Mithandir
Senior Member
In another thread somebody asked about resolution in distant focus and that tickled me into writing this post 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 many you will see pushed in lesser or smaller degree here. 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, 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 reflects of the air molecules themselves and sends them in all directions: it scatters them. Many of those get eventually scattered back to the ground, but it 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 light, for example) and what's left is yellow and red. So why's the resulting image not tinted like that? 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.
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. 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.
--continued in reply--
--
Mithandir,
Eternal Amateur
http://www.shooting43.com/
Unless stated differently, any image I post is licensed under CC-by-nc
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 many you will see pushed in lesser or smaller degree here. 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, 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 reflects of the air molecules themselves and sends them in all directions: it scatters them. Many of those get eventually scattered back to the ground, but it 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 light, for example) and what's left is yellow and red. So why's the resulting image not tinted like that? 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.
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. 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.
--continued in reply--
--
Mithandir,
Eternal Amateur
http://www.shooting43.com/
Unless stated differently, any image I post is licensed under CC-by-nc
... )?
