Question about focusing by moving glasses

[Argus]

I'm hoping that somebody here will be kind enough to answer a question that I have about optics. If a glasses wearer with a minus prescription for distance vision (and no cylinder) has slightly blurry vision but is able to see clearly by moving the glasses a few inches away from the eyes, does this mean that the prescription is too strong or too weak? My guess is that it's too weak. I figure that moving the lenses farther away allows the light rays to diverge farther, correcting the inward distortion of the eye itself that's focusing the rays before they reach the retina.

That's the main thing I wish to know. Also I'm curious, though, about whether there's a formula that would calculate exactly how much off a particular prescription is by how far the glasses have to be moved. Thanks in advance for answers to either of these questions.

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[still_confused]

get your bvd adjusted!

 

[Eve:)]

A patient who is myopic and cannot see clearly with lenses that are too close to his/her eyes but CAN see clearly when the lenses are a few inches away from the eyes means that the prescription is too strong. That is why contact wearers will usually get a weaker script for their contacts (because they are closer to the eyes).
On the other hand if the prescription was too weak, they would actually have to bring the lenses CLOSER to their eyes in order to focus better.

In terms of a forumula, I have no idea. I would suggest asking someone on the Optometry student forum as they probably know more about optics than any one of us anyways.

For all I know...I could be wrong

Hope that helps

 

[Argus]

Thanks, Eve. I've been searching the net and have found some explanations, mostly under "vergence" and "effectivity" (or "effective power"). They all confirm what you said, which is just the opposite of what I'd assumed when I asked the question. For example, "For spectacles, pushing a minus lens closer to the eyes increases the effective power of the lens (more -)." http://www.medrounds.org/optics-review/2006/05/8.html

That site and some others include a formula for determining equivalent correction when switching from spectacles to contacts or in the opposite direction. It should apply to the situation I described, though measuring the exact distance in millimeters might not be easy. PDF file - http://cfao.ucolick.org/EO/resource...eglasses -microscope "distance from the eye""

 

[Argus]

Still_Confused, your BVD (back vertex distance) shows up at some sites too. When I first read your post -- "get your bvd adjusted!" -- I assumed that it was just a humorous reference to underwear, and related to my metaphorically uptight concerns about spelling in the other discussion. According to Google "Back vertex distance" occurs on 321 net pages, but it's not mentioned under BVD in the online anagram dictionaries that I checked.

 

[diirooz]

Would this type of question be on the OATs?

 

[r_salis]

Moving to the main optometry forum to get more responses.

 

[Shroom]

Does this mean that a plus script for reading gets weaker when the glasses are moved closer to the eyes?

 

[PBEA]

I think your playing dumb, Argus. I cast your sheeps clothing aside. Ich bin der Almacht.

 

[jefguth]

Does this mean that a plus script for reading gets weaker when the glasses are moved closer to the eyes?

Thats correct, often times when you see a person (presbyope) that wears their reading glasses (plus lenses) positioned way down on the tip of their nose (I'm thinking of Whoopie Goldberg here) it indicates that they need a higher power lens. Though I suppose they may also wear their glasses like this for the ease of having uncorrected distance vision while also being able to look slightly down through their readers for corrected near vision.

 

[sethuel1]

Would this type of question be on the OATs?

no

 

[drmhyde]

It's MAAAAGIC!

F=L'+L

 

[still_confused]

Still_Confused, your BVD (back vertex distance) shows up at some sites too. When I first read your post -- "get your bvd adjusted!" -- I assumed that it was just a humorous reference to underwear, and related to my metaphorically uptight concerns about spelling in the other discussion. According to Google "Back vertex distance" occurs on 321 net pages, but it's not mentioned under BVD in the online anagram dictionaries that I checked.

eh? BVD is widely known as back vertex distance, and that is usually the problem if someone gets glasses that seems right via the perscription but still has trouble seeing (aka the opt or tech simply needs to adjust). I am a funny guy but for once i was being serious =)

not sure about BVD not being mentioned in online dictionaries, someone in school or current opt needs to back me up! BVD is commonly used no?

 

[planex]

eh? BVD is widely known as back vertex distance, and that is usually the problem if someone gets glasses that seems right via the perscription but still has trouble seeing (aka the opt or tech simply needs to adjust). I am a funny guy but for once i was being serious =)

not sure about BVD not being mentioned in online dictionaries, someone in school or current opt needs to back me up! BVD is commonly used no?

With contact lenses the difference in power needs to be compensated with refractive errors of +/-4 diopters and greater, when refracting from the typical 13-14mm vertex distance with the refractor. Most of Us probably do that

I typically use the BVD as kind of a "rule of thumb" when it comes to high magnitude refractive errors with frame style changes.
By changing frame types (metal to zyl or vice versa)there is a change in back vertex distance. BVD in metal frames with nosepads being greater than zyl frames without nosepads and can vary by several mm between frames.
The effective power difference wouldn't be noticed by most (engineers excluded )refractive errors below +/- 6 diopters. Greater than or equal to that however, usually requires compensations be made for the change in vertex distance and therfor change in effective power. The change is similar to that of contact lens power changes based on measurements made in the spectacle plane with the refractor, but of smaller in magnitude. So using the vertex compensation chart- another rule of thumb might be half of what the chart says above +/-6 diopters in the direction of change in vertex distance.
Hope this is helpful

 

[Shroom]

Thats correct, often times when you see a person (presbyope) that wears their reading glasses (plus lenses) positioned way down on the tip of their nose (I'm thinking of Whoopie Goldberg here) it indicates that they need a higher power lens. Though I suppose they may also wear their glasses like this for the ease of having uncorrected distance vision while also being able to look slightly down through their readers for corrected near vision.

Ah, this helps me quite a bit. I recently got computer/reading glasses and I notice that if they slip down my face that things start to get blurry. Have to make sure I keep them close to my eyes.

 

[rpames]

I use the rule-of-thumb or 4-6-8-10-12 rule. At +/-4, add or subtract about 0.25D, at +/-6, add or subtract about 0.50D...etc. It works pretty well for contact lenses.

 

[Argus]

>"I think your playing dumb, Argus. I cast your sheeps clothing aside. Ich bin der Almacht." [PBEA]

Nein, ich bin ernst. I really did want to know the answer. I'm writing this post now only to reassure those who took the trouble to answer that they weren't fooled. I'm not an optometry student, just somebody who had trouble with his glasses and was curious about what was happening. My own guess about the explanation sounds very plausible to me, and I appreciate the help of those who said that this isn't so.

 

[luvthemhogs]

It's MAAAAGIC!

F=L'+L

It's F=L'-L. Right?

 

[Opii]

No, it won't be on the OATs. Yes, it will be on the NBEO I and/or II.
Like said earlier in various posts it's all about the vertex distance...

 

[Elfego Baca]

Spectacle prescriptions effectively have more plus power or less minus power as you move them farther from the eye. If you are nearsighted and wearing a minus Rx for distance, the image will normally blur as you move the glasses away -- unless the Rx is too strong. If you are farsighted and wearing a plus Rx for distance, the image will blur as you move the glasses away -- unless the Rx is too weak. If you are wearing a plus add for near, such as a bifocal, moving the glasses away will effectively make the power of the bifocal stronger, which will normally make the near vision clearer.