The determination of stellar and nebular parameters of planetary nebulae such as effective temperature, luminosity, distance, extinction and elemental abundances is often very difficult, especially if the star is completely hidden by its circumstellar material (CSM). In this case, the CSM absorbs the stellar light, and only reprocessed radiation can be observed. The stellar parameters must then be determined indirectly, e.g. from the emission lines. In the literature there are often simple approximations used but a crucial point is that these approximations deal with the assumption of a constant electron density. Using a set of optical observations, we compare the results that are obtained for the stellar parameters from the constant density nebula model with a simple spherical wind model. We emphasize the differences that arise for some of the stellar parameters with special focus on the elemental abundances in the nebulae.