We study the equation of state of quenched strongly coupled QED with fine structure constant α, extended by a chirally invariant four-Fermi interaction with strength parametrized by G, along the entire critical line in the ( α, G) plane as described by the Schwinger-Dyson equation for the fermion self-mass. From there we extract the critical indices β, δ, ν and γ as functions of the two couplings and find that they vary continuously from their mean field values at α = 0, G = 4 to those corresponding to the essential singularity behaviour at α = π 3 , G = 1 . At each point on the critical line, the critical exponents satisfy hyperscaling. We extract the critical exponents δ and β from quenched lattice QED simulations by studying the bare fermion mass dependence of the chiral condensate and the λ-dependence of the density of states ϱ(λ) of the spectrum of the Dirac operator at the critical point. We find that the effective δ obtained from numerical fits depends sensitively on the value of β c taken as the critical coupling. Our best fits favor β c = 0.257(1), δ = 2.2(1), and the exponent β = 0.8(1). For the particular value of the critical exponents obtained from the lattice simulations we find a point on the critical line. Since the exponent δ depends on the anomalous dimensions of the fermion composites, we make a connection between lattice measurements and Schwinger-Dyson equation results.