the proper decimals), which is why you are confused
about the power.

I would just try on some OTC readers at the store, focusing on the same
distance as your monitor (take a tape measure with you), and pick out the
right power that way. Assuming that your reading Rx is +1.25, then your
guess of +.75 or +1.00 is probably correct for computer distance, but you
can easily verify that at the store.

 

I think you're slipping decimals. 1.00 Diopter means a focal length of
1 meter. Your Rx may be 1.25, but it could be .125, there is a huge
difference. The formula is 1/d=f where d=lens power in diopters and
f=focal length in meters.

 

I would say that the goal of reading glasses is to supplement his
flagging accommodation in order to bring the plane of focus closer to
the retina, thereby reducing the area of blur or defocus.

DrG

 

Does not what I suggested above do just what you say needs to be done?

Bill
-- Fermez le Bush--about two years to go.

 

Yes, Bill, but the devil is in the details. The conditions at
infinity are simply not the same for everybody, i.e. nearsighted folks
vs. farsighted folks.

Sure, I 'm being picky, but that's my job.

DrG

 

Presumably, the 1.25D he already uses takes care of spherical correction he
needs to compensate for his position along the myopia-hyperoia axis. The
remainder allows him to see his close computer display clearly. This is
simple physics--not complicated biology.

Bill
-- Fermez le Bush--about two years to go.

 

Simple physics assumes a linear 1:1 relationship between accommodative
stimulus and accommodative response. It also assumes that what the
patient perceives as clear is the same as having the circle of least
confusion exactly at the plane of the retina. If you are trying to
paint a mental picture, it's nice to be able to have an understanding
of the eye's physiology, which is physics and biology.

DrG

 

Educate me if I am wrong. Do you not fit spectacles so that there is a
distance for which accommodation effort is not required? For example, does
an emmetropic eye use accommodation to view objects at infinity? Why not
design (fit) spectacles so that no accommodation is required at the working
distance to the display?

Bill
-- Fermez le Bush--about two years to go.

 

The average amount of tonic accommodation for an average eye at
infinity, i.e. no stimulus to accommodation, is approximately 0.50
diopters. At 40 cm., or a stimulus of 2.50 diopters, the amount of
accommodation in a non-presbyope would be approximately 1.75
diopters. In an early presbyope, it may be 1.00 diopter of
accommodation. Therefore, in a patient with 1.00 - 1.25 diopters of
accommodation at 40 cm., should I give a 2.50 diopter add, or a 1.25 -
1.50 diopter add?

DrG

 

Thanks for your replies, everyone. Yes, that was a typo, I meant to
type. +1.25. Bill, are you saying I should get 3.0 glasses for the
computer? That makes no sense to me as they would be much stronger
than my reading glasses.

William I took your formula, and with a bit of algebra I deduced the
following:

- assuming my 1.25 readers are adjusted for a reading distance of .
355m (14 inches - I guess that's about right)

- my screen is .61m, IOW .255m longer focal length than my readers (.
61-.355=.255)

- 1/d = f is 1/1.25=.8

- this can be rearranged to 1/f=d (if you multply both sides by d and
divide both sides by f)

- I want my computer glasses to f+.255m

- .8 + .255 = 1.055 (focal length adjusted for screen)

- 1/1.055 = .947 = d

So I should get 1.00 for the screen then? That I can get at Walgreens
et al. If I get +0.75 I would have to buy online cause I haven't seen
any that power on the shelf.

 

You are obfuscating by using jargon. Do you do that to your patients? The
meaning of the term "tonic" is not clear to me. My best interpretation of
what you say is that an average [normal] eye is slightly hyperopic and
"tonic accommodation" adds +0.5D of additional power to see objects at
infinity clearly.

In any event, if it is good practice to include the effects of tonic
accommodation at infinity, what I said still holds. The original poster told
us, perhaps incorrectly as suggested by previous responders, what his
prescription was. He also told us what the distance to the computer screen
is. Adding the reciprocal of that distance to his prescription moves that
screen to infinity and he still uses the tonic accommodation to function
correctly.

