Today’s Question: When doing time lapse of shooting stars my lens will often fog up as the temp changes. I solved the fogging by using those disposable hand warmers that you open and expose to the air. I use a rubber band to hold it wrapped around the front lens area. Works great.
Tim’s Quick Answer: This is a great tip! I heard variations on this recommendation from several readers who use hand or toe warmers on their lenses or in their camera bag in order to warm up their lenses and prevent fogging up in humid environments.
More Detail: Today’s “question” is obviously not actually a question, but rather a tip submitted by a reader (and echoed by several other readers). I thought this tip was worthwhile sharing here, even without the usual “question and answer” format.
As noted in the Ask Tim Grey eNewsletter from May 9th (http://asktimgrey.com/2018/05/09/anti-fog-wipes/), in my experience anti-fog wipes don’t actually provide any benefit in terms of preventing your lenses from fogging up when you take them from a cold environment to a relatively warm and humid environment. Instead, I suggested either waiting for the lens to warm up to the ambient temperature, or to use a hair dryer to warm up the lens before heading out (as recommended by a reader).
Using a hand or toe warmer for this purpose obviously provides a number of advantages. To begin with, you could simply put a warmer into your camera bag before you head out, so that your lenses will have a chance to warm up before you start using them.
In addition, attaching a warmer directly to the lens can help ensure the lens remains warm over an extended period, to help prevent fogging. In short, anything you can do to safely warm up the lens to match (or exceed) the ambient temperature will help prevent condensation fogging on that lens.