An attempt is made to deconvolve the contribution of the central star from that of nebular emission, in small and large Planetary Nebulae. The use of medium-band Strömgren photometry is possibly a more powerful tool than that of broad-band photometries to achieve meaningful stellar parameters, specially by means of a four-channel photometer. The various methods used to subtract the nebular contribution are analysed and the results of the first ubvy and HΒ observations are presented.