This is with a Canon 400mm f/2.8 IS paired with a 1D mark IIn. When I shot football in very hot sunny clear sky weather, I get some strange "swirly" or double-line bokeh with individual lines in some of the photos. It sort of depends which angle I shoot at (as it seems to appear more on photos where the background includes the audience). I am not shooting behind a net or anything.
I shot soccer last week with the other body (the 1D mark II) in quite similarly sunny but not as hot weather and I didn't get this swirly effect. I'm not sure if the body is what's causing it.... probably not. Maybe it's the heat waves rising from the grass.
Is this a normal phenomenon. All photos are f/2.8 and around 1/2000 shutter speed on AI SERVO. I used image stabilizer in football but I don't remember if I did for soccer. I am using the standard drop-in filter Canon gave me. I read some stuff on IS and bokeh and maybe it has something to do with it? http://www.bokehtests.com/Site/Stabilization_and_Bokeh.html
I posted this on other forums and the consensus was that it was heat haze and possibly IS. Inputs welcome.
IS could not cause this phenomenon without causing a similar blur in the areas that are in focus. It is most likely due to a turbulent atmosphere caused by extreme heat. The sensitivity of the lens to air turbulence in areas outside the DOF is greater, much like how diffraction around a grating placed close to the lens while focused far away does not significantly affect the subject in focus, but will show up in the bokeh.
"Haze" is not really an accurate description of this phenomenon, since it implies a sort of more-or-less uniform kind of veil that obscures visibility. This is more akin to a type of distortion or unevenness in the air. If the variation in air density (and thus refractive index) were more smooth, it could create a mirage-like effect.
It's funny you bring this up. I had similar results with my 70-200 mkII. I had no idea what caused it. The bokeh is right out of the camera. I was sorta thinking it was from my 1.4TC.
It's definitely heat coming off the fake turf field. This will be much less of an issue on natural grass. Unfortunately most schools/pro teams are moving away from natural turf to FieldTurf. While FieldTurf is better than the old Astroturf, it still makes it very difficult to get well focused images with long lenses on hot, sunny days (or even some overcast days). Standing instead of kneeling will reduce the sharpness degradation to some degree, but the only way around it is to shoot action closer to you, which is why you don't see it in the tightly composed sample.
A little story: In 2007 Canada hosted the U20 FIFA World Cup. All of the fields were FieldTurf. After the first couple day games, many European photographers complained about not being able to get sharp images shooting far field action, something they could always do previously. The difference: most top level teams in Europe play on grass fields (also supposed to be easier on the athletes).
When shooting birds in higher altitude, there are days when I can't seem to capture anything sharp - usually when there are alternate sunlit and shaded (cool) air between me the subject.
