Lensometer Optics

When a plus lens is inserted at the lens stop, the light rays leaving the lens and the eyepiece are converging, causing the mires to blur. As the lensometer drum is adjusted to clear the mire image, the lighted target (that creates the mires) moves toward the standard lens. When the light rays entering the lens being measured have the same divergence as the converging power of the lens, parallel light will exit the lens and eyepiece and the image will once again be in focus.