  We propose a procedure for defining all single-photon observables in terms of
Positive-Operator Valued Measures (POVMs), in particular spin and position. We
identify the suppression of $0$-helicity photon states as a projection from an
extended Hilbert space onto the photon Hilbert space. We show that all
single-photon observables are in general described by POVMs, obtained by
applying this projection to opportune Projection-Valued Measures (PVMs),
defined on the extended Hilbert space. The POVMs associated to momentum and
helicity reduce to PVMs, unlike those associated to position and spin, this
fact reflecting the intrinsic unsharpness of these observables. We finally
extensively study the preparation uncertainty relations for position and
momentum and the probability distribution of spin, exploring single photon
Gaussian states for several choices of spin and polarization.
