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Moonrise over Seattle's Space Needle and the Cascade Mountains

Heat distortion and zoom lenses

Have you ever taken a photo with a long zoom lens where you thought your technique was perfect, only to come home and see an image that is mushy and seemingly out of focus?

Blurry images can be caused by heat distortion when there is a significant temperature difference between the surface and the air. You’ve probably seen heat distortion on a hot summer day, with heat waves rising from a road. Or perhaps you have seen it in boat, vehicle, or airplane exhaust.

A photographer’s first reaction will be that something is wrong with their technique or equipment, but no fancy gear or flawless camera technique can fix photos ruined by heat distortion.

The example at the top of this article is from a moonrise over Seattle on an abnormally warm winter day. There was nothing wrong with the lens, focus, or tripod, but the big temperature difference between the cold water of Puget Sound (~47 °F) and very warm air (~75 °F) caused heat distortion that ruined the picture.

In this photo of Seattle’s Space Needle at sunset, the 500mm lens is tack sharp and definitely in focus. The camera was on a sturdy tripod, and a timer triggered the shutter. Both the equipment and technique were perfect, but the resulting image is a squishy mess.

Seattle's Space Needle - severe heat distortion
Strong heat distortion of Seattle skyline taken from over 7 miles of water. Air temperature ~ 75 °F, water temperature ~47 °F. 500 mm, 1/400 sec, f/9, ISO 1250.

When will heat distortion occur?

Heat distortion is caused when light is refracted through air of differing densities. Hot air is less dense than cold air, so light waves are bent differently in hot versus cold air. The result is visible heat waves when there is a significant temperature difference between the ground and the air above it.

On a hot, sunny summer day, you will see waves of heat distortion above roads, but it can occur on just about any landmass where the sun heats the ground to a temperature higher than the surrounding air, such as an open field or beach.

Bodies of water can cause the same phenomenon when the water is significantly warmer or colder than the air above it. A warm ocean with cold air above it may show heat distortion, as will a cold ocean with warmer air above it.

Container crane ship on Puget Sound
Cold 38 degree air over Puget Sound at 50 degrees causes heat distortion

Heat distortion is not restricted to hot summer days. It can also occur in arctic temperatures during winter, where the sun warms the land or mountainside to a temperature well above the air temperature.

Distance makes heat distortion worse

The further away from the subject of your photograph, the more heat distortion will be present. The further distance means the light travels through more air before reaching you. Therefore, it gets refracted more in areas where heat distortion is present.

A long zoom lens usually means photographing subjects at a greater distance. That greater distance increases the chance that heat distortion can ruin your images. Heat distortion is most prevalent at ground level.

Mt. Rainier & ferry - heat distortion
Heat distortion is more prevalent near ground level. Notice the strong heat waves above the water, but a sharper view of Mt. Rainier above those heat waves.

Zoom lenses have the added disadvantage of bringing the detail of the heat waves closer, making the result more significant and more evident in your photo. Photographers who spend over $10,000 on a monster 600 mm or 800 mm prime lens will want to remember this before getting angry and wanting to return the lens for repairs.

This photo shows a ferry that is ~1 mile away from the camera, taken at 200 mm. The focus was perfect on the ferry, but there was some softness due to heat distortion. The city in the background is ~7 miles away. The heat distortion is very severe at a greater distance since that light has to travel through more refracted air.

Walla Wall fery and Seattle skyline - heat distortion
Heat distortion is slight on the ferry 1 mile away but severe on the city that is 7 miles away.

How can you avoid heat distortion?

In many cases, heat distortion is unavoidable, and you can do nothing to fix it, but there are a few different ways to avoid it.

  1. Move closer to your subject. Reducing the distance that light travels through the refracted air will reduce the heat distortion you see in photographs.
  2. Avoid photographing over surfaces easily heated by the sun, such as a road, beach, or similar.
  3. Shoot near sunrise before the ground has heated up or near sunset, once the ground cools. There is less or no heat distortion during these times.
  4. If photographing over a large body of water, try to take photos when the air temperature is similar to the water temperature.

Soft, out-of-focus images can be caused by poor technique or the wrong camera settings, but once you have ruled those out, don’t forget about heat distortion.

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