Does anyone know how to convert diopters to magnification power? I mean, if I have a 4x magnifying glass, how many diopters is that? Or if I have a 30 diopter plus lens, what magnification power is it?
Does anyone know how to convert diopters to magnification power? I mean, if I have a 4x magnifying glass, how many diopters is that? Or if I have a 30 diopter plus lens, what magnification power is it?
For simplicity's sake, it's times 4.
4x magnifier = 16 D
30 D lens = 7.5x
(If I remember right, it's not exactly 4, but it's the easiest way to calculate it quickly.)
Last edited by mlm; 02-26-2008 at 01:01 AM. Reason: more explanation
From what I remember there are 2 standards and Asian and a German standard both of which have different conversions but both fall in the 4-5D range.
Good Luck
Thanks Guy's I was wondering this myself, excellent question!;)
The Scientific conversion: magnification reference (comparative) distance is 25cm (10 inches). 4D = 1X magnification.
This means that a lens that permits an object to be in focus at 10 inches away yields an approximately-equivalent retinal image size to an un-aided eye that focuses at the same distance.
If there reference distance is changed, say to 8 inches (approx 20 cm), then 5D = 1X magnification
Then, add in marketing by magnifier companies that couldn't explain or find persuasive the concept that a 4D lens only yields 1x mag, and they decided to create the formula:
Magnication (of a simple maginifier) = Dioptric power of lens/ 4 "+" *1* = magnifying power.
In this example, a 4D lens = 2X.
This is all for "linear" magnification. Angular magnification is something else.
FWIW
Barry
This IS actually one of the few cases where it´s not (only) marketing, but there is some true optics behind it.;)
The first formula applies to the case, where the eye itself is focussed to infinity. The "+1" enters for the more realistic case, where the eye is already focused to 25cm and you use of the magnifier, meaning you also need to move the object a bit nearer to the lens (In that latter case, actually the distance eye-magnifier lens enters, the "+1" is an approximation for the limiting case where the magnifier lens is straight in front of the eye).
小卫
I am so glad that I found help help here. There is a study guide out there that says to find the dioptric power of a magnifying glass that one should multiply the "x" power by 2.5, however several sources say to multiply by 4, so thank you all.
I do have a few questions that I am certain can be answered here.
1. If an image in a mag lens is clearer at 5cm from the page, what is the mag power of the lens?
2. If a patient has a 30D hand mag, what distance should they hold it from the page to get the clearest image?
Thanks once again
Eliminating accommodation effects and assuming magnifier held at focal plane:
a. 1/5cm = 20 D
F/4 +1 = 6X ("conventional magnification") or F/4 = 5x ("relative magnification")
b. 1/30 D = 3.3 cm
Last edited by drk; 04-25-2008 at 01:14 PM.
Brief comment on the statement by Dr. Santini: This is all for "linear" magnification. Angular magnification is something else."
Actually the magnification discussed is angular magnification. That is what the patient is interested in.
Everything I see on this thread correctly points out that the formula assumes a near point (NP) of 25 cm. For an older patient, typically with a larger NP, the generalization is angular magnification = NP x diopter + 1.
The equations for this, assuming a thin lens, are in many references on the web, e.g. http://www.phy-astr.gsu.edu/cymbalyuk/Lecture4.pdf (I have no association with this university, this was just the first reference that popped up on google.) There is a small typo in the formula for magnification, where the symbol 'q' should be 'f' but everything else is OK. Picture are nice.
Just keep in mind that the formula for conventional or maximum magnification, F / 4 + 1, assumes that the image vergence leaving the magnifier will be equal to the 25 cm reference distance. So I believe that the patient will need to either wear bifocals with close to a +4.00 D add power or make up the difference with ocular accommodation (if available).Originally Posted by heritage972
The attached article provides some of the specific mathematics involved:
Best regards,
Darryl
Darryl J. Meister, ABOM
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