Addendum:
After seeing the example photos that were not available when this answer was originally written, there are a few qualifications that should be made regarding the original answer that appears below the line following the newer answer immediately below this explanation.
- It contains accurate information with regard to how the AF system of the 7D and similar AF systems in other cameras operate and how they must be understood to be used effectively.
- The issue that caused the OP's images to be soft near the end are not really addressed in the original answer, but someone else describing their AF issues similarly may in fact be dealing with what is dealt with in the below answer.
- We choose to leave the below answer intact in order to make that information available to those who may find it of use.
- Immediately below is our response based upon examination of the sample images now included in the question.
The first couple of images are reasonably in focus with regard to the intended subject. The third frame includes water in the foreground closer than the previous images and Zone AF, as is often the case, chose to focus on the closest thing it could find. From there the camera totally lost focus and spent the rest of the sequence hunting at distances much shorter than anything in the frame.
All of the images were taken at 270mm and f/6.3 (The EXIF info is viewable at the flickr link in the question). This is the lens' maximum aperture at 270mm which means all AF operations were also performed with an f/6.3 aperture. The 7D's AF system is rated to operate with lenses having an f/5.6 or wider maximum aperture. Many third party lenses, presumably including the Tamron 18-270mm in question, get around this limitation by reporting the maximum aperture of the lens as f/5.6 regardless of the focal length to which the lens is set and the actual maximum aperture at that focal length. But just because the AF system tries to AF doesn't mean it can succeed. As the point of aim of the lens is lowered there is progressively less and less total light in the frame with which the AF system has to work. This is what likely led to the failure of the AF system to focus correctly on anything in the final frames.
The obvious solution is to shoot with a lens having a larger maximum aperture or at least at a focal length that allows a wider maximum aperture, specifically one that is at least f/5.6 or wider. At focal lengths of 169mm and longer the Tamron 18-270mm f/3.5-6.3 is limited to f/6.3. At focal lengths of 168mm and shorter the lens maintains an f/5.6 or wider maximum aperture.
But even then, you still need to practice, Practice, PRACTICE to master the AF system of the 7D!
(Original answer posted before example images were included in the question)
First off, select an AF mode other than Zone AF. In my experience, as well as that of many others'others, it just doesn't work well when shooting action with the 7D. The camera will likely focus on something, but it probably won't be what you wanted to be in focus. Try either Single-point AF or AF point expansion. Set up your camera so that nudging the little joystick on the back of the 7D can change the manually selected focus point on the fly without taking your eye from the viewfinder and use it to track your subject in the frame. This will take lots of practice!
If you have done everything suggested above (Learned the ins and outs of the 7D AF system and practiced, practicedPracticed, PRACTICED, PracticedPRACTICED) and are still experiencing a high rate of poor AF performance you might consider using the 7D's AF Micro Adjustment feature. If your focus most often misses in front of your target your camera/lens combination is front-focusing. If your camera/lens combination most often misses behind your target they are back-focusing. Please see Do the issues with sharpness I am seeing require AF fine-tuning? for more. Also What methods can be used to micro-adjust autofocus of a camera body to a particular lens? and Which offers better results: FoCal or LensAlign Pro? If your camera/lens combination misses focus about equally in front of and behind your target, then you may have found the limits of that camera/lens combination's AF performance. For more please see this blog entry from Roger Cicala of lensrentals.com.
Addendum:
After seeing the example photos that were not available when this answer was originally written, there are a few qualifications that should be made regarding this answer.
- It contains accurate information with regard to how the AF system of the 7D and similar AF systems in other cameras operate and how they must be understood to be used effectively.
- The issue that caused the OP's images to be soft near the end are not really addressed in this answer, but someone else describing their AF issues similarly may in fact be dealing with what is dealt with in the above answer.
- We choose to leave this answer intact in order to make that information available to those who may find it of use.
- Below is our response based upon examination of the sample images now included in the question.
The first couple of images are reasonably in focus with regard to the intended subject. The third frame includes water in the foreground closer than the previous images and Zone AF, as is often the case, chose to focus on the closest thing it could find. From there the camera totally lost focus and spent the rest of the sequence hunting at distances much shorter than anything in the frame.
All of the images were taken at 270mm and f/6.3 (The EXIF info is viewable at the flickr link in the question). This is the lens' maximum aperture at 270mm which means all AF operations were also performed with an f/6.3 aperture. The 7D's AF system is rated to operate with lenses having an f/5.6 or wider maximum aperture. Many third party lenses, presumably including the Tamron 18-270mm in question, get around this limitation by reporting the maximum aperture of the lens as f/5.6 regardless of the focal length to which the lens is set and the actual maximum aperture at that focal length. But just because the AF system tries to AF doesn't mean it can succeed. As the point of aim of the lens is lowered there is progressively less and less total light in the frame with which the AF system has to work. This is what likely led to the failure of the AF system to focus correctly on anything in the final frames.
The obvious solution is to shoot with a lens having a larger maximum aperture or at least at a focal length that allows a wider maximum aperture, specifically one that is at least f/5.6 or wider. At focal lengths of 169mm and longer the Tamron 18-270mm f/3.5-6.3 is limited to f/6.3. At focal lengths of 168mm and shorter the lens maintains an f/5.6 or wider maximum aperture.
But even then, you still need to practice, Practice, PRACTICE to master the AF system of the 7D!
