Hi everybody
Can anyone clarify me the diference between Magnification caused by a spectacle lens and a Hand Magnifier?
Thanks again!!
Edgley
Hi everybody
Can anyone clarify me the diference between Magnification caused by a spectacle lens and a Hand Magnifier?
Thanks again!!
Edgley
Well a spectical lens is tailored to the individual eye it is placed in front of.
This is my own way of thinking about this so It may not make sense!
A spectacle lens prescribed to correct refractive error will not magnify an image in the same way as a magnifier would because it's purpose is to render the eye emmetropic. So a person who requires a +6.00 DS prescription for ametropia will not recieve a magnified image as such, just an image of the same size as an emmetrope would see. However if the same +6.00DS was used as a spectacle magnifier by an emmetrope or by an ametrope corrected by,say, contact lenses, they would benefit from 1.5X magnification.
So I dont regard a plus powered rx as a magnifying lens (although it does magnify!) as it is only adding the neccessary power to the eye rather than making the image any bigger than the image the next guy sees.
Is this the sort of thing you meant?If not and this is patronisingly simple please accept my apologies!If I am totally wrong and this is tripe I also apologise!!:hammer:
If held the same distance from the eye and the object being viewed the magnification is the same (aproximately 1 X per four diopters of power). And Sara, I am afraid the spectacles to magnify and minify. This is the reason that image size is such a problem when powers differ greatly. However the closer to the eye the corrective lens is the less this effect is noticed. Hense contact lens wearers can stand more difference in power than spectacle wearers (aided of course by the optical centers moving with the eye reducing distortion).
Chip
:( Thanks chip and sorry Edgely, apparantly the only accuracy in my post was the part about tripe!!
...and this bit, specifically the letter part of the sentence(sorry, i just had to do it):D :D :Dsarahr said: This is my own way of thinking about this so It may not make sense!
curiosity killed the cat...well, in that case i should be dead soon
Aren't you supposed to be revising or something???;)
Hi people
First of all, I would like to thank you all for the answers.
I think chip anderson gave me the answer that I was expecting to get, what is getting confused is that a lens +4,00D with:
Base curve:+6,00
Tc:6mm
Index: 1,499
Stop Distance: 27mm
This lens give us 9,5% of magnification, is it? why not 1x? :hammer:
So, I understood that in this case, not only the lens correct the vision problem ( sending the image to the retina), but also, magnify the image by 9,5%. Am I completly wrong?
Thanks you guys:) for the helping hand.
Edgley
yes, but this was too tempting.:bbg:sarahr said:
Aren't you supposed to be revising or something???;)
curiosity killed the cat...well, in that case i should be dead soon
Edgley,
Spectacle magnifiers (used as low vision aids) magnify by proximity. That is, it is the closeness of the object that creates the magnification, not the lens. The lens merely allows you to hold the object very close. The standard from which the amount of magnification is calculated is an object distance of 250 mm. The image is considered normal size (1X magnification) at this distance. If you hold the object at 125 mm it will appear twice as big. Since 250mm is the focal length of a +4D lens the spectacle magnification is the power of the lens divided by 4. So a +16D lens creates 4X spec mag and will be held at the focal length of the +16D which is 62.5mm (or 4X as close as 250mm). This all assumes that the person is emmetropic. If they are not then the magnification is dependent on how much of the lens power is the addition. For example if a person with an Rx of +4.00D had a spectacle magnifier of +16D they would hold the object at the focal length of a +12D (since 4D of the lens power is used up in correcting the hypermetropia) anfd the spec mag would be 3X (+12 divided by 4). A hand magnifier or stand magnifier magnifies the image in a totally different way. They create an erect magnified virtual image of the object.
Spectacle mannification, on the other hand, is the change in the retinal image caused by spectacle lenses used to correct ametropia. As Chip says, all lenses create this effect. There are two factors responsible for the magnification, the shape factor and the power factor. The power factor is 1/1-dF where d is the BVD in metres. The shapefactor is 1/1-tF1 where t is the lens thickness in metres and F1 is the power of the front curve. The total spectacle magnification is the power factor times the shape factor.
I hope that this helps.
Regards
David
Hi David
Thanks for your explaination.
Just to check if I understood it in the right way... spectacle magfication and magnification cause by a magnifier has nothing to do with each other, is it?:hammer:
Thanks for the helpping hand..
Best regards
Edgley
Hi Edgley,
That's right. Spectacle lenses used to correct ametropia will create a magnification effect (spectacle magnification). It is usually relatively minor, less that 5% in most cases. Spectacle magnifiers are intended to magnify the image usually for low vision patients. They do this by allowing the object to be held very close.
