Got any eyeglass experts here?

The focal length in diopters (D) is the inverse of the focal length in meters. So 1.50D readers have a focal length of 1/1.50 = 0.67 m = 26 inches. Objects that far away will appear to be at infinity so if you are presbyopic and otherwise corrected to focus at infinity that is the working distance. If you're otherwise nearsighted or farsighted then you need to add that power into your calculations. Sounds like you must start out a bit nearsighted if 1.50D readers force you to get too close.

Assuming you are starting from perfect (or perfectly corrected) vision at "a long distance" (infinity, or at least the mountains across the way) the focal length is related to diopters as the inverse in meters -

0.5 diopter will change "infinity" to 2 meters (and everything beyond 2 meters will be out of focus), 1 diopter will change "infinity" to 1 meter (and everything beyond 1 meter will be out of focus), 1.5 diopters will change "infinity" to 0.75 meters (and everything beyond 0.75 meters will be out of focus).

Since 0.75 meter is roughly 29.5 inches, it sounds like you should start with a visit to an optometrist if you can't see the lathe at 22 inches with 1.5 diopter glasses.

The stronger the power, the closer it will have you be. If 1.5 is too strong, try 1.25 (the test stand at Wal-Mart works just fine...).

"Gerry" wrote: (clip) Looking for something with a focal length of maybe

22" so I can stand at a comfortable position. Does that mean a stronger lense? ^^^^^^^^^^^^^^^^ First. I believe your nomenclature is wrong. The focal length is not the distance at which your eye focuses comfortably on the work. Lets drop that term, and talk about diopters. A flat piece of glass has zero diopters, and does nothing to improve your vision. If you go to a store that sells cheap reading glasses, you will find an assortment of strengths, all labelled in diopters. The higher the number, the closer you can focus. Just try on different ones until you find a pair that makes you comfortable at the 22" working distance you seem to like. They won't be safety glasses, though. For that you will need to give some money to your optician.

I keep a pair around for removing splinters, etc, that are about 3 diopters. Withg those I get a working distance of about 6-8" (guessing.)

I have another pair that are about 1.75 diopters that I wear under my welding hood. It's really hard to line up tri-focals through a welding hood. (Don't sneer. You'll be my age someday if you live long enough.)

snip---

I wear both ---reading glasses, with safety glasses on top----and it works well enough for me. I'm a weird dude, so it surprises no one.

I have a few pairs of 3.25 diopters. For close-up work, I'd love to have some even higher, but I've yet to find them. I often wear an Opti-visor along with a pair of 3.25 reading glasses.

Told you I was weird! :-)

Sigh!

Of all the things I've given up with advanced age, the loss of my vision is one of the most troubling. I used to have 20/15 eyes and could see anything of interest without aid. I attribute my struggle with welding to my loss of vision-----and fully understand your plight. The only thing that brings me comfort is it's better than being dead.

Harold

Your math is a bit shaky.

Focal length in meters = 1 / d, where d is the strength of the lens in diopters. 1 / 1.5 = 0.66666..., not 0.75.

0.667 meters = 26.2"

The OP might get along just fine with 1.75- or 2.00-diopter drugstore reading glasses.

I like the 3 at 14" for bench and lathe work but the 2 fits your request better.

jw

What you may want is a binocular loupe as used by neurosurgeons, dentists etc. COmpound lenses, allow higher magnifications at comfortable distances. Some on ebay for under 100.00. For many times higher prices, available from medical / dental supply firms with what might be considered to be safety glasses designs.

Because of the high magnification, they're sensitive to the innate magnification of each eye, and if your eyes are different, these things will focus at different distances - not good for those with tendencies toward seasickness etc.

Check with your dentist etc. for advice if you have a full wallet, otherwise ebay / mark

Gerry wrote:

Nominal magnification is inversely related to the normal reading distance of 10" so a 1" focal length lens used as a magnifier is rated as x10.

