Re: Lens condensation question

"Skip M" <shadowcatcher@xxxxxxx> wrote:
>"Floyd Davidson" <floyd@xxxxxxxxxx> wrote in message
>news:87k6czu7fs.fld@xxxxxxxxxxxxx
>> "Skip M" <shadowcatcher@xxxxxxx> wrote:
>>>
>>>Nope, didn't say that. It just works in reverse, going from hot and dry to
>>>cold and wet. Happened to me on Saturday, going from cool, damp outside to
>>>the relatively warmer and drier interior of a venue.
>>
>> It never works in reverse. It is *always* the cooling of air to a
^^^^^^^^^^^^^^^^^^^^^^^^

What happened to you on Saturday is *not* "just works in
reverse".

>> temperature below the dew point. The dew point changes, of course,
>> depending on the temperature and the relative humidity.
>>
>>>But what I was referring to was in SoCal, if it is truly hot, it is probably
>>>a Santa Ana, which can produce relative humidity in the singe digits, and if
>>>it is damp, it is winter, and relatively cold.
>>
>> In fact it can happen at *any* temperature and relative humidity
>> combination. The point in every case is that if the air touching
>> the camera is rapidly dropped to a temperature below the dew point,
>> it *will* result in condensation.
>>
>> Air with single digit relative humidity is not immune, it just
>> has a lower dew point.
>>
>
>Well, it sure happened to me, going from the rain to the inside of the
>reception hall, my lens, and viewfinder, fogged up. Logic told me that it
>couldn't happen, but it did.

Logic should have told you that it *would* happen. That is
*exactly* the classic case that has been explained here several
times in great detail.

Your camera is outside, at a "cool" temperature. You bring it
into a "warm" room with relatively moist air. The _warm_
_moist_ _air_ that contacts the surface of the camera is cooled
below its dew point, and moisture condenses out and forms water
droplets on your camera. (I've put quotes around "cool" and
"warm" because those are relative terms, and this happens at any
temperature between freezing and boiling.)

>Probably because, even though the lens was
>already damp,

The lens being damp has no effect.

However, objects being damp suggests that the outside air's
relatively humidity is very nearly at 100%. That same amount of
moisture is where air that enters a building starts at, so
obviously cooling the inside air to that same temperature is
certainly going to put it close to the dew point. However, it
gets worse (unless the room has controlled humidity) because the
air inside is warmed and can hold more moisture... which is
provided by all those people inside breathing! (Not to mention
anything else they do, such as cooking.)

Th moisture produced by people breathing is significant. Just
take something like a pair of eye glasses, and breath on them.
In most environments that will fog them at least a little because
human breath is *very* moist, and usually warmer than the glasses
will be (obviously that won't happen if very warm places).

For folks that ride around in a car filled with people, it can
be fairly annoying too, because the only way to rid the car of
the moisture is either open a window or turn up the heat. The
best solution is usually heating external air. (That can also
be a problem too, if water or snow is sucked in and the vapor is
then blown directly on the front windshield!) Sometimes, with 4
or 5 people in a car, it becomes a choice of fogged windows or
being uncomfortable with an open window or overheated interior.

>the lower temp of it condensed what moisture was in the inside
>air. I guess.

Well, that is no surprise, is it? Everyone who has attempted to
explain it has said that is what causes it.

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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