Welcome to question of the day #141

Eyetools question of the day #141

One of my patients is trying to use a stand magnifier for reading. She tells me that she can’t see to read at all when she uses her distance vision glasses with the stand magnifier and when she tries to read with her reading glasses she has to lift the stand magnifier off the page in order for the print to become clear. She can’t hold the magnifier off the page for more than a few seconds at a time. How can I help her?

I have a lot of experience of working with people who need stand magnifiers to help with reading. I have used and recommended stand magnifiers from many of the major manufacturers. Reading for longer periods of time usually requires the use of a stand magnifier. A stand magnifier consists of a plus lens mounted on legs that fix the distance from the lens to the object, which is usually print on a page. Some of the magnification comes from the plus lens itself and some comes from the close distance the patient can get to the lens and the print. The closer a person can get to something the bigger it is-this is relative distance magnification. The closer the user holds a stand magnifier to the eye or glasses the better the field of view.

Nearly every stand magnifier I have used is constructed so the legs that hold the lens (the stand) are shorter than the focal length of the lens. This means that light from the object (print) emerges from the lens is diverging. This is often described as ‘the emergent vergence is divergent’. I have asked many people who are knowledgeable about optics why this is and the best they could come up with is ‘that the optics works best this way and the image will be clearer.

In order for a person to focus on divergent light, they need to be able to accommodate (children) or use reading glasses (presbyopes). Both of these will focus the divergent light on the fovea. When a person using a stand magnifier is lifting the stand magnifier off the page they are reducing the divergence of the emergent vergence allowing the reading glasses that they are wearing to focus the divergent light on the fovea.

While there are some mathematical techniques to determine which reading addition is suitable for each stand magnifier, in my experience, trialling reading lenses works well. When determining which stand magnifier provides the optimum visual function in practice, use a trial frame and the distance refractive correction and then manipulate the reading addition until the person can see the print clearly at a reasonable eye-to-lens distance, without having to lift the device off the page. If this occurs with the current reading glasses great, if not then new stronger reading glasses will need to be supplied.