  Strain has a strong effect on the properties of materials and the performance
of electronic devices. Their ever shrinking size translates into a constant
demand for accurate and precise measurement methods with very high spatial
resolution. In this regard, transmission electron microscopes are key
instruments thanks to their ability to map strain with sub-nanometer
resolution. Here we present a novel method to measure strain at the nanometer
scale based on the diffraction of electron Bessel beams. We demonstrate that
our method offers a strain sensitivity better than $2.5 \cdot 10^{-4}$ and an
accuracy of $1.5 \cdot 10^{-3}$, competing with, or outperforming, the best
existing methods with a simple and easy to use experimental setup.
