We consider photometric reverberation mapping, where the nuclear continuum variations are monitored via a broad band filter and the echo of emission line clouds of the broad line region (BLR) is measured with a suitable narrow band (NB) filter. In the first part of this contribution we investigate how a incomplete emission line coverage by the NB filter influences the BLR size determination. This includes the two basic cases of 1) the symmetric cut of the blue and red part of the line wings and 2) the filter positioned asymmetrically to the line center so that essentially a complete half of the emission line is contained in the NB. We find that symmetric cutting of line wings may lead to an overestimate of the BLR size which is less than about 5%. The case of asymmetric line wing cutting, as for our data of the Seyfert-1 galaxy 3C120, yields the BLR size with less than 1% bias. In the second part of this contribution we model the BLR of 3C120. We use well sampled light curves with quite sharp features, which were provided by photometric reverberation mapping at the Universitätssternwarte Bochum in Chile. The aim is to determine the geometry type of the BLR. We find that a spherical BLR yields only a poor fit of the light curves, but a thin disk-like BLR with extension from 22 to 28 light days and nearly face-on inclination i = 10 ±5 reproduces all of the features of the light curve with great significance. To summarise, any BLR size bias due to narrow-band line cut in photometric reverberation mapping is usually negligible. Thanks to the well sampled light curves efficiently obtained by photometric reverberation mapping it was possible to determine the BLR geometry for 3C120.