This rotary slide rule Conversion Disc allows calculation of the equivalent effective power at alternative distances between the cornea and the lens. This is particularly useful to show the power differences caused by trial lens and spectacle lens distance. It is essential to have the means of assessing the power compensation required to corrections over 5 dioptres. Can be supplied as part of our Vertex Distance Gauge Set (SGR-271-VDS) which also includes our Vertex Distance Gauge (VDS-217-VDG).