Lens fogging is one of the many curses in astrophotography. You don’t really think about lens fogging until you have wasted a good night because of it.
In this article, I’ll teach you why lens fog and how to avoid this.
What Is Lens Fog?
If you use glasses, you are familiar with the whole fogging conundrum.
Lens fogging is no different, and it is a well-known problem when shooting in certain ambient conditions, particularly in humid environments.
Why Does Condensation Occur On A Lens?
Assuming you are not shooting for astrophotography under the rain or in thick fog (and why would you, anyway?), several factors determine whether your camera lens will fog or not:
- The Air Humidity
- The Air Temperature
- The Lens Temperature
- The Dew point
While this section may look like a tedious and technical one, this information will help you to have a better chance of success when doing astrophotography.
This is a lesson I learned the hard way myself, and I suggest you keep reading.
Because the argument can become very technical very quickly, allow me to quote and break down some info I found while searching Wikipedia for Dew Point.
It should be clear by now that the key to everything is the Dew Point and how it compares with air temperature, humidity, etc.
This is how the dew point is defined:
“The dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water (dew) […]”. From Wikipedia.
And these are the implications when considering the air temperature and surface temperature with respect to the dew point:
“[…] When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface. […]”
“[…] When the temperature is below the freezing point of water, the dew point is called the frost point, as frost is formed rather than dew. […]”
And finally, to complete the picture, here is the relation between the dew point and humidity:
“[…] The measurement of the dew point is related to humidity. A higher dew point means there is more moisture in the air.”
Ok, let’s cut to the chase now: when a photographic lens will fog?
To put it simply, a lens will fog when the temperature of the front glass of the lens (or the photographic filter you have on it) dips a few degrees below the dew point.
The cold glass will make the water vapor in the air condense on it, particularly if there is no air movement to help reduce the condensation.
Here a couple of examples to understand better why condensation occurs:
- Take your glasses or a lens and breathe on them. Since your breath has high humidity, its dew point is well above the ambient temperature. When your hot and humid breath meets the colder glass, the water in it will condensate on the glass.
And this is how we clean our glasses or fog our lenses to create artistic effects.
- The same principle is responsible for the fogging of your bathroom mirror when you take a hot shower.
Why Is Condensation a Problem?
In photography, there are two different aspects to consider when it comes to condensation: the gear and the image quality.
Condensation Vs Your Gear
You do not want condensation to form on your sensor, inside your camera, and inside your lens and you have to reason in terms of temperature difference:
- Exposing your gear to a colder environment does not create condensation. This is because your camera is hotter than the air around it, thus well above the ambient dew point.
- While working, your camera generates heat: this will prevent condensation from forming inside your camera body, but you may have problems with your lens.
- Exposing cold equipment to a hotter environment will cause condensation to form. Bringing your camera in your home after hours spent shooting stars on a cold night is the worst thing you could do.
The proper way to avoid condensation inside your gear is to put your camera in your camera bag at the end of the session, before going back inside.
Alternatively, put your gear inside a zip bag and seal with some cold air trapped inside.
This allows your gear to slowly thermalize while avoiding condensation. For extra protection, you can throw in your camera bag some silica bags to help absorb moisture.
Wait 12/24 hours before taking the camera out. If you are in a hurry to see your photos, before putting your camera away, take out the sd card.
Condensation Vs Image Quality
While in the field, condensation can cause your lens to fog, thus dramatically reducing the image quality.
The faintest stars will disappear from view, the brightest one will create halos, and the image will get a general soft look.
How To Prevent Condensation On Your Lens
Preventing condensation to form on your lens is the way to go. To defog a fogged lens will be more difficult.
And when it comes to preventing fogging, the key is to control the temperature of your gear, so that it stays warmer than the ambient dew point.
Serious astrophotographers using telescopes often make use of high-end equipment to prevent fogging their optics: 12V controllers, 12V power tanks, and heating strips.
For us astrophotographers on a budget, using camera lenses and small telescopes, there are a couple of simpler (and cheaper) solutions.
5V DC USB Heating Strips: The Best Solution
This is by far the best solution to keep your lens from fogging.
All you need next is a decent power bank to power them via a USB 5V port.
This solution is a very lightweight and flexible solution, by far the one I recommend the most.
Here there are a couple of tips for using heating strips:
- Always use the lens hood: this alone helps to prevent condensation, but it also provides a place to mount the heating strips without obstructing the view.
- Be careful not to move the lens focusing ring while installing the strip. This may be a problem with lenses with a very smooth focusing ring, and you may need to tape it down. Don’t be lazy and always take a test shot to confirm your focus is still spot-on.
- You do not need to cover all of your lens with the strip: just make sure the strip wraps nicely tight around the lens near the front element and the lens hood.
- Once you have the strip, don’t be lazy and install it every time you are doing astrophotography: summer nights can get cold and humid, and lens fogging can still happen occasionally.
Hand Warmers: A DIY and Backup Solution
Using a hand warmer is a more DIY solution to prevent lens fogging. Simply strap one or two of those to your lens using a rubber band.
The downside of this solution is the cost of the long period and the possible seasonal availability of the hand warmers.
Nonetheless, have a couple of hand warmers on your camera bag, in case your power bank is empty or your heating strip is not working.
Too Late: How Do You Get Moisture Out Of A Camera Lens?
Has your lens fogged up?
Do not start wiping the lens trying to remove it: you may scratch the lens if you have dust or dirt on it, and condensation will form soon after.
If you don’t have anything with you to warm the lens above the dew point, call the night off and if you have one, place them in a dehumidifier storage cabinet.
If you forgot to install the heating strip or hand warming, install them on the lens and wait for the condensation to evaporate.
Blowing on the lens or using a cordless hair dryer can speed up the evaporation process.
How Knowing The Dew Point Improves Your Images
You may think I could have discussed lens fogging in a single sentence, something like: get a heating strip fitted on your lens, plug it to a power bank, and you will never have your lens fogged again.
This may be true, but interestingly, by understanding why a lens fogs up, you will be able to improve your astrophotography.
How is it so? Because part of the answer to “why lenses fog up?” is the answer to a different, and equally important, question: “will it be foggy out there tonight?
If you, like me, are forced to drive up to 150km one way outside the city to avoid light pollution, you may want to know what chances are it will be foggy out there.
Whether apps such as Clear Outside can tell you whether you may find fog at your location. Clear Outside also gives you forecasts on the Dew Point for your location.
These are, of course, forecasts, but if you see the local temperature is flirting with the dew point, there will be a good chance fog and haze will rise during the night.
This is particularly true if you are close to bodies of water, such as lakes and rivers, or you are inside or near a valley or ground depression.
If you can, use this information together with your knowledge of different locations to pick the one that gives you the best chances to success:
- Pick a location for which the temperature is well above the dew point.
- Avoid places near water (sea, lakes, rivers, marsh, etc.).
- Favor higher ground rather than staying inside a valley.
- Favor “open” locations, where the breeze can move the air and reduce the ambient humidity.
Lens fogging is one of the many little problems that we face in astrophotography, and that can ruin an entire imaging session.
A simple heating strip powered via USB with a power bank is a great way to solve this problem.
Factors such as humidity, wind, dust, etc, will affect the overall seeing for a given location, and the principles responsible for lens fogging are the same behind the formation of fog and haze.
Learning how to use those principles to estimate the chances of fog or haze will ruin your imaging session is crucial, particularly if you need to travel to a dark location.