Fogged lens

[Colin Southern]

In that case Colin. I hope you are very careful when you wash your hair, as the active ingredient (sodium laureth sulphate) in dishwashing liquid is the same as in shampoo and for that matter as in liquid soap. Most of the rest of the chemicals make it look and smell nice.

Luckily there's no metal in my head.

[Hero]

Luckily there's no metal in my head.

There is actually quite a lot of metal(s) in your head, but that's a whole different biological topic...

[pnodrog]

Yeah, haemoglobin is red because of the rust.

[Colin Southern]

Yeah, haemoglobin is red because of the rust.

Too big a word for me I'm afraid ... heck, I can't even spell hemaglobin, let alone erythrosite! (little medical humour there )

[Glenn NK]

I've worn eyeglass for 54 years, and for most of those years I lived where winter temps were often well below freezing (minus 30 C was common).

Every time I came in from the cold outdoors, my glassed fogged up, sometimes badly.

They never fogged up when I went outdoors.

This could provide some insight as to what to do with a lens to prevent fogging.

Glenn

[pnodrog]

Yeah I have the same sort of problem every time I open the oven door with my glasses on they fog up. I solved it by heating them up on the hot plate before wearing them - my optician and doctor are making fortunes.

[pnodrog]

Too big a word for me I'm afraid ... heck, I can't even spell hemaglobin, let alone erythrosite! (little medical humour there )

Talk about big words I was looking it up when I came across erotic so I got distracted. Hope they don't lock me up in a red cell.

[Inkanyezi]

I've worn eyeglass for 54 years, and for most of those years I lived where winter temps were often well below freezing (minus 30 C was common).

Every time I came in from the cold outdoors, my glassed fogged up, sometimes badly.

They never fogged up when I went outdoors.

This could provide some insight as to what to do with a lens to prevent fogging.

Glenn

There is one problem there. Eyeglasses are a simple optical system, which consists of a pair of simple lenses, both of their sides exposed to the environment. When they are cold and you come into a heated space, thee water that is stored in that air will go below the dew point close to the glasses and set off tiny little drops, that cloud your vision.

When the glasses attain room temperature, that water will evaporate. If you then go out, when the glasses are warm, no fogging will occur, because your eyeglasses are above the dew point for the water contained in the air outside. Also when they cool down, no fogging will occur, because they will not be cooler than the surrounding air.

A 400 mm lens however is a container, filled with air. When this air cools down from its previous indoor temperature, the stored water will condense when and where the temperature goes below the dew point. What we do with the 400 mm lens is that we take it with the air contained within from the heated space to outdoors, and as long as it is still hot, no condensation occurs. But when inner areas of the lens cool down below the dew point of the air contained within, droplets will form and set off at the cooler surrounding areas. This process is only a tiny bit different from when we take cold objects into the house. The long lens is some of that indoors air which is taken out. It will lose its stored water when it gets cold, and that water will set off somewhere.

When you take the cold lens indoors, just as your eyeglasses, it will be fogged as well. This should be avoided, and it can be done in several ways. If you are going to continue shooting outdoors and can store the lens in a cold place, it is better not to take it in at all. If you must take it in, keep it in its purse, bag or where you usually store it. If it is very cold, you can wrap your equipment in plastic bags, to avoid humid indoors air to get at the camera and lenses until they have reached indoors temperature.

The fogging of a lens that has been taken from indoors to outdoors is due to condensation of water stored in the air within. Some long lenses are single component lenses, and with those, it can be sufficient to leave the rear end open, without a rear lens cap. Then humidity may escape that way as you take it outdoors before you put it on the camera. However more complex systems might need to be kept in dry conditions in order to avoid fogging of the inside when they get colder.

[Glenn NK]

I don't think that the complexity of the lenses changes the physics.

If the camera lens is warm (including the interior elements) the interior will not fog up inside when cold air is drawn into it (as happens when zooming and/or focusing). Reverse the temperature and procedure and trouble will follow.

As for taking lenses/cameras from a cold environment into a warm one, one should always leave the gear in a closed bag (resist the urge to open it), and let it warm up slowly. I did a lot of photography in Northern Saskatchewan in the winter (as I noted, -30C was common) - never had the interior of a zoom lens fog up using this technique.

Glenn

[pnodrog]

The thing we have to be aware of is a lens can fog internally even if it is above the ambient temperature.

As you have all guessed I store my camera and lens in hothouse at 30 deg C and 90%RH. When I take it into my house at 23deg C and 30%RH by the time my camera and lens cools to 28deg C the humid air in the lens will saturate and condense on the coldest surface it finds. The trick is if I am lucky I will be able to pump my lens and exchange the high humidity air in the lens with the lower 30%RH room air before the camera cools to 28deg C and I may avoid any condensation.

It actually ends up as a bit of a race between the lens temperature drop and the exchange of the moist air in the lens with ambient air. If we take a warm lens with internal high humidity air into a cold foggy environment we are going to need to pump the lens sufficiently to exchange the air but not cool the lens to much and may the force be with you. (it might just work}

[Colin Southern]

As you have all guessed I store my camera and lens in hothouse at 30 deg C and 90%RH.

Why?

I store mine in their leather pouches in my camera bag - in winter they probably get down to a few deg C - in summer probably up to 30 deg C - never - ever - a single problem / issue with optics.