Magnification vs diopters
Yesterday we had a customer that wanted a magnifier that would magnify 5x but most of our magnifiers list their power in diopters.
I looked in the catalogs to find a coversion between diopters and magnification, but it doesn't make sense to me. In B&L's catalog they say that an 8 diopter is the same as 2x, but Coil says that an 8 diopter is the same as 3x. Why the difference? I've checked with several people but nobody can explain this.
There are various definitions of magnification, but, it the field of low vision and magnifiers, head borne aids (such as hyperocular) use the formula power divided by four. Manufacturers of hand magnifiers use what is often referred to as industrial magnification which is power divide by four plus 1. This is the formula that COIL has used. B & L have adopted the spectacle borne formula (even though it is a hand magnifier) which gives a more conservative measurement. COIL would, no doubt, point out that the B & L formula gives the rather anomalous measurement of 1 X for a +4 D lens. Clearly a +4 D lens will produce some magnification and not merelty keep the image the same size as the object which is implied by 1X.
Spectacle (head borne) magnifiers magnify in a different way to hand held magnifiers. Spec magnifiers magnify by proximity (the closeness of the object) with the lens merely making the highly divergent light parallel. Hand magnifiers produce a magnified virtual image.
I hope that this helps.
In addition to David's excellent response, I thought that I would also note that the "Diopters divided by 4" rule is based upon a reference distance of 25 cm (1/4 m). This distance, which is roughly 10", is known as the least distance of most distinct vision. David also pointed out the spectacle lenses produce magnification by "proximity" -- another good point. This essentially means that the wearer is able to hold reading materials closer, effectively increasing the image size on the retina. For instance, a +4.00 D lens allows the wearer to hold a near object at 25.0 cm. A +8.00 D lens allows the wearer to hold the same near object at 12.5 cm. Consequently, the retinal image produced by the +8.00 D lens will be twice as large as the same image produced by the +4.00 D lens, since the same near object is only half as far away. When compared to the 25 cm reference distance, the +4.00 D lens produces 25 cm / 25 cm = 1x magnification (or +4.00 D / 4), while the +8.00 D lens produces 25 cm / 12.5 cm = 2x magnification (or +8.00 D / 4).
Two Lens System
With two ultimate explainations by David and Darryl,
I could easily excavet the Defination of Magnification taught in 5th or 6th Std. It said : the Ratio of Focal Lengths of the Objective to the eye piece is Known as Magnification.
In case of Headborne systems ( Single Lens other than eyelens itself,), as Darryl says,the Least distance of Most distinct vision (LDMDV), refers to the focal length of the natural Eyelens.
This LDMDV may vary from person to person and probabaly it's more of a Generalised and refaral standard.
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