Seeking some advice. I am looking for a fast mid-range lens for sport type photography; in particular to shoot dogs in motion. I currently have two lenses in my kit the Nikkor 18-200mm and the Nikkor 50mm f/1.8 coupled with the D90. Neither of the lenses are exactly fulfilling my needs for this particular type of shooting. The zoom lens is wonderful in that it allows me to get in close and fill the frame, but I will very quickly run out of light to get the shutter speeds I need. The 50mm allows me to keep the shutter quick but is a little too far away and I'm losing pixels to cropping.
I can't afford the 70-200mm f/2.8.
I also enjoy nature photography and have been using the zoom lens for close-ups, but am hoping that the mid-range may also suit for that purpose; though it isn't my primary reason for this lens. But it does explain how I ended up looking at both the Nikkor 85mm f/1.8 and the Tamron 90mm f/2.8 macro. I've read that both are good for portraits (though the only portraiture I do is outdoors with dogs). Primarily I am concerned that being a macro lens the Tamron may be slow to autofocus for motion shots, so am seeking feedback on that.
Would also appreciate it if there are any other lenses that I have overlooked that might be more suitable or at least worth a look.
Any advice would be greatly appreciated.