Regards
David
Hi!David Wilson said:
A hand magnifier or stand magnifier magnifies the image in a totally different way.
I think that in a way there is no difference between spectacles and hand held magnifier.
reading spectacles work with print held at the focal
point of the lens and the image is at infinity
Hand held magnifier in the simpliest applicaton is used with distance glasses and held above the print at the focal point of it and the image is at infinity similary to the spetacle.
Best regardes
care
Hi Care,
They are different. When using a hand magnifier the object is held within the first principal focus of the lens an an upright virtual image is formed (but not at infinity). Indeed, manufacturers calculate ,and in many cases tell you, the distance of this image. For example, Eschenbach calculate the 'Eye to image' distance (normally 400 mm). The lens is designed to produce the image at this distance so that the user can use the lens with their reading correction. On the other hand, spectacle magnifiers magnify using the proxinity of the object. Low vision clinics decide whether htir patient will be more comfotable with a head borne lens (with the object held very close) or a hand held lens, with the object at a comfortable position.
A hand magnifier can be used like a spectacle magnifier, merely bu
y holding the lens up to the eye, but then it is effectively a hand held pair of spectalces, like a lorgnette.
Regards
David
Hi David!David Wilson said:
Hi Care,
They are different. When using a hand magnifier the object is held within the first principal focus of the lens an an upright virtual image is formed (but not at infinity).
I actualy wonder what made you decide that"vitual image is formed,but not at infinity"???
All my favorite authores on low vision probably would disagree with you.
1)"Primary care of optometry" by T.Grosvenor O.D.,Ph.D.(3 ed)
writes:
Page 524"If a hand magnifier is held at a distance from a printed page equal to its focal length, the patiant will see an enlarged,erect image of the print.The image will be formed at INFINITY so it will be seen clearly through patient`s DISTANCE CORRECTION without the need to accommodate."
2)"Primary low vision care" by R.W.Nowakowski O.D.,Ph.D.
writes:
p.89-90" A hand mag-r or hand-held mag-r is a plus lens that is literally held in the hand, usually by means of handle...If the object is located in the focal plane of the lens, the image is formed at OPTICAL INFINITY, parallel light leaves the lens, and the image will be in focus when viewed through the best DISTANCE CORRECTION"
3)"The art and Practice of low vision" by P.B.FREEMAN O.D.(2 ed)
writes:
p256"Hand-held mag-rs ...can be placed anywhere in space..In the simplest application they are used with DISTANCE SPECTACLES and held above the print at the focal point of the lens. The image is at INFINITY and SIMILAR TO THE SPECTACLE the magnifier lens provides F equal to the power of the lens itself"
Would you please, David ,write where is the sourse of yours abovemention statement.
Best regards
Care
Hi Care,
I take your point, and I did say that a hand magnifier can be used like a spectacle magnifier, that is, with the object held at the focal point. You will also note that your quotes begin with "If". This is because there is more than one way a plus lens can magnify. However, hand magnifiers are intended to have the object held within the principal focus. The evidence of this (I will get to some authors in a moment) is that stand magnifiers, which are a variation of hand magnifier, do not have the stand set at the focal length but shorter so that the object is within the principal focus. Also, these manufacturers design the image to be set at the distance of most distinct vision (around 400 mm) and requiring accommodation or a reading add.
As to my references (most of my books are at home at the moment but I do have two rather good books here). Mo Jalie (Principles of Ophthalmic Lenses), says"It is more reasonable to suppose that the object will be positioned so that the image also lies at the distance of distinct vision, q. The magnification which is obtained under these conditions can be deduced from Figure 368" This figure he refers to shows an erect virtual image being formed at the distance of distinct vision, not infinity.
Freeman (formerly Fincham and Freeman) Optics 10th edition. His diagram (Figure 6.14) shows an erect image being formed at the distance q, and erect virtual image. Freeman also arguse that thas image is formed at a distance,q. He does point out that the image could be formed anywhere between infinity and q.
I agree, Care, that if a magnifier is used like a spectacle magnifier (with its object held at the focal length, the image will be at infinity. But most magnifiers are not used this way and, indeed, stand magnifiers are not designed to be used this way. You will find that an advanced presbyope trying to use a stand magnifier with their distance correction will need to lift the stand magnifier off the page, making the stand rather pointless.
In short, Care, I don't think that we are really disagreeing all that much. Have a look at some of Eschenbach's printed material to see what I mean by the eye to image distance. They have some quite good stuff.
Regards
David
Last edited by David Wilson; 11-25-2002 at 08:43 PM.
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