Which lens provides greater field of view with Slit Lamp, +90D or +78D?

Which lens provides greater field of view with Slit Lamp, +90D or +78D?

• A. +90D
• B. +78D
• C. Both have the same field of view
• D. Field of view is not related to lens power

Answer: (A)

The key idea here is how lens power affects magnification and field of view when using a slit lamp. A condensing lens with higher diopter power has a shorter focal length (f ≈ 1/D). Shorter focal length means the system produces higher magnification of the retinal image. When magnification goes up, the portion of the retina that can be seen at one time through the slit-lamp setup becomes smaller, so the field of view is reduced. Conversely, a lens with lower diopter power has a longer focal length, which yields less magnification and a larger portion of the retina visible at once. So, in practice, the lens with higher diopter (+90D) gives more magnification but a narrower field, while the lens with lower diopter (+78D) provides a wider field of view. Therefore, the greater field of view is achieved with the lower-power lens (+78D) rather than the higher-power lens (+90D).

The key idea here is how lens power affects magnification and field of view when using a slit lamp. A condensing lens with higher diopter power has a shorter focal length (f ≈ 1/D). Shorter focal length means the system produces higher magnification of the retinal image. When magnification goes up, the portion of the retina that can be seen at one time through the slit-lamp setup becomes smaller, so the field of view is reduced.

Conversely, a lens with lower diopter power has a longer focal length, which yields less magnification and a larger portion of the retina visible at once. So, in practice, the lens with higher diopter (+90D) gives more magnification but a narrower field, while the lens with lower diopter (+78D) provides a wider field of view.

Therefore, the greater field of view is achieved with the lower-power lens (+78D) rather than the higher-power lens (+90D).