Weird focusing problem

jhefti

Last weekend I shot a soccer tournament, and noticed once again a phenomenon I have see before on turf fields (i.e. artificial turf) in direct sun. Basically, any shots that are taken over a long distance are blurry. Ar first pass, the blur seems like motion artifact, but is universally applied to all objects in the field, moving or not. I am shooting at 1/1000 or higher, and this past weekend I was hovering around 1/1600, with the long lens on a monopod. IS is off. It is not a matter of front- or back-focusing, since *nothing* is really in focus. (Turf fields are great for spotting the latter errors, as one will see a nice line of 'grass' in perfect focus either in front or behind the subject. I do not see this.)

Now, I only notice this effect on turf fields that are so hot one can see the heat ripples coming off of them. Several of the games were on grass fields, with no problem. My current hypothesis is that the heat ripples cause the AF to focus taking into account the small changes in refractive index of the air, which may or may not work. This past weekend, virtually all the shots over 20 yards were out of focus.

Anyone else seen this effect? I posted two examples below.

jhefti

Here is an example SOOC (~50% crop). 1/1250, f5.6, 400mm, ISO320:

jhefti

...and a shot SOOC (~50% crop) from the same distance on a grass field. 400mm, f5.6, ISO250, 1/800.

pipster

It is not a focusing problem.It is a seeing problem. The heat waves in this case. While using astronomical telescopes the moving air causes similar problems. The more air that you look through the worse it gets.

As you have seen natural grass is better to shoot over but it is no guarantee that you will not see the same thing as you do over an artificial surface.

jhefti

pipster wrote: »

It is not a focusing problem.It is a seeing problem. The heat waves in this case. While using astronomical telescopes the moving air causes similar problems. The more air that you look through the worse it gets.

As you have seen natural grass is better to shoot over but it is no guarantee that you will not see the same thing as you do over an artificial surface.

Thanks pipster--that makes sense. I do in fact see it on natural grass as well, just nowhere near as bad. You'll notice that both pix above are soft, but the top one is worse.

MileHighAko

I've noticed this too on turf fields. They get HOT and really show the heat when you're far away from the action. Only work around I've found is to get closer. I also tend to try and shoot higher (I know, breaking a rule by standing up).

jhefti

MileHighAko wrote: »

I've noticed this too on turf fields. They get HOT and really show the head when you're far away from the action. Only work around I've found is to get closer. I also tend to try and shoot higher (I know, breaking a rule by standing up).

Yep, I was shooting with two bodies that day, and the only ones that came out were the closer shots.

pipster

Because of the heat waves I try not schedule a day game on artificial turf. I always check for a high contrast edge and see if I can see the distortion through the lens. If I do I have been known to pack it in and go home.