Dew forms on the front of your camera lens when the temperature of the glass cools to a few degrees below the dew point, causing water vapour in the air to condense onto the glass.
It’s a familiar pain for photographers: your camera lens becomes misted over and any subsequent images that are taken through that lens are useless.
People often think a camera lens will only dew up during winter, but because the dew point is related to humidity levels, lenses can fog or dew up even more rapidly on a warm and humid summer evening.
For more camera maintenance, read our guide on how to clean a DSLR camera.
For help with foggy telescopes, read our guide on how to stop dew forming on your telescope.
And for more practical guides like this, visit our DIY Astronomy page.
How to dew-proof your camera lens
Keeping the camera lens glass warmer than the dew point will prevent it from fogging up, and a simple way to achieve this is to use a dew heater.
You can buy dew heaters commercially, but they are usually powered from a rechargeable power tank that can be heavy to carry around, particularly if you are shooting on location.
You can buy dew bands that run from 9V batteries, but they consume plenty of power so the batteries don’t last very long.
In addition to worrying about batteries, you also have the inconvenience of dealing with a set of cables from the dew heater to the controller and power supply.
One way to solve these issues is to make a simple dew heater that uses hand warmers as its heat source. For this project we used inexpensive reusable hand warmers, which can be purchased online.
They are filled with a liquid that turns into a solid once activated. This process is an exothermic reaction so they remain warm for a couple of hours after activation.
Once they have cooled again, you can reverse the reaction by warming them in a pan of hot water; they can then be reused again and again.
Our dew heater consists of a dew band made from a sock to hold the hand warmers, which is held in place with Velcro; the fastening is adjustable so it can fit a variety of different camera lens sizes and can even be adapted to fit around a small refractor telescope.
If you are doing a long imaging session, you can easily remove the dew heater and swap the hand warmers for a fresh pair between shots.
There is no temperature control with this simple solution, so there is a small chance that the heat may cause a slight increase in dark signal noise (unwanted artefacts) in your images.
However, the dew heater will be placed around the camera lens rather than near the camera sensor, so it shouldn’t affect the images too badly.
If it does cause a bit more noise, it is still far more successful to carry out a noise reduction step in post-processing than it is to try and gain any useful data from images taken through a fogged-up lens. For more on this, read our guide on how to remove noise from astrophotos.
The added bonus of its portability coupled with no cable management definitely make up for this.
You Will Need
- Hand warmers (2), small, reusable, 9cm x 5.5cm
- Thermal sock, wide enough to hold hand warmersl long enough to fit round your lens
- Sewing machine, optional
- Needle and thread
- Buttons (2), about 2.2cm diameter
- Elastic (2), about 4cm long each
- Velcro strip (2), 5cm loops; 11cm hooks
Lay the sock flat on your table and then cut two slits that are a bit smaller than the long edge of the hand warmers into one edge. The slits will stretch a bit during Step 2 so don’t make them too big.
Sew a basting stitch around the edges of the slits to prevent fraying, then fold the edge inwards and sew it down with a straight stitch to produce a neat edge. We used a sewing machine, but this can be done by hand if you don’t have one.
Sew a button onto the middle front edge of both slits and then sew loops of elastic onto the back edges so they line up with the buttons. Once done up, this will keep the hand warmers in place and prevent them from falling out.
Sew a seam across the cuff and toe of the sock, and also across the middle (roughly where the heel is) to form two enclosed pockets. This is where the hand warmers will be fitted. We used a sewing machine here (and Step 5) but a needle and thread is fine.
Next, sew the 5cm strip of Velcro (loops up) to the outside of the one end of the sock, then sew the 11cm strip (hooks up) to the inside of the other end, leaving a piece hanging over the edge. Fastening this will keep the heater attached to the lens.
Activate the two hand warmers, pop them into the pockets and fasten the buttons. You can now wrap the dew heater around your camera lens and secure it with the Velcro. Be careful to avoid part of the sock covering the lens as you don’t want sock vignetting!
Mary McIntyre is an outreach astronomer and astro imager based in Oxfordshire. This guide originally appeared in the April 2021 issue of BBC Sky at Night Magazine.