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Why are some Canon lenses painted white or silver?
Nearly all large telephoto L series lenses have barrels constructed from solid metal and painted off-white, rather than black plastic or black-painted metal. Canon say they do this since white surfaces absorb less heat than black when used out in the sun, and fluorite crystal lens elements are sensitive to heat - they can expand and contract, altering their optical properties. Of course, the fact that a white-painted Canon lens stands out in the crowd is probably part of the reason as well. Look at any major sports event and you’ll see rows of hefty white lenses. Though to confuse matters Nikon have also begun to sell some lenses in optional white paint.
A few lenses intended to be sold as kit lenses with silver-painted camera bodies have also been available with silver-painted plastic barrels. These include the 35-135 4-5.6 USM (to match the silver-painted commemorative edition of the EOS 10/10s which was released to mark the sale of 60 million Canon cameras), the 24-85 3.5-4.5 USM (to match the APS IX camera) and the 28-90 4-5.6 USM II (to match the Rebel Ti/EOS 300V/Kiss 5). The silver paint on these midrange and low end lenses is purely for cosmetic reasons.