# Optical Forums > Progressive Lens Discussion Forum >  Why the difference between how plus and minus is prescribed?

## oxmoon

Something I've noticed is that the docs here will give any plus wearer all the plus they ask for and even try to talk them up into more.  On the other hand, it seems like those who need a minus lens have to beg for every increment as though the docs were giving away gold.  This has always been my experience too, as I need a minus lens and cylinder, but never feel like I get what I need.  Now I know I am not alone in thinking this.  Can you tell me why it is?

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## Ghost Machine

I'm not a doctor, nor do I play one on TV; I didn't even stay at a Holiday Inn Express last night. But from my personal observation, I see this:
If you overplus a hyperope, even .25d or so, distance VA will be blurred. But you can keep pushing minus on myopes and they love it (until they see craters on the moon I tell them). 

I have worked with a couple of ODs that are convinced that every myope is overcorrected and they feel the need to right the world and get them back to where they should be (must have plenty of trial lenses on hand if you're fitting contacts for one of these types).

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## LAGUNAEYEDESIGNER

I m not a doctor but my guess is giving more plus lenses (over prescribe plus rx) = more magnifies because the plus lenses act like a magnifier = patients will easy to accept the rx = be happy
Giving more minus lenses (over prescribe minus rx) = patients will get headache, diziness, etc = not be happy

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## wmcdonald

Any extra plus will not be accepted because it will blur DVA. An increase in minus power will stimulate accommodation. We attempt to reach the point of MPMVA (maximum plus for maximum visual acuity). This has been standard practice and is just no the docs here, but everywhere.

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## fjpod

I think a lot of the "fear" of giving full minus stems from the unproven theory that minus lenses make your eyes "get worse".  There is no evidence to support this...even if someone is mildly or modestly overminused.  The worst that can happen is that overminused person can get a headache from the excess minus screwing up accomodation and convergence, albeit temporary, as the visual system adapts in most cases (assuming pre-presbyopic).  On rare occasions, such as in a divergence excess, a prescriber may actually choose to over minus a bit in order to "force" better alignment of the two eyes.  

If there were any evidence that minus lenses made your eyes get worse, the FDA would have stopped us from prescribing them by now.

On the plus side...in my experience, giving too much is asking for a re-do (same as underminusing).  Hyperopes are habitually used to not wearing correction, they are habitually under-plussed.  You might say, they are over-minused...yet they survive...and will argue til the cows come home, that they do not need their distance correction.  In rare cases, a prescriber might over plus a patient, usually at near to decrease esophoria/tropia, but again,...these are special cases and there is no golden rule.

Prescribing is an art.  The science of refraction is easy once it's scientific principles are learned.  Applying the science to the human organism is what really counts, and takes years of clinical training and experience.

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## chip anderson

Over minused or underplused (both the same thing if you _think_ about it) causes the patient to accomodate when viewing at differences when accomodation should not be needed.  Something like keeping a muscle flexed when it should be relaxed.  Causes fatique.  Causes patient to need even more plus at near when they approach presbyopia.   

Chip

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## oxmoon

Hi Chip,

You say "Causes patient to need even more plus at near when they approach presbyopia"  Please tell me why this is.

Three years ago, before I got into this field, I talked my OD into upping my minus "against his better judgement" and found that I could see well to drive at night for the first time in my life.  Always before it was just taking a chance and hoping for the best.  From this perspective it seems that giving the least minus is doing no favors.

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## chip anderson

Oxmoon:   Please understand I am not an OD or OMD and not too knowledgeable about refraction.  But What I have read and been told is that the eye (no designation or hyper or myope) is 1/2 diopter more myopic at night.   This is probably why you saw better over minused at night.    Of course I was also told that "good doctors take this into account."

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## Barry Santini

Its called night myopia, Chip, and the found amplitude in an individual has to with an amalgam of factors:

Size of pupil/ sph. Abb. Of cornea
Empty field accomodative response (stronger in younger people)
Purkinge effect
And other, more minor factors
B

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## drk

All posts are right, here.

I think the best way to achieve a good sphere endpoint is the RG Bichrome in a dark room.  Me likely.

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## Off Axis

All true. I think the length of the testing room also plays a part here. Ideally you want the equivalent of 6 metres or more and optoms often may walk their patients out of the test room into open space with a trial frame just to finalize what the patient will be experiencing.

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## Barry Santini

> All posts are right, here.
> 
> I think the best way to achieve a good sphere endpoint is the RG Bichrome in a dark room. Me likely.


Dark room = trouble with Corneal SA, and often results in latent hyperopes getting too much plus.

B

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## listenclose23

I'm learning to refract right now and the first thing  explained to you is that we are looking for the MOST plus for a patient or the LEAST minus....as an optician I have had great success  measuring the seg a tad higher on hyper-opes and a tad lower for myopic patients...obviously this insures the myopic patient not getting into the intermediate too soon and being robbed of some minus and the hyper-ope getting in to just alittle more plus which they tend to want anyway

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## skt

90% of redos here are hyperopes being over-plussed...the complaint is always the driving issue....
I'm great at 20' but beyond that I can't see".

