The three lenses that I believe every photographer should have are a wide-angle zoom, a telephoto zoom and a 50mm prime.
One telephoto option is a 70-200mm telephoto zoom. If you follow professional photographers, you will often hear them say that this zoom range is a necessity for portraiture. And of course, if you are shooting sports, like basketball, then the long reach will be a great help. (If you shoot other sports, say football, then this telephoto range will not be enough to reach the other end of the court...)
There are four varieties: the f/2.8L and f/2.8L IS and the f/4L and f/4L IS. The f/2.8 lenses will give you a very creamy bokeh and are one stop faster. However, some, actually a lot, may find the price for either lens really steep. Yes, the f/2.8L is almost the same price as the f/4L IS, but the image stabilization brings a lot of benefits. If you constantly shoot with speedlites, then maybe you can do away with IS, but for me, IS is something that I simply must have.
Aside from the price and IS, another advantage of the f/4L IS is it's simply lighter. The f/2.8, either of them, are really good but they are quite heavy. The f/4L IS is no lightweight but it is only half the weight of the f/2.8. The f/4L IS weighs 1.7 pounds.
The Canon 70-200 f/4L IS USM is a joy to use. When shooting events, I prefer to be at least some 10 feet away from what is happening; I don't want to do "in-your-face" photography so this telephoto lens is really what I need. (As opposed to people who use their iPads or tablets to photograph something really up close thereby getting in the way of other people.)
The only thing missing? I wish that this lens came with a tripod collar. The lens is not that heavy but, for such a long lens, it's almost 7 inches long, it would be more stable on a tripod if it's mounted via the lens collar. And at such a price, you shouldn't be required to spend a couple hundred dollars more for a collar...
So, if you wish to get a telephoto zoom or if you want to upgrade your non-L telephoto lens, then I suggest you get the f/4L IS USM. It is Highly Recommended!
Get yours here!
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