HELP! My bifocals OK; My trifocals -- mis-alligned, mis-installed?

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David Combs - 30 Mar 2006 07:50 GMT

HELP!

I had been wearing bifocals for some years, and they worked
just fine for me.

However, my accomodation had gotten *so* small, that I now
needed tri-focals (for reading, mid-range being the computer screen,
and long for driving).

Soon after acquiring my trifocals, I noticed something:

With my TRI-focals ("executive type"), looking STRAIGHT AHEAD at a
vertical edge of a door

(ie my NOSE pointed STRAIGHT AHEAD,
DIRECTLY at a vertical edge of the door)

, HERE'S WHAT I SAW (holding my head fixed, closing one eye at a time):

TRIFOCALS -- LEFT LENS, LEFT EYE: TRIFOCALS -- RIGHT LENS, RIGHT EYE:

+----------------------------------+ +----------------------------------+

| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |

+------------------.---------------+ +--------------.-------------------+

| . | | . |
| . | | . |
| . | | . |

+---------------------.------------+ +------------.---------------------+

| . | | . |
| . | | . |
| . | <NOSE> | . |
| . | | . |
| . | | . |
| . | | . |

+----------------------------------+ +----------------------------------+

NOW, I *experimented, trying to get the vertical to line up STRAIGHT
in each of the two lens.

Indeed, I *could* get that to happen -- but only ONE eye+lens AT A TIME,
by either:

(a) ROTATING my head (ie nose) to either the left or to the right,
which would get it to "line up vertically" (no jags)
in ONE eye+lens (but NOT both).

Rotating my head the OPPOSITE direction, I could get the object to line up
straight, but only in the OTHER eye+lens.

(b) Likewise, instead of "wearing" the glasses, I could get the same
two results by "closing" the classes (folding the "arms" up as if
I was going to insert the glasses into its carrying-case),

, and then holding them in front of my eyes (at the same distance as if I were "wearing"
them), and then, instead of *rotating* my lead one way or the
other,

SHIFTING the glasses either straight left or straight right maybe
a third or half of an inch, in which case I got the same results
as before: shifting left fixed one eye+lens, shifting right fixed
the other one.

LIKE THIS:

HEAD TWISTED *RIGHT* 15-DEGREES
(Equiv: left lens SHIFTED RIGHT ~.3IN)
LEFT LENS, LEFT EYE

+---------.------------------------+

| . |
| . |
| . |
| . |
| . |
| . |
| . |
| . |
| . |
| . |

+---------.------------------------+

| . |
| . |
| . |

+---------.------------------------+

| . | <NOSE IS HERE>
| . |
| . |
| . |
| . |
| . |

+----------------------------------+

HEAD TWISTED *LEFT* 25-DEGREES
(Equiv: Right lens SHIFTED-LEFT ~.4I
RIGHT LENS, RIGHT EYE

+-----------------------.----------+
| . |
| . |
| . |
| . |
| . |
| . |
| . |
| . |
| . |
| . |
+-----------------------.----------+
| . |
| . |
| . |
+-----------------------.----------+
| . |
<NOSE IS HERE>| . |
| . |
| . |
| . |
| . |
+----------------------------------+

---------------- THE OPTICIAN:

The optician (not the one ("optomitrist"?) who determined the
"prescription") -- but the one who ordered the lenses, measured the
distance between my eyes (via a hand-held machine he had), and
installed them into the frame, etc --

I told him about how my view through the glasses jagged vertical lines --
he said he didn't understand what I was talking about.

I tried again to explain to him exactly what I saw, and even drew him
some diagrams (similar to those below) -- he shook his head, puzzled,
then at my insistence he held the glasses up to his eyes, looked
through them at the edge of store's window (seemingly just the way I
had done) -- and said that they were just fine, nothing wrong, no
jaggies at all!

A few months later, I went again, and again tried to explain
myself to him. Again, no such luck.

You know, it seems pretty obvious to me -- you guys can
tell me where I'm wrong. Or if I'm right, then what
his problem might be...

Anyway, I'm going back in tomorrow for my yearly eye-exam, and so as
to learn the proper terminology, so maybe *that way* he can understand me,
I post this here.

(I just hope *someone here* can figure out what I'm trying to say!)

--------------------------------------------------------------

NOTE: with my BI-focals, I had no such problem -- no jags at all
(when looking straight at the vertical edge) on *either* the left OR the right
(ie both were lined up straight AT THE SAME TIME!)

(Just Like they were *supposed* to be?)

Like this:

+----------------------------------+ +----------------------------------+

| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |

+---------------------.------------+ <NOSE> +-----------.----------------------+

| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |
| . | | . |

+----------------------------------+ +----------------------------------+

Well, I hope you can make some sense of what I have here, and
tell me that either I'm right (and tell me how to describe
it "technically"), or explain how I'm all wet, and that
trifocals are different from bifocals, and just work that way.

THANKS!

David

Reply to this Message

Dom - 30 Mar 2006 13:48 GMT

David

You certainly have spent a lot of time analysing your vision.

I think the key question for you is whether your glasses are useable
while being worn normally.

If there is no functional problem with the glasses then I don't think
you have too much to worry about.

If there IS a functional problem with the glasses then your optician
will find it easier to solve the problem for you if you can describe the
problem in everyday terms (e.g. 'I get double vision when I watch TV' or
'I get sore eyes when I use a computer'), rather than a detailed
analysis of your observations under unnatural conditions like tilted
glasses, one eye closed, etc.

