Originally posted by eddie1960 the freaking enormous, heavy - 564lbs and expensive -just one element is about the same cost as a nice S500 sedan zeiss 1700 mm is white - and will require a custom made support to use it - THAT IS A HASSY 6X6 ON THE BACK
The S500 could pay for one of the smaller lens elements, maybe... I know the price, you are really far from getting close.
Obviously a white lens has several advantages - sun light is reflected. Silver and golden colors would also have an effect, but reflexes could build up. You can really get into trouble with the sun shining on one side of thelens while the other side is in the shade. This effect can also be overcome with stronger mechanical support - something done for high-accuracy surveying instruments. Over the years each major player developed its own color. NIkon black, Canon white, Pentax silver - with severla nuances in between. Pentax used to have black or white super telefoto lenses, Leica weas black at first and became silber later, Nikon offers grey lenses, ... Heat and thermal expansion are important factors in the design of long lenses.
Pentax may offer a rainbow version as well as a pink version.