We show that in the limit of a large objective (probe-forming) aperture, re
levant to a spherical aberration corrected microscope, the Z-contrast image
of a zone-axis crystal becomes an image of the Bloch states. The limiting
resolution is therefore the width of the Bloch states, which may be greater
than that of the free probe. Nevertheless, enormous gains in image quality
are expected from the improved contrast and signal-to-noise ratio. We pres
ent an analytical channeling model for the thickness dependence of the Z-co
ntrast image in a zone-axis crystal, and show that, at large thicknesses, c
olumnar intensities become proportional to the mean square atomic number, Z
(2).