Brown William L, Siemsen Dennis W
Mayo Clinic, Department of Ophthalmology, Division of Optometry, Rochester, MN 55905, USA.
Optometry. 2008 Jan;79(1):9-17. doi: 10.1016/j.optm.2007.03.016.
Stand magnifiers (SMs) are traditionally labeled with F/4 or (F/4) + 1 magnification. This study addresses whether SMs are configured so that the assumptions associated with the labeled magnification can be realized.
Three catalogs were examined to obtain the type of magnification label used for each of 66 different SMs. Image locations were acquired from published tables compiled by independent investigators when available. Otherwise, they were taken from manufacturer-provided information in the catalogs. The image location was used to determine how many magnifiers could be used in a manner that fulfills the assumptions underlying the labeled magnification.
For F/4 magnification, the page is assumed to be at the focal point of the SM and the image at infinity. No SMs met this condition. (F/4) + 1 magnification assumes the magnifier is close to the eye and the image is at 25 cm. Only 18% of the SMs have a 25 cm image distance.
Most SMs do not have image locations that meet the conditions underlying the marked magnification. The current labeling system is inaccurate, misleading, and inefficient for clinicians. Magnification labels on SMs should be replaced with markings that include equivalent power, enlargement ratio, and image location.
