The world as a bad lens: atmospheric distortion

In a forum thread somewhere, somebody asked about resolution in distant focus and that tickled me into writing this article on the influence of the environment on long tele work. I have taken several pictures of distant blobs at long focal lengths, so clearly I am the undisputed expert and my words should be taken as gospel (and if you believe that, I have several renowned landmarks for immediate sale as well).

Disclaimer: I'm not a scientist. Also, some of the science in this post has been simplified.

So on to the main question: why is it so hard to get a good picture of something far away? Well, there's many reasons, really, and some you will see pushed in lesser or smaller degree on various forums. Camera shake is the obvious scapegoat: at 400mm, so the mantra goes, you should shoot at no slower than 1/400s.

Okay, fine, you encase your camera in concrete and tether this to the largest gyroscope you can find, then you put industrial shock absorbers on the gyroscope (just because you can) and to be really sure you shoot at 1/2000s ... and your picture is still a bit blurry and lacking in contrast. What gives? That's the SHG 90-250 you're shooting with. It's optically perfect! What's going wrong?

The answer is simple, and known in the technical litterature as "stuff in the way", or, if you prefer, "atmospheric distortion".

The culprit here is not the lens, nor your camera, nor even the dreaded AA filter: it's humanity's thoughtless decision to evolve on a planet with an atmosphere. Truely, humanity has no appreciation for good long tele photography.

Al right, so what's happening? Basically what's getting in between you and good pictures is the very air we breathe. The more distant your subject, the more air there's between you and them. "What's the problem?" you wonder? "Isn't air transparent?" Well .. no, not really. But even if it were: so is glass and look at what your lens is doing to the light. The air between your subject and your lens is a natural lens in its own right, and one of rather dubious quality. Look up right now. What do you see? Right, the ceiling. Fine. Look up through the window, what do you see now? The sky. Now the sky is a colourful beast, it can be blue, red, white or gray (or black or even green or purple if you live where the aurora is visible, work with me here). All of those colours show different defects of air as a lens.

Blue noon, red sunset

The sky is blue. Except it isn't. The earth atmosphere is, like water, mostly colourless. What makes the sky blue is called "scattering". See, light is a wave and blue light has a shorter wavelength than red light. This means that red light can move around molecules more easily than blue light. The blue light bumps into the air molecules and gets sent in all directions: it scatters. Many of the blue light rays get eventually scattered back to the ground, but they comes from all directions: we get blue light from the sky.

The same thing happens during a sunset. I hear you protesting "but the sky is RED at sunset!", and so it is. But what's happening here is that because the sun is much lower on the horizon, the light has to travel to a lot more of the atmosphere to get to us. The blue light gets scattered away (to the places where it's still day, for example) and what's left is yellow and red.

Now what has all that got to do with long tele work? Well, it means that even under perfect conditions light will be scattered by the air molecules between you and your subject. Furthermore this light loss is predominant in the blue part of the spectrum. So why's the resulting image not tintedred ? In fact don't distant mountains get more of a blue sheen?

Well, that's the annoying bit. While the blue light from your subject is stolen you get a portion of the blue light from other sources that's bouncing around in the air. This is a primary loss of contrast. Think of it as a glowing diffuse filter between you and your subject.

There's nothing you can do against this, other than move closer. This is why you don't simply mount a 22" telescope to your camera for wildlife photography: after a while the atmospheric distortion balances out any increase in resolution you could get from getting a bigger tele.

Water water everywhere, but not a drop to see

Now that was on a perfect day. Most days aren't perfect and lots of things conspire against your pictures. Take water: it's everywhere! Look at your weather station and read off the humidity. It'll read anything between 0% and 100%. This indicates how much water is suspended in the air. If it's 100% you're basically walking through clouds, also known as fog (or as a sunny day, in the british iles).

Now all this water does the same the air does, but pushes it to 11. Light gets scattered, blue light first and then all the rest. As it becomes more severe, it's even going to affect your metering and you might want to switch to spot metering (actually, you might wanne do that anyway when taking pictures of distant things).

A similar effect is seen when there's a lot of dust in the air: take the Serengeti for example. All this dust in the air gets between your lens and the subject and steals your light! The result? A dramatic loss in contrast and sharpness.

It's hot and cold and under pressure

It's often said that hot temperatures cause more atmospheric distortion, and to some extend this is true: hot air molecules move about more. However hot air is also less dense than cold air, which balances that out quite a bit.

The problem is when you have hot and cold(er) air close to eachother, and this is because of this difference in density. The light refracts as it passes from dense to not-so-dense air, in much the same way as it does when going from air to glass in a camera lens. Now you might think that the difference in refractive indexes is going to be slight, and you'd be right, except there is a LOT of air. This density refraction is what causes mirages, just to show how sever it can get. This will be most severe where hot air meets cold air, for example right above a hot asphalt road (or other black surfaces).

Also differing air densities makes the air move (wind), which is the shimmering you sometimes see in mirages.

That's a lot of complaining ...

Now I've outlined some problems which doesn't help you much. You want solutions (and at a guess, solutions that do not involve selling the atmosphere to the spaceballs empire).

Well, that's the tricky bit ... there's not too many of those. There's "haze" filters, but those aren't quite as useful in digital photography. Haze filters are basically UV filters that are meant to filter out the UV light that gets scattered even more than blue light does (even shorter wavelength), but there's already an UV filter in front of your sensor so that's not gonne do much.

What remains is simply being aware of your environment. Check the weather station: if humidity is great, try and get closer to your subject. Be aware of air temperatures (and thus densities): depending on the time of day the air above water may be colder (noon) or warmer (morning/evening) than the surrounding air. Similar with dark surfaces (like roads). Be aware of dust: if you're shooting wildlife from a car, wait a bit for the dust from the car to settle some. Shoot RAW: this will let you recover some of the lost contrast without causing posterisation. Try to shoot when atmospheric conditions are better: you have to balance the variable temperatures of noon with the smaller light spectrums of evening/morning here (usually noon loses).

And most of all: don't always blame the camera. Sometimes even a perfect camera would not manage a shot.