Trying to figure out the link between magnification and focal length.
I need glasses to be able to read and see what I''m doing clearly on
my lathe but the 1.50 safety glasses I have force me to get too close
to my work. Looking for something with a focal length of maybe 22" so
I can stand at a comfortable position. Does that mean a stronger lense?

Gerry writes:

Trying to figure out the link between magnification and focal length.
I need glasses to be able to read and see what I''m doing clearly on
my lathe but the 1.50 safety glasses I have force me to get too close
to my work. Looking for something with a focal length of maybe 22" so
I can stand at a comfortable position. Does that mean a stronger lense?

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.

In article . com,
Gerry wrote:

Trying to figure out the link between magnification and focal length.
I need glasses to be able to read and see what I''m doing clearly on
my lathe but the 1.50 safety glasses I have force me to get too close
to my work. Looking for something with a focal length of maybe 22" so
I can stand at a comfortable position. Does that mean a stronger lense?

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.

--
Cats, coffee, chocolate...vices to live by

Gerry writes:

Trying to figure out the link between magnification and focal length.
I need glasses to be able to read and see what I''m doing clearly on
my lathe but the 1.50 safety glasses I have force me to get too close
to my work. Looking for something with a focal length of maybe 22" so
I can stand at a comfortable position. Does that mean a stronger lense?

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.)

"Leo Lichtman" wrote in message
...

"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?
^^^^^^^^^^^^^^^^
snip---

They won't be safety glasses, though. For that you will need to give
some money to your optician.

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 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 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! :-)


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.)

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

In article , Ecnerwal wrote:

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.

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.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.

On Sep 18, 9:43 pm, Gerry wrote:
Trying to figure out the link between magnification and focal length.
I need glasses to be able to read and see what I''m doing clearly on
my lathe but the 1.50 safety glasses I have force me to get too close
to my work. Looking for something with a focal length of maybe 22" so
I can stand at a comfortable position. Does that mean a stronger lense?

http://www.doneganoptical.com/optivisor.php

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:
Trying to figure out the link between magnification and focal length.
I need glasses to be able to read and see what I''m doing clearly on
my lathe but the 1.50 safety glasses I have force me to get too close
to my work. Looking for something with a focal length of maybe 22" so
I can stand at a comfortable position. Does that mean a stronger lense?

On Tue, 18 Sep 2007 18:43:25 -0700, Gerry
wrote:

Trying to figure out the link between magnification and focal length.
I need glasses to be able to read and see what I''m doing clearly on
my lathe but the 1.50 safety glasses I have force me to get too close
to my work. Looking for something with a focal length of maybe 22" so
I can stand at a comfortable position. Does that mean a stronger lense?


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:
....
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. ...

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

In article ,
Bob Engelhardt wrote:

Harold and Susan Vordos wrote:
...
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. ...

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

I frequently buy 4.00 and once in a great while up to 4.5 from flea
market vendors. Last Sunday I bought some 4.00

Chuck P.

In article ,
"Harold and Susan Vordos" wrote:

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.

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.

--
Cats, coffee, chocolate...vices to live by

On Wed, 19 Sep 2007 04:34:49 GMT, "Leo Lichtman"
wrote:


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.

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

Ecnerwal writes:

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.

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.

On Sep 20, 11:35 pm, Joe Pfeiffer wrote:
...
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

On Sep 21, 5:32 am, Jim Wilkins wrote:
On Sep 20, 11:35 pm, Joe Pfeiffer wrote:

...
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!

On Fri, 21 Sep 2007 08:03:24 -0700, Gerry wrote:

On Sep 21, 5:32 am, Jim Wilkins wrote:


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-----