Explanation of Canon lenses

[PKW]

Hi,

I recently purchased the Canon EOS 550D with the kit lens as my first DSLR and am now looking at a zoom lens. I have seen some lens with IS and some with USM. So what is the different between Image Stabilizer and Ultrasonic lens? And what is better?

PKW

[NRoberts]

It's not an either/or situation - many lenses have both.
IS - an Image Stabiliser - helps reduce the effect of camera shake to give you less blurry pictures. It can't reduce motion blur or blur caused by the subject being out of focus, but given the choice, it's very useful to have.
USM stands for UltraSonic Motor - it's the lens focus motor that works, well, ultrasonically. The effect is that it's pretty much silent, and very quick in operation. Nice to have, but not critical unless focus speed is paramount for you i.e. you take mostly action shots.