Which lens provides greater magnification with Slit Lamp, +90D or +78D?

• A. +90D
• B. +78D
• C. Both equal in magnification
• D. Magnification cannot be determined from diopter alone

Answer: (B)

The amount of magnification you see when viewing the retina through a slit-lamp lens comes from how the lens collimates and relays the retinal image into the slit-lamp eyepiece. A lens with lower dioptric power forms a larger retinal image at a given observation distance, and that larger image is then magnified by the slit lamp’s eyepiece to a greater extent. A higher-power lens (more diopters) has a shorter focal length, which tends to produce a smaller retinal image at the same distance and, after the eyepiece magnification, a smaller overall image. So, using the lower-power lens (+78D) yields greater magnification than the higher-power lens (+90D). The trade-off is that the higher-power lens often provides a wider field and brighter view, while the lower-power lens concentrates more on magnification of fine details.