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This is a big problem when photographing field sports on artificial turf fields on warm, sunny days. And is a situation where AF systems don't work well when trying to track action, compounding on the heat haze problem. The solution is a natural turf field. Unfortunately, those are becoming rarer and rarer. Having photographed many football games in such conditions, I rarely have focus issues at games played on natural turf. On artificial turf I have noticed that focus consistency improves considerably whenever clouds block the sun, or the sun is lower on the horizon. Noon/1pm games are frequently horrendous due to the compound effects of high-noon sun with harsh shadows (the solution is to shoot back-lit) and the intense heat haze from the sun baking the small black rubber pellets used as cushioning for the fake turf.
At some venues, photographers are required to kneel so as not to block the view of the paying spectators. This naturally worsens the heat haze issue. Standing reduces it somewhat, but of course not completely. One would think that a cold, crisp late fall or early winter football game would be ideal, and it generally is, with the exception of the propane heaters used to heat the players benches, and the waste hot air that wafts across center field. Depending on the wind direction, random image blurriness is frequently the result....
Back in 2007 Canada hosted the FIFA Under 20 World Cup. All of the matches were played on artificial turf. I clearly recall how after the first couple afternoon games the visiting European photographers were stumped about why all their across-the-field images were soft. They were used to matches played on natural turf and were unaware of the heat haze issues on fake turf...