Question: I shoot a lot of night sky photography but can’t stop my lens fogging up. What can I do to get around this?
Answer: Condensation on the front element is a real problem for astrophotographers, especially when there’s a heavy dew. It’s usually most severe on low ground or near water. In severe cases, a lens can be completely fogged up just five seconds after being wiped clean, making longer exposures almost impossible to achieve.
Fortunately, it is possible to alleviate the problem. The first thing to remember is that sudden changes in temperature can cause condensation to form, so allow your camera to acclimatise to the ambient temperature before you shoot.
The next step is to invest in anti-fog gel or wipes. I’d also recommend a battery-powered hand fan to keep a flow of air moving across the end of the lens. This discourages droplet formation. Serious astro shooters often use heat straps, which are battery-powered heating elements that stop dew from forming on the end of the lens. Hand warmers secured with elastic bands also work well as a budget alternative.