Hi, Yes It is done with large glass. Heat is one of the reasons Canon and Minolta and Pentax used a light/white coating in the lens barrels. One interesting factoid is that these lenses also have a variable infinity so that when the temps do get extreme the lens can still focus. Even on my 70-200 2.8L I have seen differences in the focus during hot weather... Also having the glass at ambient temp means fewer condensation issues from extreme/fast changes in temp humidity etc. Pelican cases have a pressure relief valve built so that you can slowly break the seal after flight and altitude/humidity/air pressure changes that can effect the lens. Les Baldwin -----Original Message----- From: owner-photoforum@xxxxxxxxxxxxxxxxxxxxxx [mailto:owner-photoforum@xxxxxxxxxxxxxxxxxxxxxx] On Behalf Of Gregory Stempel Sent: Monday, April 05, 2004 12:14 AM To: List for Photo/Imaging Educators - Professionals - Students Subject: Thermal Balance In astronomy, we have to bring our scopes to thermal equilibrium with the ambient air temperature to stabilize the image. Anyone ever here of having to do this with our big glass like the 500mm??? Should the glass be at the same temperature as the ambient air before use? Take care, Gregory david Stempel FIREFRAMEi m a g i n g www.americanphotojournalist.com
