We investigate the impacts of zero loss peak (ZLP) removal and retardation effects altering the electron energy loss spectrum on the optical properties obtained by using Kramers-Kronig analysis and on the determination of the bandgap. For this purpose we use amorphous SiN(x):H having a bandgap of Eg(SiNx:H)= 5.5 eV. We demonstrate that for bandgap determination not only the accurate removal of the ZLP is crucial, moreover also retardation losses have to be taken into account. The same is valid for an accurate determination of the optical properties of semiconductors which can be done if the retardation effects are treated correctly or avoided at all before Kramers-Kronig analysis is applied. Beside the detailed study on using SiN(x):H we discuss the impact of the retardation effects on several other semiconductors and insulators, like GaP.