Bill
-- Fermez le Bush--about two years to go.

 

Gee, Bill, I'm just trying to be accurate without obfuscating you. I
would like to say that you over-simplify a complex biological system
because you are an engineer. Words such as infinity do have
meanings. So much for trying to expand your vocabulary.

DrG

 

Never take my advice in lieu of a vision professional. In addition, from
your post quoted above, I get a different picture of your situation. Your
first post gave me the impression that the 1.5D corresponded to your every
day "street" glasses. No I get the impression that the correspondence is to
reading glasses. It is all too easy to be ambiguous.

I still have difficulty understanding you.

I think you are saying that your reference is that 1.5D reading glasses work
well at 14inches = 0.356m. You then ask, "What is needed to work well at
0.61m?"

If you had a perfect eye, in the sense that it sees objects at infinity
clearly without any accommodation from your eye, you would need a 1/0.356m =
2.812D lens. Because you are using a 1.25D lens to get that effect, your eye
already supplies the difference of 2.812-1.25 = 1.562D.

To see clearly at 0.61m you need 1/0.61 = 1.639D if your eye were perfect.
Because your eye is not perfect and supplies 1.562D, the lens has to supply
only 1.639-1.562 = 0.077D. That is almost no lens at all.

I would conclude that you need no lens at all to see a screen well at 0.61m.
Alternatively, you have not given an accurate description of what is needed.

Bill

-- Fermez le Bush--about two years to go.

 

I am saying that 1.25D works for me at the usual reading distance,
which I have assumed to be about 14", or .355m. I want another pair
that works at 24", or .61m.

I don't get your calculations. I do know that I get blurry vision and
headaches while reading the screen at 24"/.61m. So, if 1.25D works for
me at .355m, what will work for me at .61m? That is my question.

 

I think your last paragraph quoted above says that you want the same thing I
thought you wanted as in my second paragraph quoted above.

I do not think that the presumption that your 1.25D provides clear vision at
0.355m can be justified without actual testing. Certainly 1.5D at 0.355m
would only work if your eye's accommodation provides the additional 1.562D I
calculated above. (The high precision in my calculation is there just to
prevent accumulation of roundoff error. Do not take it too seriously.)

I was suggesting that you do not need glasses to view your screen at 0.61m.
1.25 glasses will only blur your screen.

Bill
-- Fermez le Bush--about two years to go.

 

That is correct. The other Bill's assumption that we would prescribe
that which would make your eye not have to focus at all is incorrect.
We only do that when there is zero accommodative amplitude or when there
is a significant binocularity issue (esophoria) that demands it. In
by far most cases, we only prescribe what is actually needed, and most
patients will not accept such "full correction" as the Salmon Egg suggests.

 

Salmon Egg wrote:

That would only be true if he had zero accommodation (an absolute
presbyope), or had special binocularity issues. I think it's pretty
obvious that neither of those are the case. If you mean he "needs" to
supply 1.639 of total focusing convergence (which can be supplied by a
combination of accommodation, depth of focus, and spectacle lens), then
you are correct. Since most eyes have some accommodative response, and
all have some depth of focus, we rarely prescribe the entire convergent
demand for any distance, except for the occasional binocularity issue.
Why not prescribe that "full" amount? It's not wearable for most
people, because it is stronger than what is necessary and the stronger
the spectacle power, the narrower the depth of focus becomes.

 

I can believe that he is comfortable supplying 1.5 accommodation at 14
inches, for occasional reading at that distance, but is not comfortable
supplying the entire 1.5 sustained focusing at computer distance without
some additional "help". Not only do we usually prescribe less than the
full convergent demand, but we also usually prescribe enough to take up
some of the demand when appropriate. If he is emmetropic then the 1.00
lens he calculated will not blur that computer monitor.

btw I calculate 1.9 years remaining, and the accumulation of error is
increasing daily.

 

I was tested just over a year ago and that was the presecription I
received. Looking back I should have had my eyes tested for the
computer screen distance as well.

They don't blur the screeen, they make every pixel stand out. In fact
they are too strong, at least that's how it feels.