Back in 1949 Scherzer suggested an optimum defocus condition which occurs at:
this value is now called "1 scherzer".
Note: sometimes the value of 1.2 scherzer is called "Scherzer defocus" (we are going to call the value of 1.2 scherzer as "extended scherzer" throughout this manual). "Extended scherzer" provides even broader band of transmittance with CTF still close enough to -1.
Figures below show CTF's for different defoci values for a 200 keV