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## chip anderson

Jack Coleman used to say:  "Always crowd plus!"

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## optilady1

> I think a lot of the "fear" of giving full minus stems from the unproven theory that minus lenses make your eyes "get worse". There is no evidence to support this...even if someone is mildly or modestly overminused. The worst that can happen is that overminused person can get a headache from the excess minus screwing up accomodation and convergence, albeit temporary, as the visual system adapts in most cases (assuming pre-presbyopic). On rare occasions, such as in a divergence excess, a prescriber may actually choose to over minus a bit in order to "force" better alignment of the two eyes. 
> .


My uncle is something like a minus 6, and his 'eye doctor' gives him a -4.00 so that his eyes will work harder and not get worse.  
I've told him to tell me when he's driving so I won't be one the road at the same time as him.

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## rdcoach5

Why don't a lot of Doctors use this ? I always found it to be very useful?

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## Barry Santini

The main problem with this whole Most plus/least minus approach is that a typical exam is done:

1. With a larger than "normal" (daytime) pupil
2. Does not take into account any "pupil" mapping or corneal topography
3. Does not consider how "too much plus' Is advantageous for adds greater than +2.25 used indoors.
4. Does not differentiate between sunwear, used primarily for DV,and general use eyewear used indoors
5. Age of patient
6. etc.

B

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## drk

> Dark room = trouble with Corneal SA, and often results in latent hyperopes getting too much plus.
> 
> B


Care to put some flesh on that, Barry?

You must mean: "dark adaptation" for "dark room".  You don't refract in the dark room, you turn off the lights for the RG bichrome.  There's no time for dark adaptation of even the pupil, usually.

Corneal spherical aberration?  What are you talking about?  Some higher-order aberration that's selectively influenced by pupil size?  

Why would only latent hyperopes get too much plus?  What IS too much plus for a latent hyperope?

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## drk

Barry, as you learn to refract you will see that most of these things are simply cluttering up your mind.  It's not that complicated.

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## Barry Santini

> Barry, as you learn to refract you will see that most of these things are simply cluttering up your mind. It's not that complicated.


Too much plus is *anything* they gives a blur at distance.

Yes, larger pupils = Corneal SA.

And...there's a certain amount of accomodative "bias" in early presbyopes...no matter whether they're hyper or Myper.

B

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## DragonLensmanWV

> My uncle is something like a minus 6, and his 'eye doctor' gives him a -4.00 so that his eyes will work harder and not get worse.  
> I've told him to tell me when he's driving so I won't be one the road at the same time as him.



OMG! Time for a new doctor who doesn't think like an asian doctor. That practice is pretty common over there, I've read. And of course, we all know that won't work, AND he gets the pleasure of eyestrain all the time, except for near.

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## drk

Barry, you should stick to the Hubble telescope and not worry about Bubba.

Truth is, you may be right about corneal spherical aberration and other higher-order aberrations, but they are negligible. 

Now, don't show me a Zernicke polynomial that shows some -0.62 SA in a 8mm pupil myope. It just isn't something you need to factor in. Now, if you have some kind of problem, it's good to consider such esoterica, granted.

But in general, once you consider crystalline lens aberrations, tear film variability, a fingerprint on the dang front surface of the lens, a little POW slip here, a little variable pupil size there, a little diurnal variation, etc. and you are going to freak out.

Just refract to 1/4 D and make them to half that standard and we will all be fine.

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## Barry Santini

drk,

in all my years, I've finally reached a point that I have more than a little confidence about a few things.

This is one of them. But I'm always open to learn.

Barry

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## kws6000

> Jack Coleman used to say:  "Always crowd plus!"



Jack Coleman was wrong...

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## drk

In all your years of refracting?

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## HarryChiling

> Its called night myopia, Chip, and the found amplitude in an individual has to with an amalgam of factors:
> 
> Size of pupil/ sph. Abb. Of cornea
> Empty field accomodative response (stronger in younger people)
> Purkinge effect
> And other, more minor factors
> B


Three conditions:

Photopic
Mesopic
Scotopic

Fix one?

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## Barry Santini

> Three conditions:
> 
> Photopic
> Mesopic
> Scotopic
> 
> Fix one?


SCotopic. But "empty-field" myopia can also be present in Photopic and mesopic conditions as well.

B

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## uncut

> Jack Coleman was wrong...


 
I feel a poem coming on........

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## drk

There once was an optician near York.
The ultimate geeky lens dork.
He'd make glasses clear
by fine-tuning the sphere
and the cylinder axis he'd torque.

(We love our Barry)

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## Barry Santini

> There once was an optician near York.
> The ultimate geeky lens dork.
> He'd make glasses clear
> by fine-tuning the sphere
> and the cylinder axis he'd torque.
> 
> (We love our Barry)


Why thank you, drk!

B

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