September 02, 2014, 10:33:23 AM

Author Topic: History behind the white lens  (Read 6197 times)

unfocused

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Re: History behind the white lens
« Reply #15 on: January 17, 2013, 04:40:03 PM »
...why is the 200 f2.8 prime black while the 70-200 f4 is white?

Not denying that some of this is marketing, but in the specific case above, the 70-200/4 has a fluorite element, while the 200/2.8 does not (fluorite is more thermally sensitive than glass).  But then...the 70-300L has no fluorite, nor do the 300/4 IS or 400/5.6 (although the 100-400 does), which is why I'm not denying some of this is marketing...

As usual, you know the technical specs far better than I ever will. But you also get my point. If I'm paying $1,300 for a lens, I want it in white and with a red ring, dammit!
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Re: History behind the white lens
« Reply #15 on: January 17, 2013, 04:40:03 PM »

kubelik

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Re: History behind the white lens
« Reply #16 on: January 17, 2013, 05:04:50 PM »
The interesting thing about white vs black is that if a white and black lens are both at the same temperature after being in the sun for some period of time then the black lens will return to a room temperature quicker than the light coloured one if they are both then taken into an air conditioned building.

has this been tested? I assumed the black vs. white thing only helps regarding heat absorption from radiation, but once you bring it into a room it should be cooling via conduction in the air, which shouldn't have anything to do with its color.

http://littleshop.physics.colostate.edu/activities/atmos1/ColorAndCooling.pdf

interesting, so radiation is more important than conduction in cooling. thanks for the link!

Zen

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Re: History behind the white lens
« Reply #17 on: January 17, 2013, 05:40:49 PM »
I don't know the answer re the testing of the theory on lenses, but I DO know from experience, that the white top on my race car, a '63 Morris Mini Cooper in SCCA "C Sedan" class, which was a metallic dark blue elsewhere, kept the interior much cooler than was the case in the cars that had darker colored tops. So, based on my experience, the lens story is entirely believable. ;D

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LDS

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Re: History behind the white lens
« Reply #18 on: January 17, 2013, 06:33:53 PM »
http://littleshop.physics.colostate.edu/activities/atmos1/ColorAndCooling.pdf[/url]

I believe you got it wrong - reread this passage:

"But silvery metals don’t work like this. They reflect visible light and they reflect infrared too. They don’t absorb it—and, more importantly, they don’t emit it! So, in this experiment, the two cylinders will cool at different rates. The bare aluminum cylinder radiates less and cools rapidly; the white cylinder (and the color doesn’t matter—it could be any color at all!) will radiate more and so it will cool off more quickly."

As long as you paint the lens metal, black or white - it will cool in a different way than unpainted metal. It's the paint that radiates. But the black lens will absorb much more energy - heat - than the white one... and a polished bare metal lens would be a nightmare

kubelik

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Re: History behind the white lens
« Reply #19 on: January 17, 2013, 06:43:18 PM »
LDS, I read and re-read that paragraph, but I think that they're missing the word "less" in between "cools" and "rapidly". it should read:

"the bare aluminum cylinder radiates less and cools LESS rapidly; the white cylinder will radiate more and so it will cool off more quickly."

the statement about the color not mattering seems to be relative to an unpainted metal surface, not relative to other colors. compared to anything painted, reflective material will heat and cool more slowly. but between different colored materials, their relative level of emission will matter in comparison.

neuroanatomist

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Re: History behind the white lens
« Reply #20 on: January 17, 2013, 09:33:03 PM »
http://littleshop.physics.colostate.edu/activities/atmos1/ColorAndCooling.pdf[/url]
I believe you got it wrong
Nope, dilbert is correct about the cooling rates, although the statement about silver vs. any other color in the link he posted may be a bit misleading.  While it's true that painted metal will cool faster than the unpainted metal, metal painted black will cool faster than metal painted white.  But, dilbert also makes an incorrect assumption, which renders his conclusion invalid:

The interesting thing about white vs black is that if a white and black lens are both at the same temperature after being in the sun for some period of time then the black lens will return to a room temperature quicker than the light coloured one if they are both then taken into an air conditioned building.
A black lens and a white lens in the sun for the same long period of time will NOT be the same temperature - the white lens will be cooler.  In the sun, the temperature of each lens will rise until the heat gain from absorption reaches equilibrium with heat loss due to emission and conduction/convection.  That equilibrium will occur at a lower temperature for the white lens (as much as 30-40% lower), due to lower heat gain because of the reflectivity of the white paint.  So, when both lenses are taken into that air conditioned building, although the black lens will cool at a slightly faster rate, the white lens has less heat to lose, and thus will reach room temperature sooner.
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Re: History behind the white lens
« Reply #20 on: January 17, 2013, 09:33:03 PM »