Star-trail technical trouble

Redundo

My lens keeps fogging up!


Since we've had great clear weather and it's approaching time for the new moon, i've been taking the opportunity to do some Star Trail photography. I thought i had a good shot framed out by Dunhill castle, with everything set-up. But when i came back an hour later i found the lens had completely fogged over with condensation due to the cold lens.

So has anyone any experience with this, or know of a solution? I'm looking to go out again on Friday night/Saturday morning and i'm not losing another couple hours sleep for a fogged over photo!

I was thinking of some of those microwaveable hand-warmers that the hill walkers and mountaineers use. I just wonder would it last the 3-4 hours from microwave to shutter release?!

Any suggestions?

Calina

Allow the lens to reach ambient temperature before taking the photograph I suspect would be the best thing. Am guessing you're talking it from a warm environment to a colder one?

Let it cool down then clean the front of the lens would be my suggestion.

Redundo

The problem isn't that the lens isn't cold enough.... it's that its too cold!

As the lens is taken out of the car, the temperature is going to be above that of the dew point of the air. So to begin with, no condensation will form.

As the lens cools the longer it's been out, it gradually gets below the ambient air temperature since it's made of glass and metal. Then the dew forms as the air in contact with the lens becomes too cold to support the moisture.

Thats my understanding of the problem. So the only way too avoid fogging is to keep the lens somewhat warmed.

magicbastarder

fogging usually happens when you take a cold object into a warm environment, though...

Redundo

Thats because the cold lens has cooled the air too much to support the amount of moisture it carries. And so you get fogging.

It's really still the same problem, the lens becomes colder then the air so it has to be kept slightly warmed to prevent fogging.

kensutz

I was awake at 4 this morning and was so tempted to try it. Only problem is that I was in work for 8

magicbastarder

Redundo wrote: »

As the lens cools the longer it's been out, it gradually gets below the ambient air temperature since it's made of glass and metal. Then the dew forms as the air in contact with the lens becomes too cold to support the moisture.

the camera shouldn't get below ambient temperature - what may be happening though, is that as the air itself is cooling, it can no longer hold as much water vapour, and this condenses out on any object which is not above the temperature which will re-vapourise the water.

nilhg

Some reading, looks like the first thing to try is a dew shield

Redundo

I thought it was the camera itself getting colder then the air. Because from handling it and the tripod it definitely seemed a lot colder! Either way its getting too much condensation.

I'm not sure if a foam insulator around the lens would work for me. I was using a 17mm wide-angle on a film camera to cover quite a portion of the sky. I think there would be too much vignetting if i were to collar the lens with foam.

I'll probably be going back to Dunhill again Friday night as the weather reports make it look like a perfectly clear night again. It seems to be a perfect location given that you can get the castle in silhouette against the pole star.

magicbastarder

Redundo wrote: »

I thought it was the camera itself getting colder then the air. Because from handling it and the tripod it definitely seemed a lot colder! Either way its getting too much condensation.

it feels colder because it's better at absorbing heat from your hands.

Redundo

That makes sense to me.

I think i'll try to find some of the hand-warmers about town, and hope an elastic band or two can hold them around the camera lens. I think its just going to be a case of trial and error.

Or how about a hot water bottle? That should stay warm enough for a few hours to keep the dew away....