[@Z2_1; @Z2_2; @Z2_3; @Z2_4; @Z2_5; @RBC]. In addition we make use of a judicious choice of momenta carried by the incoming and outgoing quarks off the electromagnetic vertex, which helps in reducing statistical fluctuations in the form factor measurements. In Sec. 3 we present the results of pion form factor measurements, and in Sec. 4 we analyze the data as a function of the momentum transfer squared and pion mass. In particular we examine consistency with the predictions of chiral perturbation theory. Finally, in Sec. 5, we discuss our findings closer to the physical point including the results of our test calculation on a $64^4$ lattice with the quark masses tuned to the neighbour of the physical point. We end this work with conclusions in Sec. 6. A preliminary report of this work was presented in [@OanhNguyen].

Methods
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Pion electromagnetic form factor
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The electromagnetic pion form factor $G_\pi(Q^2)$ is defined by $$\label{eq:fund4}
\left< \pi^+(\vec{p'})|J_\mu|\pi^+(\vec{p})\right> = (p_\mu + p'_\mu)G_\pi(Q^2),$$ where $Q^2=-q^2=-(p'-p)^2$ is the four-momentum transfer, and $J_\mu$ is the electromagnetic current given in $N_f=2+1$ QCD by $$\label{eq:fund3}
J_\mu = \frac{2}{3} \bar{u}\gamma_\mu{u}-\frac{1}{3} \bar{d}\gamma_\mu{d}-\frac{1}{3} \bar{s}\gamma_\mu{s}.$$ In the limit of vanishing four-momentum transfer $Q^2=0$, the form factor equals unity due to the 