The Science of Lens Condensation

You gaze out the window of your beach front hotel room as the sun gently fades on the horizon.  At the last second, the sun and clouds converge in a brilliant display of light across the water.  You grab your camera, rush out onto the balcony, and prepare to capture the magic.  And then it happens. Slowly and imperceptibly at first, your image begins to look a little cloudy through the viewfinder.  And then suddenly, the entire image disappears into a dense fog.  You stand there, fuming with disgust, as the sun fades across the horizon.  You gaze down sappily and helplessly at the tiny water droplets which have rendered your thousand dollar lens useless.

You are the victim of lens condensation, an affliction which affects millions of photographers.  There is no known cure, but understanding the cause of the phenomena will help mitigate the symptoms. For the 85% of the world population who reside in the northern hemisphere, lens condensation occurs during two times of year.  The worst time is during summer when cold air conditioned lenses are taken outdoors into warm, humid air.  Condensation can also occur in winter when, after shooting outside in cold air, you return inside to warm, moist air.

Dewpoint Science

To understand why condensation occurs, we must first understand dewpoint.  Ambient air always contains a certain amount of water vapor.  We measure it in terms of relative humidity. The reason it is relative is that hot air can hold more moisture than cold air.  So an 85% relative humidity at a given temperature means that the air is carrying 85% of the moisture that it could hold at that temperature.  But if the air is allowed to cool, the relative humidity will rise – despite the fact that the actual moisture content has not changed.  The lower moisture handling capacity of the cooler air just means that the air is closer to its saturation level and thus its relative humidity rises.  If the air is cooled enough, it will reach 100% relative humidity or 100% saturation.  Beyond this point, the air can no longer hold all of the water vapor contained within it, so moisture begins to condense onto exposed surfaces in the form of dew.  For that reason, this saturation temperature is known as the dewpoint.

The diagram below shows an example of how relative humidity increases as a parcel of air is cooled.  For this specific case, dewpoint is reached at a temperature of 85° F.

Lens Condensation

So how does dewpoint affect lens condensation?  Whenever a cold lens is subjected to warm, moist air, the local environment around the lens creates excellent conditions for condensation.

Condensation will happen any time a cold lens is exposed to warm, humid air.  Specifically, condensation will occur whenever the temperature of a lens is lower than the dewpoint of the ambient air.

As shown in the image below, the local air surrounding the lens is cooled to an extent that the air can no longer retain the moisture within it.   Excess moisture is then deposited on the lens in the form of dew droplets.  But that is just the beginning of the problems. A uniform layer of transparent water on the lens may still be manageable.  The real problem with lens condensation is that water does not deposit uniformly.  Because of the surface tension, water molecules would rather bind to themselves than to the glass and the result is millions of bulbous “dew droplets.”  It is these droplets that create the fog effect that prevents imaging.

 

Dealing With Lens Condensation

How can you deal with lens condensation?  There are two commercial methods for dealing with condensation.  The first is to use heaters to ensure that the temperature of the glass surface of interest never drops below the dewpoint of the ambient air.  This is the solution used in many of the “fog-free” mirrors sold today.  Unfortunately, localized heating spells trouble for the thousands of tiny parts used for precise lens optics.  The battery life hit alone keeps this method out of commercial lenses.

A second method is to coat the lens with a substance that doesn’t prevent condensation but rather alters the surface tension of deposited water molecules. By providing a better binding surface, water molecules deposit uniformly instead of balling up into droplets.  Although the result is not optically ideal, it is a workable alternative to an opaque fog.  There are many fluids sold on the market which can achieve this purpose.  You may see them marketed next to ski goggles as “anti-fogging” spray.  You can also achieve the same goal with common household items such as dish soap.  A well known method for dealing with fog in swim googles is the line them with a thin layer of spit.  Yep – it works.  But while all these methods work for ski or swim googles, it is probably not advisable to rub spit or soap onto your thousand dollar lens.  There are better commercial solutions in the works, such as this one, which involves permanently binding thin layers of molecules to a glass surface.

Until a better solution is at hand, the best technique seems to be just planning ahead and waiting for your lens temperature to stabilize with the ambient.  While there are normally no adverse effects from just letting your camera fog up and then defog, you will get to your end result quicker and cleaner by protecting your camera.  It is basically just a waiting game in allowing your lens temperature to rise above the dewpoint temperature.  If you are planning a shoot in a dewpoint-challenged environment, prepare for condensation in advance.  Before going from cold to hot, wrap your camera and lens in a ziplock bag or trashbag.  You can also use a more expensive dive bag just for this purpose.  Once you’re in the warmer environment, let the camera acclimate for about twenty minutes before opening the bag.  Any condensation will then build up on the bag instead of your camera.

Size Matters

Be aware that the acclimation time will be affected by the size of your lens.  The graph below shows temperature readings taken with a thermocouple mounted to the front element of two different lenses.  As you can see, a tiny kit lens reached the dewpoint of the day (85°F) in 5.5 minutes while the much larger 400mm lens took 7.5 minutes.  It will also take a few minutes for all the water molecules to desorb, even after the dewpoint temperature is exceeded.


Whatever you do, don’t change SLR lenses while they are still acclimating to the temperature.  Changing lenses will expose the SLR mirror and rear lens elements to sudden condensation which are more challenging problems than the simple fogging of the front element exterior.

When to Bother?

This may seem like a lot of trouble to go through for a few photos, so when should you bother with condensation protection?  A quick rule of thumb for outdoor shoots is to check your local weather provider (weather.com, etc.) and note the dewpoint.  If the dewpoint is lower than the present “indoor” temperature of your lens, then don’t worry about condensation.  If the dewpoint is higher, then bring out the ziplock bags.  If it is close, better to be safe than sorry.

While science has not provided a perfect solution (yet) for overcoming condensation, understanding the mechanism can certainly help you deal with it.  Here’s hoping the fog doesn’t ruin your next shoot!

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