Hi, If you will look at any large Canon lens you will find that the infinity locator has a range built into it. The lens will focus past infinity. This is done so that the natural expansion and contraction of any material can be compensated for during extremes of temperature. Canon builds it lenses white to try and reflect as much heat as possible. Heat buildup with in the lens can have very adverse reactions on it's ability to function within the tight parameters of the lens. The engineering built into the glass, barrel and mount are designed to ward off those changes, and still render very sharp images. I have had the lens use that extra focus range to create sharp pictures even in 110+ heat. The camera body was actually too hot to touch without a cloth. I hope this explains the situation. Les Baldwin -----Original Message----- From: owner-photoforum@xxxxxxxxxxxxxxxxxxxxxx [mailto:owner-photoforum@xxxxxxxxxxxxxxxxxxxxxx] On Behalf Of Bill Wood Sent: Tuesday, April 06, 2004 6:36 AM To: List for Photo/Imaging Educators - Professionals - Students Subject: RE: Thermal Balance >>> fotofx@xxxxxxxxxxxxx 4:27:22 AM Monday, April 05, 2004 >>> Even on my 70-200 2.8L I have seen differences in the focus during hot weather... Les Baldwin I don't see how this can be since the autofocus still has to focus to a sharp image. All it means is that the lenses would move farther out (or farther in) than in a cooler enviroment. Isn't the sharpness determined at the sensor plane? Please enlighten me. Bill
