Sorry to create a brand new thread, but I am just so excited! I just ordered my first DSLR, a Canon Rebel XSI! After stalking this board for a month, stalking reviews, looking at flickr, I finally saved enough to get it!! Does anyone know of any "teaching" websites that can explain in laments terms what aperture means, shutter speed etc?? Especially the advantages and disadvantages of the different lenses!
I think i'm going to get the nifty fifty, but if it doesn't zoom, what makes it so great?? thanks for listening!! 2 more days until my camera gets here!!!
I agree with ukcatfan, but I'll try take it a step further. Ann hit on it a little bit too, so I'll add to what she said.
You'll notice on your "kit" lens that there is a series of number 3.5:5.6 (in addition to the focal length range such as 18-50mm). These numbers are the aperture range of the lens (aka f/stop) So in this example the lens would range from f/3.5 to f/5.6 depending on where your zoom to. At the widest point of the zoom the biggest f/stop available is f/3.5. At the tightest point of the zoom the biggest f/stop is f/5.6. So the more you zoom in the smaller your f/stop becomes.
With the 50mm f/1.8 (aka the nifty-fifty) the widest aperture if f/1.8.
On an 18-55mm kit lens the widest aperture will be f/5.6. That is 3 1/3 stops slower than the f/1.8 on the 50mm prime lens. To help understand it a little more that is the same stop difference between 1/640th shutter speed and 1/60th or between ISO 160 and ISO 800. This is a HUGE difference when it comes to low light and being able to get usable shutter speeds.
Keep in mind that f/stops are fractions. So just like 1/5th is smaller than 1/2, f/5.6 is smaller than f/1.8.
Here is a comparison chart to physically show the difference in the f/stop physical size:
Additionally, more often than not, prime lenses are sharper than zoom lenses. Especially true in comparing prime lenses to "kit" lenses. The 50mm f/1.8 lens (both Canon's and Nikon's) are VERY sharp, especially between f/2.8 and f/8. Where as the "kit" lens (18-55) would be sharpest starting around f/8 to f/11 and even still, in general, isn't as sharp (when in proper focus) as the 50mm f/1.8 prime.
When it comes to low light this lens, because of its low cost and wide aperture, is highly recommended especially when you can't use a flash. You can combine a high ISO setting in camera and a wide aperture on the lens and in many cases get fast enough shutter speeds to prevent camera shake or motion blur. Keep in mind that this is a general statement and you wont get results in all situations, but at something like many of the indoor no flash rides at Disney you can get a lot more usable images. Night time parades, indoor concerts or theater type plays/shows (depending on where your particular seats are) are other places when this type of lens will get you better results than a "kit" lens. Again, it wont work it all situations, lighting is still the biggest factor, but his type of tool will help.
Depth of field is also MUCH greater at a wide aperture. With a wide aperture you can get a much shallower DOF. Take a picture of a cup on a table with a wide f/stop and everything behind it will be out of focus. (notice how many people squint to read or see something more clearly, they're making their eyes smaller, when their eyes are wider they find more things are out of focus). With this lens you have MUCH greater control over how much DOF your image can have.
Looking forward to seeing some of the pictures you take and even though a few months ago I became a Nikonian with the purchase of my first dSLR I'm still a Canonian (is that even right?? 
) at heart.
