This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
Figure 2593 shows the ZLPs (zero-loss peaks) of a CFEG (cold field emission gun)  at various emission currents as well as the monochromatized ZLP  in logarithmic vertical scale. The inset lists the extraction voltages V1, probe currents Ip and the FWHMs (full width at half maximum) of the ZLPs. Note that the FWHMs quantify the energy spread. In general, the lower extraction voltage and probe current gives smaller FWHMs of the ZLPs, meaning that the higher energy resolution can be obtained, while the one with a monochromator still has the lowest FWHM.
Figure 2593. ZLPs of CFEG under various emission conditions as well as the monochromatized ZLP.
Adapted from 
 Koji Kimoto, Kazuo Ishizuka, Toru Asaka, Takuro Nagai, Yoshio Matsui, 0.23 eV energy resolution obtained using a cold field-emission gun and a streak imaging technique, Micron 36 (2005) 465–469.
 Kothleitner, G., Hofer, F., 2003. EELS performance measurements on a new
high energy resolution imaging filter. Micron 34, 211–218.