  We study 2d Ising Field Theory (IFT) in the low-temperature phase in
lightcone quantization, and show that integrating out zero modes generates a
very compact form for the effective lightcone interaction that depends on the
finite volume vacuum expectation value of the $\sigma$ operator. This form is
most naturally understood in a conformal basis for the lightcone Hilbert space.
We further verify that this simple form reproduces to high accuracy results for
the spectra, the $c$-function, and the form-factors from integrability methods
for the magnetic deformation of IFT. For generic non-integrable values of
parameters we also compute the above observables and compare our numeric
results to those of equal-time truncation. In particular, we report on new
measurements of various bound-state form-factors as well as the stress-tensor
spectral density. We find that the stress tensor spectral density provides
additional evidence that certain resonances of IFT are surprisingly narrow,
even at generic strong coupling. Explicit example code for constructing the
effective Hamiltonian is included in an appendix.
