Does the temperature of the lens affect image sharpness?

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Bluetail

Bluetail

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When shooting birds in a field on a hot sunny afternoon this summer, I noticed that the barrel of my 300mm f4 PF lens was getting quite warm to the touch; and after watching Steve's latest video on how to shoot sharper wildlife images and the effect of heat diffraction, I'm wondering if--in addition to the heat rising from the ground surface--heat affects the internal mechanics of the lens, thus compromising image sharpness? (Is this why, maybe, Canon and Sony use white on their long lenses?). Interested to hear your thoughts on this! Happy Thanksgiving to all in the US.
 
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I don't know the answer to your question however I do know that Canon does promote the white paint as being a special paint that lowers the temperature of the lens.

From Canon website:
Increased Heat Shielding Measures for Reliable, Longer Shooting Outdoors
Even when used outdoors in the heat for extended periods of time, the EF 600mm f/4L IS III USM delivers consistent and reliable performance you can trust. A new paint formulation reflects even more heat than before, while a two-layer barrel structure helps reduce the thermal effects on the lens elements. Together, these technologies help maintain image quality and lens reliability even during long shoots under the hot sun.
 
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Whilst it is certainly true that a black lens will get hotter than a white one whether it would get hot enough to affect the internal mechanics of the lens is impossible to say, unless you have the details plans of the lens itself. Even then, calculating things like the differential expansion between materials in contact with each other, etc., would likely need a pretty complex computer program.

I would expect that if the lens got hot enough then the first thing you would notice would be the feel of the focus or zoom ring changing - possibly getting tighter or looser.

Would any of this affect the image quality to a noticeable degree? Who knows? Personally, I would expect the effects of heat diffraction to far outweigh the effect of heat on the lens itself.
 
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I don't think there is much danger from temperature extremes as long as you don't go too far. I use lens coats on my big glass and they do a good job of keeping the lens cool in the sun as well as warm in the cold (at least until the cold gets thru the coats). The bigger danger is rapid and severe temperature changes.
While it would seem possible that extreme temperatures could affect sharpness, I haven't seen any evidence to support it. I do know it changes AF Fine tune a little, I usually have to adjust the long glass when the temps change.
 
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If you calibrated your lens in a warm climate you may want to check the calibration if you plan to use it in a very cold climate when you get there and vice versa.
 
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My experience had been that sudden large temperature changes like going from a warm indoor environment to a cold (below freezing) environment may cause a temporary problem. The lens design is a big factor in the sensitivity. I have found some of the old mirror lenses are more susceptible to temperature effects than other designs. I have an old 500mm mirror lens that I always leave in the car overnight to chill if I plan to use it on the next day when temperatures are near or below freezing. (The photographer also has a problem with the sudden temperature change but he does NOT stay in the car overnight.) For slower temperature changes during the day or being in the sun, I've not noticed any effects from temperature changes.
 
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