Unfortunately that’s pretty normal. Only certain expensive L series lenses are sealed to prevent air from entering. All others have a lot of cracks and openings where air - and dust - can easily enter. Zoom lenses which extend in length when you alter the focal length are particularly vulnerable to this problem, since air gets sucked in every time you move the lens barrel.

Luckily a little bit of dust inside a lens isn’t going to make much difference, so don’t worry about it. It may be alarming to see the dust specks when you hold the lens up to a bright light, but it’ll cost an awful lot of money to have a camera repairperson dismantle the lens and clean each internal element. And there’s no guarantee that the elements will be properly aligned when he or she gives it back to you. So unless the lens is coated with a dusty grey film of dust you shouldn’t have any problems.

There’s a scratch on the front glass of my lens!

A tiny scratch or chip on the front glass of a lens, alarming as it may look, won’t actually make much difference in image quality under most circumstances since it’s far enough from the film or image sensor plane not to be in focus. It can, however, affect lens flare, so it’s usually worth filling in the chip with black pen. Having said that, a large chip (more than a few millimetres long) is obviously undesirable. And chips on the rear glass of a lens are more of a problem.

I’ve seen ads for teleconverters or extenders. Can I put one of these onto my, say, 50mm lens and magically turn it into a 100mm lens?

Yes and no. The answer to this is complicated, but leans mainly towards the “no” side.

Teleconverters, called “extenders” by Canon, are optical accessories which fit between the camera body and the lens. They’re essentially tubes with a few glass lens elements inside which multiply the focal length of the lens in use - typically by 1.4x or 2x. So a 50mm lens with a 1.4x teleconverter (TC) would take the same photographs as a 70mm lens, and a 100mm lens with a 2x TC. Think of TCs as magnifying glasses - they enlarge the central portion of the image and cut off the periphery.

Unfortunately you can’t get something for nothing. And in the case of TCs there are three major tradeoffs.

First, using a TC cuts the amount of light entering the camera. A 1.4x TC costs you a stop of light and a 2x TC costs you 2 stops. This is particularly problematic if you have a slow lens. Since most Canon cameras (pro cameras notwithstanding) can’t autofocus with lenses slower than f/5.6 you may lose autofocus, or at least reliable autofocus, if you use a TC. You can sometimes get around this by taping over the teleconverter’s extra pins, thereby fooling the camera into thinking there isn’t a TC on it at all, but obviously that’ll only really work if there’s enough light coming into the camera for the autofocus mechanism to function. Manual focussing will also be difficult if the view through the viewfinder is dark, as it will be with slower lenses.

Second, there’s the question of compatibility. Canon manufacture two teleconverters - Extender EF 1.4x and Extender EF 2x - but they are specifically designed to work only with the handful of expensive telephoto lenses listed below. These extenders have protruding front elements and so physically can’t attach to most EF lenses - the protruding element simply gets in the way. You could work around it by sticking an extension tube between the TC and the lens, but this would cut even more light and would also mean you lose infinity focus.

You can avoid this problem by eschewing Canon and going third-party for your TCs. Tamron and Kenko sell their own TCs which don’t have these protruding elements and so can physically mate with any EOS lens (though TCs in general don’t work very well optically with lenses which aren’t telephotos). These third party TCs come in varying levels of optical quality. The better quality (more expensive versions, such as the Kenko Teleplus Pro 300 DG) models are generally held as having decent optics, though not quite as good as the Canon products.

Third, all TCs degrade image quality somewhat. First, you’re adding a bunch more glass between you and the scene you’re photographing and second, you’re using only part of the centre of your lens. 2x TCs enlarge more of the middle of the lens than the 1.4x TCs, which makes 2x converters worse optically. Now, in the case of a fancy Canon L series lens and a Canon Extender, this optical degradation will be fairly minimal. However, if you take your typical cheap consumer zoom lens and slap a third-party TC on it you’ll find that the results will be less than stellar. In fact, in such cases you’re probably better off simply cropping and enlarging part of a photo taken without a TC and leaving it at that. The quality would be higher and you’d save money.

So. The answer to this question really depends. If you have a professional lens and a quality TC then, yes, you’ll be able to increase your focal length at the cost of some light. But if you have your typical consumer lens then there’s probably no point buying a TC - you’ll end up with fairly crummy photos.