Shooting the night skies is a very rewarding field of photography, especially after you’ve sat in the dark cold night for 5 hours waiting for a time lapse or a startrails sequence to complete. But if you are shooting in a cold location (or a terribly cold night) you may lose the sequence to dew or condensation.
Once the camera hits the same temperature as the ambient temperature, some mist (or worse, dew) can start building up on the lens. If you are lucky it builds on the outside of the lens, creating a fuzzy blur to the photos. If you are unlucky, it can create drops of water inside the lens.
Mark Peter Thorpe (a.k.a pixelhobo) has a great description and a great solution:
This is something we don’t see until about an hour into the shoot which has been going handsomely. We then start to see a black fuzzy area appearing in the center of the resulting image, Whaaat the (insert expletive of choice) is thaaaat? Is the initial thought. Yeah, condensation. It happens once the glass element of the lens reaches the same temperature as the ambient temperature, until this point if the camera had been pulled from a nice warm backpack it is the residual heat in the camera and lens that has been fighting off the build up of mist on the lens. Once it starts there’s no immediate remedy, the glass will have to be heated but that takes some time to ward off the immediate presence of mist
There are commercial products that heat up the lens to avoid condensation but they are not the cheapest. Instead, Pixelhobo suggests using Heated hand packs to keep the lens warm and avoid condensation.
First you use the adhesive on the pads to stick two of them on the bottom part of a lens (heat should rise to warm it all)
Then you wrap a piece of elastic Velcro to keep them on the lens and to better utilize the heat.
Some of the larger pads provide 8 hours of heat, which should be enough for most shoots.
[Seeing Stars, or not! | pixelhobo]