The kids were real troopers for sure. You can see the rain droplets on the lens. The XTi came though it with out a bruise.
The photo doesn't faithfully show or do justice to how dark and wet it was.
Although this isn't a football photo it does show the abilities of the XTi. I was going to shoot FB that evening but the weather got so bad I didn't even leave the stands. It was just rain as the left over from one of the hurricanes came through Kansas. They played and finished the game! But here is the little XTi doing its thing with an ef 24-105mm f4 lens. Dark, wet, cold pretty tough conditions. So, it can be done but you won't get every shot for sure.
I am afraid this situation requires an exposure triangle set of parameters that your current lens and camera just can't provide. I don't think you will get any useful shots involving football action below 1/500 shutter speed but you could try to experiment with 1/250 to see. The shots you posted have severe motion blur from a too slow shutter speed and there is nothing that can be done in post to clean that up. Try going full manual mode setting the shutter to 1/500, the aperture to F4, and the ISO to max (3200) and with the Tamron lens don't "tele" past 100MM which is as far as you can go and get F4 aperture. If one of your other lens provides a little more reach at F4 then use that one but I suspect the Tamron is the best choice given what you have now. With your ISO set to 3200m see how under exposed that combination of parameters provides. If two stops or less it should be somewhat usable and maybe three stops when shooting in RAW depending upon how the sensor responds in your camera. It will have a lot of noise but noise is less destructive than blur.
Before the next game, find a similar light environment to the field and get your son to run some practice rollouts so you can experiment and see if those settings with your camera will provide usable photos. They won't be perfect but they should get away from the worst of the motion blur. For your son, you can shoot a few of him at a slower shutter speed (keep the aperture wide open with the lens only extended as far as it will go staying in the F4 range). The QB is going to freeze briefly sometimes while reading the field and that is your chance to get a sharp photo of him without motion blur, of course you will also have to stay dead steady while tripping the shutter release.
I hope this helps. Given that he is in the playoffs, have you considered renting a lens for a weekend from one of the rental companies? It is a little expensive but some will credit the rental towards a purchase of the rented lens so this is another way to work your way into a good value after you rent and try it out. I rented a 400 2.8 before I bought one, given the price of the 400 I wanted to make sure I was comfortable hand holding it before actually buying one.
Below is an example of capturing the QB frozen in position and if the lighting were bad you could get this shot at 1/160 if you hold the camera steady. I shot this at the standard parameters I use for that field (1/640, F2.8, ISO 6400) however it is the worst illuminated football field I shoot on and on several of the fields 1/640 and F2.8 yield a 2000 ISO as the third leg of the triangle and you would be in good shape with 1/500 F4 on such a field. I know that you want to shoot for the entire team but make sure to get some good photos of your son which you can do with a slower shutter speed that will put you more into the "sweet spot" for your camera and lens combination.
Rodger
