# PING Phil Hobbs... Optics Question

• posted

Phil,

If +2.00 readers have me focused at 12" what do I need for focus at

24"?

Or is that inadequate information to determine it?

Thanks! ...Jim Thompson

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| James E.Thompson                                 |    mens     |
• posted

May I help?

12 inches is about 1/3 m, which means a total refraction of +3.3. This implies that your own refraction is +1.3, which corresponds to about 3/4 m. You're asking for 2/3 m, which needs a refraction of +1.5, so you'll need additionally +0.2. The nearest standard value is +0.25.
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-Tauno Voipio```
• posted

Gack! From (often faulty) high school memory, lenses placed in proximity have effectively the same focal length as the would be obtained by adding their diopters (1/fl).

So (shirt cuff), if your current lens+eye is yielding roughly 3 diopter (12" ~= 1/3 of 1 meter) then you want to change this to 24" ~= 2/3 of

1 meter (1.5 diopter).

So, my *guess* is 0.5 diopter.

This assumes the relative locations of your eye's lens and the corrective lens can be treated as a single compound lens. (I suspect that is *not* the case).

Have you tried a +1 pair of readers to see where that moves your focus?

(Or, have I got that exactly backwards?)

Should be easy enough to try! Finding 0.5 readers may be hard...

• posted

Den mandag den 24. november 2014 21.44.30 UTC+1 skrev Jim Thompson:

A diopter is the reciprocal of the focal length in meters

-Lasse

• posted

I'd just try it out. Here on top of the vector impedance analyzer is an assortment of Dollar store glasses lined up, 1.50 and up. I just grab whatever fits the job at hand. 1.75 or so for computer work at 20-30" distance, 2.25 to 2.5 for SMT soldering at shorter distances and when it gets tougher I use various visors (2.5x to 5x with added fold-over loupe).

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Regards, Joerg

http://www.analogconsultants.com/```
• posted

I have forgotten all my school-boy optics. While I enjoyed the class, it's been ~55 years :-(

I suspect that +1 should be about right. I'm going to order "clic's" from speert.com so wanted to avoid guessing. ...Jim Thompson

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| James E.Thompson                                 |    mens     |
• posted

Not quite that long for me -- closer to 45. But, I found optics fascinating and disarmingly "simple" (previously, I'd thought some amount of *magic* involved in figuring out how they worked, etc.)

Wander into local Walmart, Costco, CVS, Walgreens, etc. and try on a pair of +1's. I'm not sure you'll find +0.5 at such a place -- though you could obviously make your decision based on the performance of the +1's and place your order with a bit more confidence.

[I've had bad Rx's twice in the past. It is perhaps the most frustrating experience imaginable: "Why can't I *see*??"]
• posted

I had cataract surgery and set one eye to 10" fl and the other to 22". So I can read a newspaper or work on a computer without glasses. My brain does the processing so everything looks sharp from roughly 8 to

24 inches.

I also bought an ebay trial lens set, so I can design my own prescriptions and order glasses from Zenni.

Why not go to a drugstore and try a bunch of reading glasses to see what works best?

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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  ```
• posted

On Mon, 24 Nov 2014 13:25:17 -0800, John Larkin Gave us:

Get a custom pair of "Superfocus" glasses (now defunct?) where you can adjust each lens individually.

I think they went out of business because of their price point. About \$300 to high, and then they got greedy on frames and other accoutrements too. Just the feeling I got. I did not check it out past seeing those outrageous prices.

• posted

The Zenni glasses, with a nice geeky frame and astigmatism prescription, cost me \$22 a pair. Sunglasses are a bit more.

Glasses in the USA are an immense ripoff.

