This is an article I wrote for Associated Content (later acquired by Yahoo and became Yahoo Contributor Network). Guess Yahoo did not think it is worth their while to host and assign resources to it and they are shutting it down end July sending many self employed writers into survival mode. I am posting it here so folks can continue to read this article.
Why Camera Lens Fog
The reason why lens fog up is very similar to why our eyeglasses fog when when we step from a nice warm car into a colder outdoor or vice-versa, when we step outside from an air conditioned car to a hot outdoor. If left unattended, this will lead to mold growth on the lens interior (and exterior) and by then most remedies are too late and the lens is basically useless. If your lens fog often, there is usually more to it than just the fogging. The lens is probably rather old or has probably seen some serious wear and tear . Moisture tends to creep more easily inside an old lens that has not been well cared for when it is subjected to extreme temperature shock. If that is what inflicts your beloved lens frequently, I am afraid it is time for you to think about getting a new lens when the next big sale come around.
Let's start with steps you can take to reduce fogging:
Always put your camera/lens in a Zip Lock bag before and after using it and zip it airtight. It does not matter whether you were using it indoor or outdoor. It must become a second nature to you if you care about your camera. This is especially necessary if you have been shooting in the cold and are returning to your warm car or going indoor where the temperature is warmer. The air inside the Zip Lock bag acts as a cushion and will allow the lens to adjust to the temperature change more slowly. Any condensation due to extreme temperature change will accumulate on the inside surface of the zip Lock bag. Try to give it some time to adjust before you pull the camera out again to shoot indoor. Be gentle with your camera, great photography often comes from time spent composing the image in your mind and not from an impulsive shoot.
Shooting winter/snow scene, being outdoor on a chilly day for many hours. If you plan on doing outdoor photography the next day, think ahead. Put your camera and lens inside a large Zip Lock bag. Zip it tight and leave it in the trunk of your car (I would only do this if my car is parked in my garage and can be locked up of course). This will mean the lens should be at about the same temperature as the outside temperature when you pull it out to use the next morning. Also if you are stepping out of a warm car into the chilly air to take a snapshot when you are driving and your camera is coming right out of your backpack in the warm and toasty car, keep your camera close to your body inside your fleece jacket until you are ready to use it and do so quickly. Have a Zip Lock bag handy, put the camera in the Zip Lock bag and zip it tight before you get back into the car.
An old lens will be more prone to fogging as mentioned above (unless it is the expensive Canon L lens with weather sealed lens elements). This can become a big problem over time and mold can infest the inside surface of the lens. Can that be fixed? you ask. Yes, but at a price that is prohibitive and highly uneconomical. The camera shop technician will likely advise you to get a new lens from his colleagues in the sales department instead due to the cost of repair. There is also no guarantee that the mold will not return once you had the lens clean.
What to do if you have a foggy lens?
If your lens fogs up because it has just gone through temperature shock. Remove the lens from your camera bag. Use a clean lint free cloth (lens cloth) and wipe the dust and moisture off the exterior of the lens. Dry off all other parts of the lens well and place the lens on its side about 10-12 inches from a reading lamp. Rotate it 45 deg every 20 minutes or so to allow the lens to warm up and dry evenly. The lens should clear over time so relax. Don't check on it every 5 minutes. It will likely take anywhere from 1-3 hours for the lens to clear completely depending on how big the lens surface area is and how bad it has fogged. I am assuming that is the only problem you have. Once the lens has cleared. Hold it up against the light to see if there is any other dirt or problems. If you see dusts hopefully on the outside, clean the exterior lens surface gently using lens cloth and cleaning solutions.
If you don't already use a lens filter on your lens, get one! A UV/Haze filter will do the job. It protects the front element of the lens in case you accidentally drop the lens. More importantly, the filter adds an extra insulation between the cold air and the actual lens surface and often that will prevent fogging of the lens as the air between the filter and lens surface helps protect it from the temperature shock. Changing lens. If you need to put a new lens on, make sure the camera power is turned off before you remove the lens and put the new one on. Static will draw dusts onto the camera sensor - the most delicate and important part of the camera. This will result in black round spots appearing on your images. The dust spots will first show up on your landscape photos (when you are shooting at smaller f/stops, e.g. f/11, f1/13, f1/16) and they will become more and more apparent and will eventually show up on all your digital shots, ruining the pictures. Do not attempt to clean your digital camera sensor yourself, no matter how much you want to. Any mis-step can ruin your camera for good. A reputable camera shop will do the sensor clean for $60 a pop.
The moral of the story: take great care of your camera equipment. Give it plenty of TLC. As you know, water and electronics don't mix well so treat moisture on any part of your camera or your lens as your biggest enemy. Dry the moisture off thoroughly with lint free cloth each time. To clean the front element of the lens, spray lens cloth with lens cleaning fluid and wipe it gently, recommended after each photo session. Put the lens cap back on, never leave your camera lens exposed. Many things can happen to your camera when you least expect it. Little things like that may be a no-brainier. Simple preventive measures will go a long way in keeping the camera in ship shape condition for many years to come. I have seen many camera disasters: A friend of mine once dropped a brand new camera into a bucket of water while on a sailboat at the dock. Despite grabbing it out of the bucket in a split second, removing the batteries and wiping it dry, that camera was finished. Finito!!. Nothing would have saved that $490 Canon A640 from the water. Ouch! but to make a point, cameras don't like water!!
Footnote: For years, I wrote articles as a Yahoo contributor (formerly Associated Content) and Yahoo has just announced they are shutting that down, and all the contents and rights will be returned to the writer/contributor. I have expected it for some time now since Helium (a similar article site) "went off the air". I wrote a total of 40 articles on photography, food and How To and so I am going to be posting some of the more popular articles on my blog to make them available to readers after July 31st.