Advice, vdu glasses from reading prescription

I'm hoping for some advice to help me order a pair of specs for computer
use from an online store. I've got a recent prescription for reading
glasses that are great for focusing at around 35cm, but not very suitable
for computer work:

R -0.25/0.50X160 L +0.50/0.50X180 Add Near +1.75

My script doesn't give a figure for 'inter', and as I want to make the
order as fool proof as possible, I was going to order reading specs and
adjust the 'add +1.75' to a lower power to make them suitable for use at
around 100cm distance.

Is there a rule of thumb I can apply to get the correct 'add' figure from
what I've already got?. Hope my question makes sense.

I'm 53 yrs, if it matters!

On Sun, 28 Oct 2007 16:59:16 -0500, Mike Tyner wrote:

> "Mr Edward" <(E-Mail Removed)> wrote
>
> Something's wrong. If you're 53, it's inconsistent that a +175 add should be
> clear at 35 cm.
>
> Not impossible, just off from what we'd expect. Normally it takes about
> +2.75 to get clear vision at 35 cm.
>
> If you've actually measured it, I'd ask you to re-measure and determine the
> _range_ rather than the midpoint.
>
> I'm guessing you mean you can see _up to_ 35 cm away. How far out can you
> push that same target before it starts to blur? 40 cm? 50?
>
> -MT

Thank you for your reply....Yes you are right! I didn't measure it before,
I was repeating what I thought the optician said and I must have made a
mistake, sorry.

OK Ive measured the range of clear focus and it's 44cm to 65cm when viewing
my LCD screen, and curiously different at 34cm to 74cm when viewing the
printed page. Is that normal?

I should just add that these distances are measured with my current reading
glasses with an add of +1.50. When I had an eye test 2 months ago the
difference from my previous prescriptions was too small to warrant a new
pair of specs. that's why I'm still using specs with a +1.50, I've had
these specs for 5 years now bye the way (my first pair of reading glasses),
and in all that time not enough change to warrant a new pair.

My optician has told me I still have pretty good vision for my age.

Any advice appreciated.

On Mon, 29 Oct 2007 05:36:01 -0500, Mike Tyner wrote:

> Since you're talking about online prices, I'd recommend a progressive to
> cover a more useful range:
>
> R +0.50-0.50x160
> L +1.25-0.25x180
> add +1.00
>
> The "tops" would be clear out to 133 cm (+075 effective add), and the bottom
> clear up to about 40 cm (+1.75 add). The cheapest progressives work pretty
> well with low add powers like +1.00.
>
> -MT, OD

Thank you for the advice, progressive sounds like a good idea. Just one
clarification first please, on my prescription the CYL is +50 for both
eyes, on the version you give above, have you adjusted this to -0.50 and
-0.25, or is simply the shorthand way it's written?

As you can tell I don't fully understand the prescription and the way that
SPH and CYL appear to be able to be manipulated to end up with the same
type of correction.

Much appreciated.

On Mon, 29 Oct 2007 10:27:12 -0500, Mike Tyner wrote:

> Yours should read R +050+050x160.
>
> -MT

OK thank you for your patience, before I place an order I need to be sure
I'm not making any wrong assumptions. As always a little knowledge equals
danger.

So should a progressive prescription look like this.

R +050+050x160
L +125+025x180
add +1.00

or like this

R +050+050x160
L +125+050x180
add +1.00

I can't see why the CYL for L became +025 from +050 on the original, if
it's just part of the magic I'll accept that it should be. Sorry to have
to ask again for clarification and I do appreciate your help.

On Mon, 29 Oct 2007 10:59:47 -0500, Mike Tyner wrote:

> More seriously, it's just two ways of describing a football-shaped surface.
>
> Imagine a football. Imagine measuring the curvature along the seams (long
> axis) as say, 40 units of curvature. Along the short axis, across the seams,
> say it measures 60 units, so the difference between steepest and flattest is
> 20 units.
>
> You can describe the surface as 40 + 20 or 60 - 20 and you get the same
> shape. The "a+b" form always starts with the flatter surface and the "c-b"
> form always starts with the steepest. To convert from one to the other, you
> must also show that the starting point is no longer the 160 meridian, but
> instead it's 070, 90 degrees away. So,
>
> +050+050x160 is equivalent to
> +100-050x070.
>
> -MT

A very clear explanation, thank you. I now understand why my first
prescription expressed with -ve values, was actually the same as the next
one even though the numbers are very differnt. The optician told me they
were close enough, but without any explanations, now I get it!

On Mon, 29 Oct 2007 11:56:59 -0500, Mike Tyner wrote:

> "Mr Edward" <(E-Mail Removed)> wrote

>> R +050+050x160
>> L +125+050x180
>> add +1.00

That's great, thank you so much.

> Sure. Since you want clarity, I have to ask about the PD. The distance
> between your pupils has to be specified with the order.
>
> PD changes with working distance (you "converge") and PAL orders usually
> specify it in detail. Details like seg height and monocular PD are not as
> critical with your low prescription, and a decent pair could be made with
> just a "distance PD."
>
> -MT

With the help of Mrs Edward I've got a PD measurement of 66mm. I did read
that with a mild prescription it's not so critical, though more so with
progressives than single distance. I think 66 is reasonably accurate,
whilst I'm focusing in the distance.

I'm ready to order that well needed pair of specs which will stop either
trying to read the screen without any, or having to get too close with them
on.

Thanks again for your patience and advice, much appreciated.

In article <(E-Mail Removed)>,
"Mike Tyner" <(E-Mail Removed)> wrote:

> "Mr Edward" <(E-Mail Removed)> wrote
>
> > OK Ive measured the range of clear focus and it's 44cm to 65cm when
> > viewing
> > my LCD screen, and curiously different at 34cm to 74cm when viewing the
> > printed page. Is that normal?
>
> It is what it is. It's noteworthy that the midpoint of both measurements is
> about 54 cm.


Well, perhaps not, except to rule out measurement error. The range of
clear focus is always heavily dependent on illumination level. It
shouldn't matter for LCD work, since that distance should be fixed,
unless:


> Well, nominally if you want clear vision at 100 cm, you'd want a 1.00
> diopter add.
>
> Those glasses wouldn't be so hot for printed work, making it difficult to
> see your bank statement while reconciling checks on the screen.


you try to read and look at the LCD in the same session.