Good info above...and Minn's right...the Sigma 28-300 is a very nice "everything" lens for the price.
Couple of comments here, and some "comparisons"
Along with the "length" of the lens (28mm, 300mm, etc) there's another number that is, imo, FAR more important...PARTICULARLY for shooting in-flight airplanes:
Aperture. (or F-Stop or F-Number) That's the (x.y/a.b) looking number after the lens. For example, the lens Minn linked, the Sigma, is an F 3.5/6.3
Here's the scoop on that
Think of it like filling a glass with water. You can turn the faucet on a tiny little bit, letting in very little water (or light). This will take a long time, but will fill nice and smooth with no splashing, if you will. Or, you can turn the faucet on full blast, which will fill it really quickly, but be something of a mess.
The aperture of a lens is, essentially, how fast can you turn on the water? A smaller number (f3.5) is a larger opening (more light) than a larger number (f6.3) What this means, essentially, is that the smaller number (or larger aperture) you have, the QUICKER the camera can get the light it needs to create the picture.
In a telephoto lens (a lens that can go from one length to another..like, 28mm - 300mm), there's usually TWO F numbers, like in the 28-300 above. The FIRST one is the max aperture at the lens' SHORTEST distance (28mm). The second is its max aperture at its MAXIMUM zoom (300 mm)
The other little tidbit of info to know is that the normal human eye more or less "sees things" at about 35-40mm. Which is to say, if you have a 40mm lens, and take a picture, it'll look about like it did when you stood there. Go shorter than that (say, 20mm), and the photo will have a wider field of view, as though you were standing farther back than you were. Go longer than that (say, 100mm), and you'll be 'zoomed in' a bit.
For reference, the first hockey picture above was at 255mm focal length, and I was...oh...30-40' away. the second hockey picture was at 240mm focal length, and I was way the **** up in the TOP of the 3rd deck.
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Having said all of that, some comparisons of some of the things mentioned here.
Minn's right...the Sigma 28-300 is a decent "do everything" lens. Its advantages are, as he said, that at 28mm, you can get REALLY close to a subject, still have room to fit in in the frame, and then rack it all the way out to 200mm and draw in objects that are a good ways away. As he said...it's handy to never have to change a lens.
On its downside, it has little hope of shooting some of the things the Canon 100-300 will shoot, because it's a fairly 'slow" lens...in 2 respects. First, f6.3 is a pretty small aperture at 300mm...meaning you'll need a fair amount of time (ESPECIALLY indoors) to get the shot. With fast moving airplanes (or hockey players *heh*), that's going to result in some motion blur in a fair number of shots. it's also, ime, a good bit slower to autofocus than the Canon is. Finally, as you can see, it's a bit more expensive as well.
Of course, on the other side, the Canon bottoms out at 100mm...which makes it tough to shoot anything closer than, say, 20 feet (it can be done, it's just tough). Figure even at its bottom end, it's going to make nearly everything look 2-3x as big as it is. So, if you want to shoot, say, a fellow AND his airplane, you're going to need to be roughly in the next zipcode to get em both in the shot, especially if it's a GS aircraft. On the upside however, it'll lock up an aircraft in flight so quick that you can literally freeze the prop...even on a cloudy day.
For example:
So...figure it's less expensive, will reward you with better shots of in-flight aircraft...but you'll probably need another lens to do any still-work.
*heh*
Welcome to our wonderful world.
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One last thing...MANY of us who are serious amateurs or professionals know a little secret....LENS RENTAL!
Most local serious photography pro shops, and plenty of places online, offer a wide range of lenses for rent for anywhere from 1-30 days or more. Usually the rates are quite reasonable...for example, I'm renting one for my Red Bull Air Race trip in May that's every bit of a $1000 lens...and the rental will run me < $100 for a week.
Renting lenses is a GREAT way to do two things. First, you can LEARN LEARN LEARN. You'll be able to see what various settings, focal lengths, apertures, types of glass, etc can do for your photographs. And then, second, it gives you a great opportunity to see if a particular lens you might be interested in will meet your needs BEFORE plunking down a few hundred on it.