I love using lenses with long focal lengths (50 mm+) for portrait work. The higher (longer) the number, the more shallow your dept of field (DOF), which I know seems counter-intutive, but that’s the way the light refracts. For these shots, I used my 105 mm. Shallow DOF lets you draw attention to the part of the image you really want to emphasize.
For more from this challenge, visit Sunday Stills: Shallow Depth of Field.