Let's talk lenses, the basics

Alright, now let's talk lenses.  Being able to switch out lenses is both a gift and a curse.  I'm mainly going to talk lenses for Canon bodies since that's what I know best.

The first thing that's important to know is that there are EF lenses and EF-S lenses.  EF lenses work on both full frame and crop sensor bodies, whereas EF-S only work on crop sensors.  It's something to keep in mind as you're buying your lenses.  If you're upgrading to the 5D at some point your EF-S lenses won't carry over.  My plan is to keep my T2I when I eventually upgrade, so I think it's good to have a variety.  Anyways, before going out and buying a lens, it's important to note your shooting style...it's nice to have a lens to figure out what you need.  Prime lenses do not zoom, you need to zoom with your feet.  It's nice to have a zoom or two, but I find a lot of more professional videos will move the camera and focus a lot more often than zooming in and out.  The benefits of prime lenses is they are super sharp and give you really low F numbers at a good price.  I'll talk more about that in a moment.
Your F stop is how much light is being let into the lens.  The lower the number the better.  A stop of light is  3 clicks on the F-Stop wheel.  The very most expensive lenses go down to 1.2.  The F-Stop numbers go:
1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.5, 2.8, 3.1,3.5, 4.0, 4.5, 5.0, 5.6, 6.3 etc
You can take a look at little video I shot to illustrate the change in F-number.  My ISO is at only 100 and my shutter speed is at 50.

So, you can see the light difference between an F2.0 and F2.8 is the same as between an F2.8 and and F4.0 (and also between F4.0 and 5.6).  Primes are great for low light for this reason.  Sharpness is still a factor in video mode, but due to the output image being smaller, the F number is perhaps even more important.
Your F number also affects your depth of field.  The lower your F stop the smaller amount is in focus.  This can really give you a really professional look.
Also note, cheaper lenses have a F-number that changes, for example 3.5-5.6.  This means that zoomed out all the way, the lens will give you a 3.5, but zoomed in it will give you a 5.6.  This also means that in video mode if you zoom the image will darken or brighten to compensate which is bad.  A fixed F number is always better (and often more expensive).  However, 3.5-5.6 isn't too bad, especially if you more often than not have your camera zoomed out (my 18-55 I almost always use at 18mm which gives me F3.5 to work with which isn't bad).
Another important thing to note is that most lenses do not perform as well all the way open (at their lowest F number!).  They can have serious vignetting (loss of sharpness around the edges) and a loss of overall sharpness.  This is especially true of cheaper lenses.  That can be a problem, for example...the 18-55mm at 55mm zoom only gives you an F5.6, but to get a really good image out of it you need to stop it down to F6.8 which is very high/barely usable indoors.
So, put into practice with video, what F numbers do you need to make it work?
Well, outdoors you can set any F-Number you want.  Indoors you'll want at least an F4 aperture if not a 2.8.  At night, at least a 2.8 aperture is often needed.  Even on really cloudy days my kit 3.5-5.6 is sometimes not enough.  I think a 2.8 aperture is great to shoot for since it gives you a lot of flexibility, I'll talk about some options that don't destroy your wallet in the next lens blog section.  2.0 and below is pretty nice, but 2.8 does the trick a lot of the time.  5.6 or above won't really work indoors.
Check out a video I did of the various F numbers and the affect on light on my 50mm 1.8.

I should note that you can boost the brightness with ISO, at a certain point this adds noise though.  With a 5D and it's big sensor, you might be able to get away with an F4 indoors without too much trouble, but with a T2i you'd be introducing a fair amount of noise with an F4.

So, is it worth it to get super expensive lenses...yes and no.  Can you shoot some pretty incredible, sharp, and great videos with the kit lens that came with...yes.  Will these work fine for many people...yes.  However, better lenses can boost your quality of your video overall and will also stay with you for a long time.  Read onto the next article where I talk about the best lenses to get at various price points.