An important factor in photographic image quality is how stable the air is between your lens and your subject. Variation in air temperature with pockets of cool and warm air cause the light to be refracted as it passes through the less dense or more dense air masses and as these pockets move, they cause unstable air. On a warm day, the familiar heat shimmer rising off a warm road is an everyday example. The effect of this instability on image quality depends on its severity and how much air you are shooting through- the more distant your subject, the worse the problem. This the effects of heat-shimmer are particularly noticeable when using a telephoto lens on a subject in the distance.
Here in the Maritime provinces we quite often experience "heat shimmer". A typical scenario would be a beach with coolish water on a hot sunny day with little wind. As the water cools the air above it, the air mixes with the air being heated by the sun hitting the beach material, whether it be rocks, pebbles or sand. The shimmer is generally worse in the zone about10-20 cm above the beach- exactly where those shorebirds are standing. However, it can also affect the whole column of air above the ground to a significant height. The result of heat-shimmer is out-of-focus, blurry images. This is hard to detect through the viewfinder of the camera but if you suspect you might be suffering from shimmer, take a look at a few images on your LCD. Zoom in on them and you will instantly notice the poor image quality. Another way to tell that heat-shimmer is occurring is to but your camera on a tripod, switch on live view, zoom in using live view and focus on a static subject. As you view the LCD screen you will notice the image going in and out of focus.
Short of going home, one way to combat heat shimmer is to attempt to get as close as possible to your subject without disturbing it. Also, simply avoid the conditions that cause the problem in the first place. You need a heat source to cause the problem so avoiding sunny conditions kills two birds with one stone: no heat shimmer and beautiful, soft light.
Over the past week, I have experienced problems with heat shimmer while photographing the Semipalmated Sandpipers at Johnson's Mills. Here are some examples.
This is from the centre of an image made with Canon's 500mm F4L IS USM version II and the Canon 1D Mark IV. This should be pin-sharp!
If you see repeating lines around out of focus elements in your image it's a good bet you are suffering from heat-shimmer:
The clincher is shown here. Notice the out of focus specular highlights- they should be smooth and untextured. Not like this: