Salmon Egg schreef:
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Bill, Andy only wants his magnifiers qualified, nothing more nothing
less and if possible in a simple way.

I could explain more but Andy wants it simple.

If you want to calculate the magnification of the combination
eye-handheldmagnifier, there is more to explain.

Jan (normally Dutch spoken)

On Sun 12 Oct08 15:15, GeorgeB <nospam[at]att.net> wrote in
<news:u314f4168fbp5t4rr79odutct3te2nn57k[at]4ax.com> :

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Thank you all for the info and thank you for your patience!

I thought I had got the hang of this so I started to do the
calculations and write the result on the lens holders.

First I took a small magnifier for looking at samples of cloth. It has
a FL=4.7cm so I calculated as follows:
100/4.7 = 21.28 dioptres. 21.28/4 = 5.3 magnification.
Then another magnifier on a stand with FL = 16cm
100/16 = 6.25 dioptres. 6.25/4 = 1.6 magnification
Then a large hand held lens with FL = 28cm
100/28 = 3.57 dioptres. 3.57/4 = 0.89 magnification.

Hold on. 0.89 magnification? That seems wrong as I would expect
magnification values are greater than 1. QUESTION: Should I add "1"?

-------------

I'm starting to think that simply marking the lenses with a
"magnification value" is much harder than it seems!

As an alternative I suppose I could mark FL values. However some of
the lenses have got manufacture's markings on them stating their
magnification, so it would seem appropriate to continue with the same
convention. I just didn't expect it to be so tricky!

Do I add that "1"? Does the use of "1" differ if the lens is used
close up or used at a distance like these two examples:

distance http://www.medimax.co.uk/acatalog/magnickel.jpg
close-up http://images.orgill.com/200x200/6941637.JPG

In article <48f3b89e$0$12266$58c7af7e[at]news.kabelfoon.nl> ,
Jan <nospam[at]nospam> wrote:

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This "standard" is based upon the assumption that the largest angular
view for a "normal" is obtained at a distance of 250 mm from an unaided
eye. The magnifier close to the eye, effectively brings the "object"
closer.

Bill

--
Private Profit; Public Poop! Avoid collateral windfall!

Subject: Further details on Magnification

Magnification


The magnification of a magnifying glass depends on where it is placed
between the user's eye and the object being viewed, and the total
distance between the eye and the object. Magnifying glasses are
typically described in terms of their magnifying power, which is
equivalent to angular magnification (this should not be confused with
optical power, which is a different quantity). The magnifying power is
the ratio of the sizes of the images formed on the user's retina with
and without the lens.[3] For the "without" case, it is typically
assumed that the user would bring the object as close to the eye as
possible without it becoming blurry. This point, known as the near
point, varies with age. In a young child it can be as close as 5 cm,
while in an elderly person it may be as far as one or two metres.
Magnifiers are typically characterized using a "standard" value of
0.25 m.

The highest magnifying power is obtained by putting the lens very
close to the eye and moving the eye and the lens together to obtain
the best focus. The object will then typically also be close to the
lens. The magnifying power obtained in this condition is MP0=1/4 $B&5(B+1,
where $B&5(B is the optical power in dioptres, and the factor of 1/4 comes
from the assumed distance to the near point. This value of the
magnifying power is the one normally used to characterize magnifiers.
It is typically denoted "m$B!_(B", where m=MP0. This is sometimes called
the total power of the magnifier (again, not to be confused with
optical power).

Magnifiers are not always used as described above, however. It is much
more comfortable to put the magnifier close to the object (one focal
length away). The eye can then be a larger distance away, and a good
image can be obtained very easily; the focus is not very sensitive to
the eye's exact position. The magnifying power in this case is roughly
MP=1/4 $B&5(B.

A typical magnifying glass might have a focal length of 25 cm,
corresponding to an optical power of 4 dioptres. Such a magnifier
would be sold as a "2$B!_(B" magnifier. In actual use, an observer with
"typical" eyes would obtain a magnifying power between 1 and 2,
depending on where lens is held. An older person might obtain an
actual magnifying power of 8 or more with this lens, however, due to
the eye's longer near point distance.

Enjoy,




On Oct 13, 10:18 pm, otisbr...[at]embarqmail.com wrote:
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Subject: General Equations for Optical Calculations.

