Did you ever look at images you captured with a telephoto lens on a beautiful afternoon and wonder why they were not sharp? As I write this tip, I have been evaluating several long focal length lenses that share an attribute common with other telephoto lenses. Ever see shimmering in the distance such as where a road goes over a hill? Telephoto focal lengths magnify these heat waves and longer subject distances are more likely to be negatively influenced by the distorting effect.
If there is a heat source (relative to the ambient temperature) between you and your subject (or below that line of sight), you can expect some heat wave impact at even relatively short subject distances. Heat waves can occur practically anywhere, but this issue is primarily encountered outdoors and the sun is the primary (but not the only) cause. I frequently see degradation caused by heat waves over artificial turf athletic fields, running track surfaces and even thick green grass in the front yard. Asphalt, being dark in color and high in heat absorption/retention, is a classic source of heat waves, including the source in my "road goes over a hill" example.
Many other heat waves sources exist, including a flowing river on a cold day as illustrated in the included photo.
That image is a 100% crop from a 600mm picture of a steel railroad bridge. No, I did not use an "Art" filter on this image. Yes, the steel should be straight and sharp. No, this blurry image is not the fault of the lens.
When present, heat shimmer will create optical distortion that will diminish the quality of medium and long distance photos. It was 13° F (-11° C) on a clear, sunny morning when I photographed the distant railroad bridge. The warmer water in the river I was shooting over was creating turbulence for the light waves reaching the lens.
Know that heat waves are not limited to affecting only long distance subjects. While testing a lens at 600mm on a sunny afternoon, the strong focal length magnification made heat wave micro-distortion easy to see over thick green grass with only a roughly 100' (30m) subject distance.
The moon is a common photo subject for telephoto lenses and to photograph the moon means that light must pass completely through the earth's atmosphere. That distance leaves plenty of opportunity for light bending to occur.
Heat waves are definitely an obstacle for creating accurate outdoor lens comparisons. Generally, a clear sky is needed for consistent lighting between captures and the sun of course needs to be at least relatively high in the sky. That means the sun will be heating anything it shines on.
Note that heat waves can negatively impact AF performance as well. Because the optical irregularities caused by heat waves are presented to the camera's autofocus system(s) (both phase detection and contrast detection systems), focus distance calculations can be impacted. Especially keep this in mind when dialing in AF microadjustment
What can you do about this problem? Heat waves are an image quality factor that you generally can't spend money to put behind you. For example, a sharper lens and a better camera are not going to be helpful. Selecting a different location, a different time of day and/or a different day completely or even a different season is often the best solution. A cloudy day with low temperature fluctuation may work for your image.
Many times, the photographer does not have control of the day and time of a shoot and will need to deal with the issue. Sports photographers typically fall into this group. For example, auto racing often takes place mid-day on asphalt tracks and photographers capturing these events will encounter this distortion.
If opting to shoot through the heat waves, move closer if possible (but not dangerously so – referring to the auto racing scenario). The less air that light passes through, the less likely that heat waves will cause strong distortion. Also, capture lots of images to allow selection of the least-influenced and to give your camera opportunity to lock in proper AF distances. How Heat Waves Affect Photography Summary
The summary is short. The reason why some of your telephoto images are not sharp is because heat waves are bending the light and confusing your camera's AF system. The basic lesson here is that using the long focal lengths to photograph distant (and sometimes no-so-distant) subjects must be done with consideration to the effect of heat waves.