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Following sign convention is used for measuring various distances:
- All distances on the principal axis are measured from the optical centre.
- The distances measured in the direction of incident rays are positive and all the distances measured in the direction opposite to that of the incident rays are negative.
- All distances measured above the principal axis are positive. Thus, height of an object and that of an erect image are positive and all distances measured below the principal axis are negative.
| The rules are same as in case of spherical mirrors.
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The following table gives the sign convention for lenses:
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