Spectacle glasses are specified in Diopters which determine the way in which they modify an eye focussed on infinity. 1 Diopter is a focal length of 1 meter (39.37") and would cause a normal eye focussed at infinity to focus at 1 meter.

With normal sight your 1.5D safety glasses would focus at

39.37/1.5 = 27". Because you find your working distance is less than 22" it means that you are short sighted and need a WEAKER lens maybe 1.0 or 0.75D

Try store bought reading glasses of this power. It is possible they may not give enough improvent. If this is the case you also need correction for astigmatism (different focal power on horizontal and vertical line objects) and this means a visit to the optician for tailor made glasses.

Jim

Harold and Susan Vordos wrote: ...

It may be at higher diopters the depth of field is so small that they aren't very usable. Bob

At the risk of beating a dead horse, order some from Zenni or another inexpensive on-line optical store - or patronize your local higher-priced shop. I don't think they even start charging extra until more than 6 diopters, which is getting into whacky short-working-distance land unless you are starting from farsighted.

At $8/pair for the low-end frames and $4.95 shipping per order (no matter how many pairs you order), they are an excellent way to stock a range of custom magnifiers. The are made in China, and I suspect that many glasses that you might think are being made elsewhere are also being made in China, but that information is not being shared with the customer when the higher bill is presented.

You can have any other aspects of your base prescription dealt with (astigmatism correction, differences between eyes, etc.) before adding whatever diopter number you'd like (keeping the working distance limitations in mind, and doing the math better than I did a couple of days ago for what the working distance will be). Ask your eye doctor to provide the inter-pupillary distance (commonly not written down), which may be slightly less as you focus in close - you can try out prescription additions at the optometrist if you bring along a work-like object to look at up close. At some point the short working distance becomes a problem, and you'll want to think about dropping either $100 or up to several thousand $ on a pair of telescopic magnifiers - however, these also limit field of view a lot. A third approach is a macro video camera and a large display screen.

Not necessarily. There are stick on lenses that cling to the lower half of safety glasses. They come in different strengths to create your own safety glasses "bifocals" I don't remember what they're called, but I found them on the web within the last year. Didn't buy any though - cheap readers with spring hinges are only $3 at the local bargain outlet.

RWL

One note on this is that while I've been able to find cheap reading glasses easily (I'm wearing a pair of +1.25's from Wal-Mart as I type this), and I've been able to find cheap bifocal safety glasses, the only way I was able to find a pair of constant-correction, ANSI Z87.1 safety glasses was to have my optometrist write me a prescription for them and pay a little over $50. I'm new to the reading glasses world (I'm only 50, so I've only been wearing them for a year), but so far I've found reading glasses OK and half-glasses OK, but bifocals pretty consistently require me to hold my head at a really weird angle to look at anything close up (before somebody points out the obvious apparent inconsistency here, no my half-glasses don't put the lens anywhere near where any bifocals I've tried put the correcting part -- the half-glasses put it much, much higher, and are much more comfortable).

I didn't spend much time looking at Zenni, but neither of the magic phrases "safety glasses" nor "Z87.1" seem to appear in the descriptions of their prescription goggles.

Bifocal safety goggles with small magnifiers at the bottom are available for around $10-$12 in welding stores. The best machining arrangement for me is normal safety glasses plus an Optivisor set high to act like upside-down bifocals. This keeps my neck at a more comfortable angle when I lean over the machine to watch the threading tool, etc.

jw

Part of my problem is the fact that I've had three disc fused in my neck so upward mobility is a bit limited. I have regular bifocal safety glasses but they force me into some uncomfortable positions when running my lathe, plus I have to get to close to be in focus. I have read that "computer" glasses have a different focal point, something like 21" to 29" where regular reading glasses are 16" to

22". I've already raised my lathe 3" to cut back on the back strain from leaning over the machine. Thanks for all of y'all suggestions!

Ditto here. You don't look up at passing airplanes. You don't look down to see what's in your shirt pocket. You don't lean back in a rocking chair unless it has a headrest.

--Andy Asberry--

------Texas-----