It happens because the polarizer works better on some parts of the sky than others. And, since you’re capturing such a wide part of the sky with a wide-angle lens, you’re almost always going to run into this problem.
Sure, you could avoid this problem all together by not using a polarizer on a wide-angle lens, but then you wouldn’t be able to take advantage of the other great benefits of the polarizer: reducing unwanted reflections, and saturating colors. [Read more…] about How to Fix a Washed Out Sky In Photoshop
In a previous post, I talked about the importance of 
Sometimes autofocus can be really annoying: it’ll search and search for an object to focus on, and either never find it, or go through multiple cycles until it finally finds the right object to focus on. And by that time, you may have already missed the shot–especially if you’re photographing a quick moving subject like birds.