Sort by: Top Voted. Thin lenses in contact . https://www.khanacademy.org/.../v/sign-conventions-for-mirror-lenses Next lesson. Sign convention is a set of rules to set signs for image distance, object distance, focal length, etc for mathematical analysis of image formation. Donate or volunteer today! This is the currently selected item. According to it: Object is always placed to the left of lens. Khan Academy is a 501(c)(3) nonprofit organization. News; Thin lens sign conventions. Site Navigation. Our mission is to provide a free, world-class education to anyone, anywhere. All distances are measured from the optical centre of the mirror. Some natural phenomena. About.