This is a comparison of the Canon 300mm f/4L IS USM lens, the Tamron AF70-300mm F/4-5.6 Di LD Macro 1:2 lens (@300mm), and the Canon 70-200mm f/4L USM lens + EF 1.4x II teleconverter (@ 280mm). First of all, I like all of these lenses. I think the L lenses are two of the best bargains available among high quality lenses. The Tamron lens, likewise, is a fine value among consumer grade telephoto glass. I have used all three for sports, but I typically use the 300mm lens for the extra reach. The problem is that sometimes it's too much reach. When I shoot alpine racing, I sometimes end up standing in the woods. Likewise, for shooting waterskiing from a boat, it's a bit tight. And yet 200mm is not enough reach, so I want to try using the 70-200mm lens with the Canon 1.4x extender for sports when the light is good. I looked for examples of the 70-200mm f/4L lens used with extenders for sports in the PBase database, but I didn't find many, so for kicks, I thought I would do a quick test myself to compare sharpness and contrast between the 300mm lens and the 70-200mm lens with the extender. I purchased the Tamron lens recently for my girlfriend to use on my Rebel XT, so I tested it, as well, for this comparo. The crops in the chart below were from images shot in raw using a 30D with sharpness set to 0, using the center focal point. They were converted with Canon DPP, then cropped from the center and resized. No other editing was performed (I did not rescale the 280mm images, hence, they are slightly smaller than the 300mm images). To make it a fair fight, I used a monopod with the Canon 70-200mm and the Tamron 70-300 lenses to balance the IS of the 300mm lens. I was approximately 30 feet from the thermometer. I took 3 shots at each setting and chose the best to display here. From these pictures, I'm pretty pleased with the results. Both L lenses appear to have slightly lower sharpness/contrast at larger aperature and the 300mm lens seems a bit sharper overall than the 70-200mm + 1.4TC combo, but the difference isn't too great. The Tamron doesn't perform as well and shows some degree of CA wide open, but I think it does great considering what it costs, the range of the zoom, and how light it is. I have since field tested the 70-200mm lens with the 1.4X extender along side the 300mm lens (with and without the extender) at the Buffalo Bills training camp. All combinations seemed to be effective. To view some of these pictures, go to my Buffalo Bills Training Camp gallery. I have more recently used all three lenses to take pictures at a motorcycle race on an overcast day. All lenses did quite nicely and I will add a gallery of these images soon.