  A detailed analysis of NGST multiband photometry applied to the reference
case of the study of high-redshift galaxies has been carried out with
simulations based on galaxy SEDs derived from the currently available empirical
and model templates and on plausible standard filter-sets. In order to
correctly identify star forming galaxies in the redshift range 5<z<9 and
early-type galaxies above z>2 and avoid confusion with other SEDs it is
mandatory to have photometric information in optical bands, besides a standard
IR filter-set like F110 F160 K L M. In particular by adding the V606, I814 and
z-Gunn filters a good discrimination is obtained above z>5 for star forming
galaxies and z>1 for early-types. The case for an extension of the NGST
wavelength domain to the optical range is therefore strongly supported by this
analysis. The effects of leaks in the filter blocking have also been
investigated. In spite of rather pessimistic assumptions (a constant leak at a
level of 10^{-4} of the peak transmission over the whole spectral range or a
leak of Gaussian shape placed at 1.5 times the effective wavelength of the
filter with an amplitude of 10^{-3} of the peak transmission and a width of 5%
of the central wavelength) the effects are not dramatic: the accuracy of the
determination of photometric redshifts with a standard V606 I814 z-Gunn F110
F160 K L M filter set does not significantly deteriorate for a sample limited
to M_AB~30. In the range 5.5<z<12.5 a rather high accuracy with a typical error
|z_sim-z_phot|<=0.2 is achieved. With the filter blocking achieved by the
present standard technology, extended and continuous spectral coverage appears
to be the driving factor to maximize the scientific output.
