Variation of the line indices with stellar population age and metallicity. In each plot, the EW is shown in left vertical axis, while the right axis shows the average fractional depth below the continuum. For each feature we draw the models constructed with Starburts99 at five value of metallicity as shown in different colors. For each metallicity, the color regions represent the error due to the dependence on the IMF assumed. In this way we can highlight the dependence of the models and robustness of the indices from the IMF and stellar population age assumed.