Dom

> HELP!
>
[quoted text clipped - 197 lines]
>
> David

Reply to this Message

David Combs - 30 Mar 2006 14:59 GMT

>David
>
[quoted text clipped - 14 lines]
>
>Dom

Thanks for the quick response!

In fact, I *did* that -- "my eyes HURT after reading with these glasses":

> , rather than a detailed
>analysis of your observations under unnatural conditions like tilted
>glasses, one eye closed, etc.

(1) I was trying to figure out what the guy did WRONG,
when he cut the to-be-installed lenses out of
the "ground-to-prescription" blanks.

Like, the fact that my *shifting* left/right each lens
*fixed* the problem -- might that somehow be relevant?

(2) I "tilted" the glasses? -- Just where did I say that?

(3) *I* didn't want to accuse the optician of making a mistake --
I thought *you EXPERTS* might come to that conclusion --
or maybe to some *other* conclusion, like trifocals being different
*kinds* of optical instruments, or something, from bifocals.

The *last* thing I need to hear is to try yet a *third*
time to get him to understand my optical experience --
or might it be to *admit* to understanding it?

(4) Maybe my mistake was in expecting people to actually *read*
the post with some degree of care, given all the effort I expended
in drawing the "ascii diagrams" detailing exactly
what my eyes saw through the glasses.

Perhaps my mistake was in not placing the *bifocals* diagram
FIRST -- where it's obvious that the lenses were *correctly*
cut from the blanks, such that where my straight-ahead gaze
of my two eyes, converging at eg 20 feet distance, passed
through each lens where it (the "gaze-ray") passed through
the glass *perpendicularly* to it -- thus no jags at the "line"
(the glass surfaces of the near-middle-far regions being
exactly parallel -- AT (only) *THAT* POINT.)

Well, the way the bi/tri-focal lenses are ground, there's
one and only one place on the lens where all regions
(near-far, near-mid-far) *are* parallel (only one
place, because they obviously must have *different*
curvatures (otherwise they wouldn't focus at *different* distances).

Maybe only someone who's studied a bit of optics will see
that -- but then that's who I'm *asking*.

Here I repeat the bifocals-diagram:

.....
.....

>> NOTE: with my BI-focals, I had no such problem -- no jags at all
>> (when looking straight at the vertical edge) on *either* the left OR the right
[quoted text clipped - 27 lines]
>> | . | | . |
>> +----------------------------------+ +----------------------------------+

Thanks for the response!

David

Reply to this Message

Dan Abel - 30 Mar 2006 17:54 GMT

> In fact, I *did* that -- "my eyes HURT after reading with these glasses":
>
[quoted text clipped - 5 lines]
> when he cut the to-be-installed lenses out of
> the "ground-to-prescription" blanks.

I'll throw out my thought. I should caution you that I have never worn
trifocals and know nothing about them. I tried bifocals, but I couldn't
tolerate them. I have never tried progressives. In short, I'm a
complete layperson, with no experience or knowledge.

I don't think you are supposed to look out all three segments at the
same time. When you look at distance, the light rays come in parallel.
When you look up close, your eyes need to turn in so they both see what
you are looking at. It seems reasonable to me that the three segments
won't show the same view. Your brain needs to learn to just look out
one segment at a time. I wonder if your brain learned to do that with
your bifocals, but just isn't used to the trifocals yet.

Signature

Dan Abel
dabel@sonic.net
Petaluma, California, USA

Reply to this Message

Sibir - 03 Apr 2006 09:03 GMT

Let me expand on what you stated.

When looking through a bifocal lens with an add over +1.00, the near zone is
so blurred, you can't notice the displacement of a distant object in the
segment. A trifocal offers a lower add in the intermediate that allows a
person to see the displacement easier. It's a depth of field issue to anyone
familiar with cameras. To those not familiar with such things, the lower
power of the intermediate segment allows you to notice things you wouldn't
be able to see through the reading area.

See below for more regarding the original poster.

>> In fact, I *did* that -- "my eyes HURT after reading with these glasses":
>>
[quoted text clipped - 5 lines]
>> when he cut the to-be-installed lenses out of
>> the "ground-to-prescription" blanks.

Considering you ordered Executive Trifocals, you had better have a very
small frame. (think 50s styles,) The blanks are only about 60 to 70 mm at
their widest. It's quite likely they did a "best possible" to get the lenses
to fit. This introduces prism errors that are similar to what you describe.
The optician gave you what you wanted, but I would think they tried to talk
you into something a little more modern than a 200 year old design that is
already one foot in the grave. Anyone who thinks otherwise should just take
a look at the number of manufacturers still offering such and what size and
material combinations they are still offering them in.

I would recommend a modern lens and a commitment from you to be a living
human. (this means you are able to learn and adapt to new things. ) The only
dog who can't learn new tricks is a dead dog.
An 8x35 trifocal (or a 10x40 specialty lens) would be a perfect choice if
you are too unsure about your abilities to adapt to your changing
environment.

Carl

> I'll throw out my thought. I should caution you that I have never worn
> trifocals and know nothing about them. I tried bifocals, but I couldn't
[quoted text clipped - 8 lines]
> one segment at a time. I wonder if your brain learned to do that with
> your bifocals, but just isn't used to the trifocals yet.

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