Heat Shimmer
Atmospheric Seeing Conditions
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Del Mar as seen from a mile away at Torrey Pines State Reserve, La Jolla, California, 3:57 PM, Sunday, 28 April 2024. Nikon Z8, Nikon Z 28-400mm at 400mm wide-open at f/8 hand-held at 1/400 at Auto ISO 180 (LV 13.8), Radiant Photo software. bigger or full-resolution 45 MP © 5½ MB JPG image.
A 400mm lens is roughly similar to a 10× telescope, depending on how large you're seeing this image.
Heat shimmer is an atmospheric effect that distorts images exactly as waves do when looking under water from above. Here's a 7× enlargement from the center of the above image, which is roughly similar to looking through a 70× telescope:
1,200 × 900 pixel (6.9× magnification) crop from above. bigger or full-resolution 45 MP © 5½ MB JPG image.
If this 1,200 × 900 pixel crop is about 3" (7.5cm) wide on your screen, the complete image would print at a large 14 × 21″ (35 × 55 cm) at this same high magnification.
If this 1,200 × 900 pixel crop is about 6" (15cm) wide on your screen, the complete image would print at a huge 27½ × 41¼″ (70 × 105 cm) at this same high magnification.
If this 1,200 × 900 pixel crop is about 12" (30cm) wide on your screen, the complete image would print at a mammoth 55 × 82½″ (1.4 × 2.1 meters) at this same extremely high magnification!
For you folks on iPhones, here's an 18× enlargement, roughly similar to looking through a 180× scope:
460 × 460 pixel (18× magnification) crop from above. No, this is not a special effects filter; the air is doing this! bigger or full-resolution 45 MP © 5½ MB JPG image.
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The air's index of refraction varies slightly with temperature.
When different sections of air are at different temperatures, the differing indices of refraction make our images start to get unsharp and distorted exactly like looking down through waves in a pool or through wavy shower-door glass.
Long telephoto lenses, rifle scopes and telescopes magnify this effect, which is present essentially anytime the sun is out — or not.
We don't see it with our naked eyes unless the effect is extreme as in a mirage in the desert in the summer, or what looks like fake water on a road far away, but ask any astronomer or long-distance marksman and they'll tell you all about how atmospheric conditions are the biggest limitation to seeing clearly pretty much all the time.
The image above isn't on a hot day; it's a lovely spring day when it was a perfect 68º F (20º C)!
The reason we have this heat wave shimmer effect is because of the different temperatures of the air across long distances, where even slight changes in temperature and turbulence will have a great effect when magnified like this. The sun heats the ground and the warmer air tries to rise, and with a telescope or long telephoto we can see the waves of different bunches of air moving around.
We see the same effects above fires and hot exhausts.
You can test for shimmer by making multiple shots of straight lines far away, typically buildings. Make a few shots and play them back and you'll see random regions of blur that are different from shot to shot (moving exactly like the waves they are), as well as what should be straight lines wobbling all over the place as shown above.
Heat shimmer is a huge limitation in super-telephoto image sharpness as well as in all telescope use. Lots of science is being thrown at this by astronomers and the military, developing adaptive optics to attempt to correct for these waves in real time.
Don't blame unsharp telephoto images on you or your lens or camera or subject motion until you rule out heat shimmer, which tends to be everywhere when you have enough magnification.
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