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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  ```
• posted

I buy my 2.0 diopters glasses 10 at a time for \$1 a pair at the Dollar store. I must admit though, the quality fell to an almost below acceptable level this last batch. The plastic frame is breaking before I get \$2 use out of them. But I have enough spread around that I can always find a pair in the car, at the computer, on my dresser, by my radio, two or three pair on the boat, and the hardest one to find, the one I slipped up on top of my head! I have contact lenses 4.5 (R) and (5.25) L but if I'm on the computer

2.0 glasses help. Mikek Mikek
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• posted

Optical power in dioptres is the reciprocal of the focal length in metres. One way of looking at this problem is to notionally put a negative lens right in front of your glasses that forms an image at

0.600 m of an object at 0.300 m. The first imaging formula is:

1/d_o + 1/d_i = 1/f

where f is the focal length, and d_o and d_i are the distances from the centre of the lens to the object and image, respectively. (In thicker lenses, the points they're measured from aren't quite at the centre, but that's irrelevant for eyeglasses.) The signs are taken as positive for the case of a positive lens forming a real image (i.e. the object is on the near side of the lens, and the image is on the far side).

In this case, the object and image are both on the far side of the lens, so d_o is negative.

d_o = -0.300 m d_i = 0.600 m

so

f = 1/(1/(-0.300 m) + 1/(0.600 m)) = -0.600 m, i.e. a power of 1/f = -1.66 dioptres.

Optical powers add normally, so the lens power you want is 1.66 dioptres less than the one you have, i.e. +0.33 dioptres. For most purposes, you might just as well take off your glasses altogether.

(Of course that's high school optics, so my answering it might be considered condescending.) ;)

Cheers

Phil Hobbs

```--
Dr Philip C D Hobbs
Principal Consultant ```
• posted

No, That's not condescending ;-) that's the review I needed... and you're right I do pretty well sans glasses at the monitor distance... just some fine print escapes me. ...Jim Thompson

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| James E.Thompson                                 |    mens     |
• posted

With my two eyes at different focal distances, drugstore reading glasses don't work very well. Zenni to the rescue again. One of my reading lenses is "plano" (zero correction, for my 8" eye) and the other is 1.25 or some such (for my 22" eye, to bring it down to 8")

It's fun to play with prescriptions, at \$22 a pair. I have a lot of astigmatism in one eye, and optometrists tend to correct that perfectly, which results in glasses that are unusable if I open both eyes. My DIY glasses have a milder correction and actually work.

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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  ```
• posted

".

o

Does this mean that "perfect" astigatism correction gives you two in-focus images which you brain can't reconcile?

Can you do biocular fusion to get the depth illusions on those pairs of fla t images where segments have been moved a little to one side in one to give the illusion of depth?

I've always been able to and yet one eye was astigmatic and the other eye s hort sighted, so until I got glasses (way too late, at about 14) I was alwa ys switching between the two, hardly ever using both (when I'd have been fu sing two equally-poorly-focussed images).

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Bill Sloman, Sydney```
• posted

The unit "diopter" which eyeglass strength is measued in is dimensionally 1/metre (focual length)

reasoning on that:

If +2.00 gets you 0.3m which is 3.33 diopters, your eyes are doing 1.33

You want 0.6m which is 1.67, diopters your eyes do 1.33, so you need 0.34 diopters of correction.

I think 0.33 is a common ly agailable strength.

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umop apisdn```
• posted

On 25 Nov 2014 06:27:08 GMT, Jasen Betts Gave us:

Are you saying that is where they are focused? BULLSHIT. Most are focussed at 12 inches, and YOUR eyes make them work from a bit less than twice that in to about 5 inches.

Note how there is no "+1.0" "magnification".

I'd buy a 3 pack of each, and you'll have reading glasses for a long time to come in every nook and cranny you want them in.

• posted

He's quite right--that's the shortcut method.

The problem with drugstore reading glasses is that the interpupillary distance is usually wrong, which leads to severe eye strain and headaches. I really notice if it's even 1 mm off. You can get Zennis custom-made for \$8 each if you don't need coatings or fancy frames.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
Principal Consultant ```
• posted

Ok. So assuming my eyes can't focus much past infinity, if I add a +2 diopter set of lenses, I shouldn't be able to focus on much beyond 0.5m in front of me? Is that close to understanding the topic?

• posted

"Focus" is a loosely defined term. A perfect lens really only focuses on a *point*. In reality, there is *some* "depth of field" that we can perceive even in front of/past the "focal point". Things are obviously *not* in "perfect focus" but your eye can't discriminate between perfection and "close enough"