- 1). Mark the pupils while the glasses are on the person to be measured. Use a lens marking pen, also called a lens marker. Make a dot on the lens right over the pupil as the person looks straight ahead.
- 2). Remove the glasses and put them on the table of the lensometer. Start with the right lens. Line up the dot you made on the lens directly under the lensometer port.
- 3). Look through the eyepiece and focus the mires, the lines that show the spherical and cylindrical power of the lens. There are two sets of mires that form a cross.
- 4). Notice where the two mires cross. If the lens does not have prism, they will appear directly inside the circle you see through the eyepiece. If the lens has prism, you will see that the image of the mires cross is displaced from the center.
- 5). Observe the bullseye or target-like image of rings inside the lensometer. Prism is measured in lines of the bullseye. The prism strength increases as the intersection of the displaced mires moves away from the center of the target.
- 6). Each line of the target represents a degree of prism as base out or base in. Base out is when the displaced image is left of the center of the target and base in is to the right of center.
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