I have a string field named CHESS2016_N94S_FL1_C__20160407230029.562GMT_COLOR-8-BIT.JPG and I would like to select characters 16-18 to populate a new field. I've been trying to use the field calculator index function in ArcMap but have only been getting errors.
As commented by @FelixIP this can be done in:
VB syntax mid(field,16,3)
and as commented by @faith_dur:
it is much nicer and versatile in Python, i.e. field [16:19]