Anyone know how to order glasses with a focal length (working distance) of 20" from Zenni? I know people here have done so but I didn't see anywhere on the site telling how and I didn't find anything doing a Google search here. It didn't find any posts for zenni. Thanks Karl
Power of glasses is expressed in diopters, the reciprocal of the focal length in meters; +1 = 1 meter f.l., +2 = 1/0.5 m f.l, etc. So a 20" f.l. would be +1.97 diopters.
That being said, I'd check out some good reading glasses of +2 strength and see if they appear to fill the bill. If so, place the order.
Assuming that you wear prescription glasses, go the drug store and try holding reading glasses in front of your regular glasses. When you find the right reading glasses to have the working distance you want, just add the diopter of the reading glasses to you prescription.
I've been looking for an answer to this for a while. Thakns (even though I'm not the one who asked).
OK, I think I understand that, but let's just make sure:
I have a prescription for "computer glasses" which are no-line bifocals, the distance part focuses at 20 inches. I have new, bigger monitors which are about 26 inches away.
Prescription for the 20" right eye reads: SPH: +2.25 CYL: -1.00 AXIS: 82 ADD: +1.00
So to change that to 26" focus, I would multiply the SPH and the ADD by 20/26 correct?
By the way, of all the useful informatiuon I've ever gotten from this group, Zenni is absolutely the best. I really am a "buy American" type, but when they can deliver these glasses for $52.95 and my local guy wants something over $400 for the same prescription... Well, I have four pairs of Zenni glasses, and I doubt I'll ever go anywhere else.
FWIW, I just did what you're doing. My prescription is basically -4 with
+1.75 bifocals (or -2.25). I can see close great, far great. Computer distance is a stone bitch.
So I ordered elcheapo Zenners with +1 from my prescription(basically -3). THEY WORK GREAT. I change glasses at the 'puter. Standard pair everywhere else.
======= Do it the easy/cheap way. Stop by Walmart or Dollar General and get some reading glasses that let you focus at 20 inches. More than likely you will wind up with several pairs at slightly different diopters for use at different screen distances for different applications.
FWIW if you need the URLs
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see
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if you need prism or cylinder correction.
Put a note about computer use in the comments section. Both places have been good about answering my questions. If you get custom computer glasses ask about tints.
Unka' George [George McDuffee]
------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).
Yeah but... The Zenni glasses can be LESS expensive than drug store glasses, and they can match your prescription exactly. Good idea about putting a note in the order comments. Also, the one time I called them, they were quick, polite and informative.
Good to know! But ... the condition of one's eyes must come into it somehow. I.e., it's the combination of your eyes' focal length and the glasses that determine the actual focus, yes?
I bought 4 pairs of Zenni (one bifocal sunglasses, one night driving bifocals and 2 computer/shop glasses. 4 pairs were cheaper than one at the optometrist. I also have a set of optometrist trial lenses that I use to select the sphere that I want. As I am farsighted, the drugstore reading glass trick doesn't work.
I get an eye exam and prescription from my opto then use the trial lenses to select my near and far working distances for the different glasses I use. It has worked excellently.
I'm guessing, but I think you would change only the ADD, not the SPH. I think the SPH (and cyl) correct for distance, the ADD is additional correction for a selected closer distance. Don' t know if you can just ratio diopters, though.
Check with a friendly optometrist or optician. There may well be one that reads RCM. There used to be an opthomologist eye surgeon but haven't heard from Jim for years now.
Seems that you can ratio diopters, if Terry's formula is correct. Also, in the case of these glasses (which hang on a cord around my neck), the distance part (SPH) is for reading the computer screen, the ADD part is for reading books..
I used to have a great optician - Rode a BMW R90S, which I took care of for him. He has since retired moved to points unknown.
Depends on what you want to change. The SPH, CYL and Axis are all the primary correction items. They apply across the ENTIRE lens.
The +1.00 applies an additional correction for the bifocal area. This is added onto the base correction.
If you use the top of the lens to read the computer screen and the bottom to read books and such and you want to increase the focal length for BOTH to 26" focus you add the correction factor to the SPH number.
IF you want the correction in the distance portion and NOT the bifocal then you would subtract the correction factor from the ADD number While adding it to the SPH correction. Not sure if Zenni could do this unless they allow you to enter fractional ADD numbers?? Maybe make a call?
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