Converging/diverging mirrors/lenses: conceptual question

[tensor0910]

Homework Statement

Im having trouble knowing/remembering when my d_i and d_o should be positive or negative. Does anyone have any tips/advice for this?[/B]

Homework Equations

1/f = 1/d_i + 1/d_o, h_i/h_o = d_i/d_o[/B]

The Attempt at a Solution

No solution. Just a question.[/B]

 

[andrevdh]

Real images and objects positive, virtual images and objects negative.
Converging lens focal length positive, diverging negative.