Condensing Lenses are used in conjunction with the Binocular Indirect Ophthalmoscope (BIO) and Slit Lamp Biomicroscope. Use them to view a wider area of the retina than would be possible through direct opthalmoscopy. Each of these lenses are individually optimized for viewing different areas of the retina.
Though there are many different types of lenses...
there are a few basic designs that are considered "universal".
Simple magnification is the type most often used or mentioned in general publications or manufactures of lenses.
Genrally, the lower the power the condensing lens the farther from the eye it must be held. The stronger the power the condensing lens the closer it must be held toward the eye. The relationship is very simple, for it is based mainly on the focal length of the condensing lens. Ideally one wants the light from the condensing lens to focus perfectly within the pupil of the patient's eye. This placement will result in the condensing lens forming an image of the retina, in front of the condensing lens, which fills the whole lens.