bsprague wrote:
If you take a cold dry lens into a warm, humid environment frequently, you might want one. If you live in a Florida air conditioned, dehumidified dwelling and frequently take your lenses outside without precautions, they will steam up. It goes the other way in Alaska. If you take your cold, dry lens inside from a shoot, to your hot humid dwelling, it will steam up.
Yup, in winter Alaska, left my camera in the 20-40F "Arctic entry" most times, in Louisiana summer, spend alot of time holding cameras in front of engine exhausts until they warm up and clear up... the camera, I mean...