The passbands occur periodically with underfocus and, in fact, the first
passband corresponds to the Scherzer defocus.
Higher order passbands can be useful to access higher spatial frequencies
with less aberrations. The price we pay for this is that now we have zeros in
CTF at lower spatial frequencies.
For a certain material it is possible to find an "aberration-free"
defocus condition when all Bragg reflections will be within
"transparent" passbands in the CTF out to very high spatial
frequencies. However, any information which falls into the gaps in CTF will be
lost! For instance, if there is a defect present, we will lose all information
about the defect, since defects scatter between the Bragg peaks.