Sign Conventions

The light path follows the direction from left to right. Paths measured in the direction of light are positive and those in the opposite direction of light are negative. Paths perpendicular to the optical axis are positive above the axis and negative below it (in this case, for example, u‘ is negative)..

The minimum object distances (MOD, see charts of lenses) are determined by the maximum possible focusing limits. For extreme close-up work (less than MOD), there are two major approaches to effectively reducing these limits:

a) Inserting one or more extension tubes. The total length of extension tube(s) needed to yield magnification of an objective lens of a specific focal length is given by:

Ex tens ion tube length

b) Inserting an auxiliary positive lens. The power in diopters of the auxiliary close-up lens needed for working at a specific object distance is:

Close-up lens power