Since this subject develops on sci.med.vision -- here
are the equations.

Re: Some Optical Equations for Analysis


The power of a lens in diopters.


1 / f = P

Where

f = Focal Length in Meters

P = Power in diopters

Motivation for using "Diopter" Values:

The power of lenses can be added (within reason).

P = P1 + P2

See the thin-lens formula for reference.

===========

Magnifying Power (telescope)

Mag = - F / f

Where F is the focal length of the objective

f = Focal length of eye-piece.

================

Focal Length, Image and Object Calculations:

1 / p + 1 / q = 1 / f

Where:

f = Focal Length of Lens

p = Object Distance to Lens

q = Image Distance to lens


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Magnification Equation

I / O = - ( q / p )

Where:

I = Image Size

O = Object Size

p = Object Distance to Lens

q = Image Distance to lens

=============

Combining Thin Lenses

1 / f = 1 / f1 + 1 / f2

Where:

f = Total focal length

f1 = First lens focal length

f2 = Second lens focal length

Enjoy,




===============




On Oct 13, 5:07*pm, Jan <nospam[at]nospam> wrote:
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Andy schreef:

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Yes sir!!!

The magnification standaard for a handheld glass is the diopters devided
by 4, period.

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So in this case you have to call this handheld a 8.33/4= 2 times
magnifying glass, period.

This is a world wide standard industrie qualification.

Jan (normally Dutch spoken

Andy wrote:
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Same thing:
1000/fl = diopters, given focal length in mm.
250/fl = "magnification"

So, a lens with 12 cm (120 mm) fl is 8.33 D.
Or, 250/120 = 2.0833 X magnifier.
8.33 / 4 = 2.0833

- quote -

Just as Dave suggested in the earlier reply. This is a more useful
figure for magnification, assuming a hand-held lens. A loupe is kept at
a fixed eye distance, so the simple magnification is probably more correct.

- quote -

The answer, sadly, is "Yes"! Choose your formula, either one is
"correct". For me, assuming a hand lens, I'd say 3X

Dave

On Sun, 12 Oct 2008 10:08:21 +0100, Andy <andy[at]yahoo.com> wrote:

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We'll assume positive (magnifying) lens here ... I believe the diopter
terminology is identical, but the definition of focal lenght is harder
to understand, and my determination method does not work for negative
lens.

First, diopter is the reciprocal of the focal length in meters ... if
you have a 2 diopter lens, its focal length is 50 cm.

Magnification ASSUMES 1x for 10" (or 250mm) focal lenght. Shorter
focal lengths are higher magnification by their ratio ... a 4x
magnifier will be 2.5" focal length; a 10x will have a FL of 1".

I don't know but one way to determine focal length. If you light an
object and create its image in focus on the opposite side of the lens,
and if the ratio is 1:1 (object and image the same size), the object
and image are 4 FL apart. SO, your 12cm FL lens, placed half way
between an object and wall that are 48cm spaced, will have a projected
image the same size.

On Sat 11 Oct08 08:09, Dave Martindale <davem[at]cs.ubc.ca> wrote in
<news:gcpjf5$sqp$2[at]swain.cs.ubc.ca> :

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Dave, thank you for the information.

I think I didn't give you enough background. My query is for a
handheld magnifying glass. In such a case a think magnification is
stated as a fixed value such as "x8" (although the lens position will
determine the actual magnification that is achieved).

Now keep it simple for me please!

I have several of these and wanted to label them bytheir power. I
determined the focal length by using the sun and seeing how far away
from the lens the sharped image of the was. My example said 12 cm.

So you showed me that 12 cm focal length is 1/0.12 = 8.33 dioptres.

ISTR that 4 diopters makes one "magnification"??? OTOH I have seen
suggestions to "add 1" to the result to get magnification.

Does this lens with FL of 12 cm, have a magnification of x2 or x3?

Thank you. The info is a bit too theoretical for my ability. For
example the info at this page is a bit tricky for me to use to
simply get dioptre and magnification from focal length.

http://hyperphysics.phy-astr.gsu.edu.../image.html#c1

Is there something simpler?

Andy


On Sat 11 Oct08 05:20, <otisbrown[at]embarqmail.com> wrote[at]
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