lenses and athmospheric changes

[Ikinaa]

I came upon some questions concerning lenses...

Are they sealed? Are they filled with a special gas, not standard air?
If they are sealed, are there any noticeable changes in quality when the outer pressure and the inner pressure are different (sea-level, mountain-top, etc)
How do athmospheric changes affect the lens quality? High humidity, low humidity, high pressure, etc
I guess that when the sun shines on a (black) lens, it heats up, the air inside extends, etc.
Some say that various L-lenses are white so they don't heat up so much (white reflects light)
What about all this?

[BearSummer]

Hi Ikinaa,

the only lenses that I know to be "sealed" are things like binoculars and spotting skopes that are nitrogen purged. This means that they are sealed and are full of nitrogen to prevent internal fogging of the elements. Most camera lenses (AFAIK) are not atmospherically sealed. Some are weather sealed (see the newer canon L's) with rubber rings, gaskets etc to prevent water, sand and crap getting inside the lens. However they are not sealed to an atmospheric level. When you take a lens (even a weather sealed one) from sea level to the top of a mountain the internal pressure should change as the external pressuer changes.

I seem to remember that difraction of light depends of the difference between the density of the material the light is travelling from and to (but its some time ago so I could be wrong, anyone know for sure). If the above is correct then i wouldn't expect there to be a significant issue caused just because of the atmospheric pressure. I dont think the change is going to be significant enough to be visible. You are more likely to see other effects far earlier eg blue colour shift because of higher UV levels.

Anyway, why are canon L's grey/white and Nikons black... Some of the Canon L's are white because they use fluorite lenses that are temperature sensitive (the other L's used Low Dispersion glass or other cunning lens design, DO etc), in an effort to minimise the temperature increase they made the lenses white. Nikon lenses use LD glass which isn't as sensitive to temperature change and therfore the lenses can be black. However I do remember that Nikon brought out a grey lens a few years ago, not because it used flourite optics, but just because they could :wink:

(corrected due to my typo)

Best Regards

BearSummer

[robertwgross]

That would be fluorite, for those who need to search.

I've shot from many mountain tops, and I have never seen any effect from air pressure on a decent lens. I won't claim that there is no effect at all, but I have never seen any, and I have been to 40% of sea level pressure. UV is sure more intense up there, so some cameras will need industrial strength UV filters. At 14,000 feet or so, the UV intensity is estimated to be about three times what it would be at sea level.

To the thinking of some of us, you've never lived until you've shot a sunrise from 14,000 feet.

---Bob Gross---

[BearSummer]

Hi Robertwgross,

thanks for the correction, didn't run it through the spell checker, doh. Thanks for the extra comments regarding the amount of UV, I knew there was more up there just didnt realize it was that much, it's also nice to get it direct from someone with experience rather than someone trying to remember a physics course from way to many years ago.

Best Regards

BearSummer

[Ikinaa]

You guys are great. :P

I didn't expect so extensive information so fast...

[John_T]

What the expansion/contraction of the internal air will do is suck in and blow out air, and though the volume is small, any air pollutants may get sucked in and condense on the lenses. Over time it can become noticable.

This doesn't happen with sealed binoculars that are filled with nitrogen. Nitrogen expands and contracts minimally with temperature and altitude changes. I have the tires on my car filled with nitrogen for this reason, because a more constant pressure gives more constant and predictable performance.

[Ikinaa]

What the expansion/contraction of the internal air will do is suck in and blow out air, and though the volume is small, any air pollutants may get sucked in and condense on the lenses. Over time it can become noticable.

This doesn't happen with sealed binoculars that are filled with nitrogen. Nitrogen expands and contracts minimally with temperature and altitude changes. I have the tires on my car filled with nitrogen for this reason, because a more constant pressure gives more constant and predictable performance.

hmmm... doesn't everyone have at least 80% of the volume of the tires filled with nitrogen? 8)

[John_T]

Dunno, but I do know it costs CHF 8.- extra per tire. :wink:

[Ikinaa]

Dunno, but I do know it costs CHF 8.- extra per tire. :wink:
air is composed of 21% oxygen, 78 % nitrogen and 1% of something else...
I hope they fill the tyres with 100% N for 8CHF per tyre. :wink:

[John_T]

Gotcha. I'll ask! :x

(Whatever I've got, it's the same as they fill in F1 cars.)

[PhotosGuy]

Uneven heat from sunlight may cause uneven expansion of the lens elements which would affect resolution to a small degree. I don't expect that the designers can completly compensate for that.

A more important consideration is leaving cams locked in your trunk. I've measured temps of 140 degrees in there which can cause lubricants to migrate over time. I suspect that electronic components aren't all that happy at higher temps, either.

[Jon]

I seem to remember that difraction of light depends of the difference between the density of the material the light is travelling from and to (but its some time ago so I could be wrong, anyone know for sure). If the above is correct then i wouldn't expect there to be a significant issue caused just because of the atmospheric pressure. I dont think the change is going to be significant enough to be visible. You are more likely to see other effects far earlier eg blue colour shift because of higher UV levels.

Well, it's refraction, not diffraction, but otherwise you're right. (Diffraction is bending of light across an edge; refraction is bending of light as it passes through a medium.) Snell's law says that the angle a ray of light is bent when transitioning between two media is directly related to the angle of incidence of the light ray and the ratio of the two medias' indices of refraction. Air's I.R. is very close to that of a vacuum, so changes in air density don't have much impact within a lens. Outside the lens, hot air currents can give rise to mirages and "heat shimmer", but there you're seeing the effect of repeated refractions due to micro-thermal gradients.

[BearSummer]

Well, it's refraction, not diffraction, but otherwise you're right. (Diffraction is bending of light across an edge; refraction is bending of light as it passes through a medium.) Snell's law says that the angle a ray of light is bent when transitioning between two media is directly related to the angle of incidence of the light ray and the ratio of the two medias' indices of refraction. Air's I.R. is very close to that of a vacuum, so changes in air density don't have much impact within a lens. Outside the lens, hot air currents can give rise to mirages and "heat shimmer", but there you're seeing the effect of repeated refractions due to micro-thermal gradients.

Thanks Jon,

See, exactly what I said, just with more detail, accurate and remembered whose law it is :D . As I have always said , A little ignorance can go a long way. Only trouble is that my little ignorances are ganging together, next thing you know they will be anexing my frontal lobes. Its all down hill from there.

BearSummer