  The instant form and the front form of relativistic dynamics proposed by
Dirac in 1949 can be linked by an interpolation angle parameter $\delta$
spanning between the instant form dynamics (IFD) at $\delta =0$ and the front
form dynamics which is now known as the light-front dynamics (LFD) at $\delta
=\pi/4$. We present the formal derivation of the interpolating quantum
electrodynamics (QED) in the canonical field theory approach and discuss the
constraint fermion degree of freedom which appears uniquely in the LFD. The
constraint component of the fermion degrees of freedom in LFD results in the
instantaneous contribution to the fermion propagator, which is genuinely
distinguished from the ordinary equal-time forward and backward propagation of
relativistic fermion degrees of freedom. As discussed in our previous work, the
helicity of the on-mass-shell fermion spinors in LFD is also distinguished from
the ordinary Jacob-Wick helicity in the IFD with respect to whether the
helicity depends on the reference frame or not. To exemplify the characteristic
difference of the fermion propagator between IFD and LFD, we compute the
helicity amplitudes of typical QED processes such as $e^+ e^- \to \gamma
\gamma$ and $e \gamma \to e \gamma$ and present the whole landscape of the
scattering amplitudes in terms of the frame dependence or the scattering angle
dependence with respect to the interpolating angle dependence. Our analysis
clarifies any conceivable confusion in the prevailing notion of the equivalence
between the infinite momentum frame approach and the LFD.
