Hi,
I definitely agree that Understanding Exposure
by Bryan Peterson is a good starting point. Great book that will help you better use virtually any camera. It's especially valuable for newcomers and less experienced shooters... but even an old dog like me can learn a thing or two from it. Plan to read it several times. There's too much info to pick up and use in a single reading. This book might be the best $20 you ever spend on your photography.
I'd also suggest you get one or more of the available guide books specific for the 50D. I'm most familiar with and can recommend those by Charlotte Lowrie, David Busch and Rob Sheppard, but I'm sure some of the others are good, too. Read the reviews at Amazon, to help you choose.
There are also numerous tutorials online... including some at the Canon Learning Center, at Adorama and B&H Photo as well as other websites, and others on Youtube. In particular, to better understand the Canon AF system I highly recommend viewing this video posted by B&H Photo, along with the two other videos in this series. Each is about half an hour long and the 50D in particular is discussed (as well as other models). Each video is approx. half an hour long.
I currently use a pair of 7Ds to shoot a lot of sports and action, but in the past used a trio of 50Ds for the same purpose. I use my 7Ds in essentially the same way that I did 50D (and other models). Some general suggestions...
First, I don't use any of the "preset" modes such as the "running man" sports icon. That overrides certain settings I want to make... for example I shoot mostly RAW files, since they allow me to make better adjustments to exposure and color balance after the fact. Most of the "presets" limit you to shooting JPEGs only, and often choose other things for you.
I'll use Av or Tv or M exposure modes, whichever is appropriate or even possible in a particular situation. I prefer to "lock in" my exposure with M, but that's often not possible when light conditions are changing or a subject is moving through various light conditions. In those situations, I'll use Av or Tv.... Av if I am mostly concerned about the aperture that's being used and Tv if I want to be sure a certain or minimum shutter speed is used. Both Av and Tv are auto exposure modes, but still allow me a lot of input how the camera handles the situation. I do occasionally use P, too... but mostly just if I'm suddenly switching from one type of lighting to another and don't have time to recalculate my settings (such as mostly shooting indoors, then stepping outside to take a couple quick shots).
Many Canon shooters using the auto exposure modes (also including P) and shooting RAW files will dial in a bit of + Exposure Compensation to "expose to the right" slightly (1/3 to 2/3 stop, usually). If you search ETTR (luminous-landscape.com has some interesting reading about this) you'll find more info about it. This is simply a precaution... it's safer to slightly overexpose a RAW file, than it is to accidentally underexpose it (correcting underexposure later will increase noise in images dramatically).
Most of the time I find Evaluative metering works well. I will occasionally use other modes (Center Weighted, Partial or Spot) with the in-camera meter or a separate handheld incidence/flash meter to establish or check my exposure settings. Heck, sometimes I just set it by eye. Every so often while shooting I replay images to check the histogram, a very useful tool to dial in exposure settings.
Beyond the basic exposure settings, you also have to set up the focusing system for your intended purposes. The most basic thing is choosing between One Shot and AI Servo. One Shot is used for stationary subjects and gives you that nice Focus Confirmation when focus is achieved and locked. AI Servo is for moving subjects and is continuously updating, never stops focusing as long as you keep the button pressed. Since focusing is continous and there is no lock, there is no Focus Confirmation with AI Servo. (Note: AI Focus really isn't a focus mode at all... I ignore it and don't use it.)
Next is to choose your AF point(s). I use the center point only most of the time. That makes it my responsibility to keep the point where I want the camera to focus... instead of leaving it up to the camera to guess which AF point to use. The 50D has more sensitive "dual axis" type points throughout (all 9 points), so don't hesitate to use one of the peripheral points if it works better, but the center one is further enhanced and the best of the bunch. Incidentally, USM lenses are the fastest and most accurate focusing (it's not just the camera).
To control focusing I nearly exclusively use Back Button Focusing. This is a setup on Canon cameras that's popular among sports photographers using them. It takes a little practice, but soon becomes second nature, particularly complements AI Servo shooting, and allows some additional focusing techniques not possible when focusing is still assigned to the shutter release button. Note: if you use a BG-E2/E2N on your 50D and often use the vertical controls the grip provides, there isn't an "AF On" button on the grip, so you might want to assign Back Button Focusing to the * (normally the AE Lock) button instead. This is done by swapping the function of the AF On and * buttons.
Note: if you are using zoom lenses, nearly all today are "varifocal" designs... This means the lens doesn't maintain focus when the zoom is used to change focal length ("parfocal" zooms did and were more common in the past when we focused manually). With zooms in particular, you have to refocus... or continuously focus... when changing the focal length.
Also, using the center focus point leads to centering your subject a lot. That can be fine for some things, but other subjects might look a bit better off-center. Shooting sports, you rarely don't have time to "focus and recompose". For that reason I try to frame subjects a bit loosely... This allows me to crop the image later, for more pleasing composition. However, you also don't want to over-crop your images. It's a fine line that's learned with practice in the field and post-processing one's images. Your 15MP, 14 bit 50D gives you a lot of resolution to work with, but you still will want to keep cropping to a minimum, so that there will be plenty of resolution in the final image.
There are times when it can be useful to use multiple AF points, such as when shooting a very fast moving subject against a plain background (such as a bird in flight against a blue sky). These instances are relatively uncommon, though. IMO, it's better to keep control over the point of focus, as much as possible.
I usually set a high frame rate (6.5 frames per second on 50D, if I recall correctly), but I don't just "spray and pray" with it. I'll mostly fire single shots... But at times use short burst of two or three frames... occasionally even four or more. I simply don't like sitting at my computer editing photos any more than necessary (nor do I like wearing out my camera needlessly), so I try to time individual shots. But there are times when it's very hard to time a shot and that's when a short burst might be necessary. It takes some practice to learn the "feel" of the camera's shutter release button and be able to leave it set to a high frame rate, but only take individual shots unless and until you instantly want to take a short burst.
Finally, digital cameras are great in that we can set white balance to each situation (with film cameras we had to change film and/or use color correcting filters). Much of the time I simply use AWB or Automatic White Balance. It works very well outdoors in daylight or overcast. It can be a bit bluish in shadow, overly gold in tungsten light indoors, or give a somewhat sickly color cast to fluorescent lighting. In those cases, it might be better to set a Custom White Balance or use one of the presets... However if there are mixed light sources, it's often not possible. This is another reason I shoot RAW files, since I can pretty easily adjust white balance after the fact, in post production.
There are other nuances that you'll learn in time and with practice. The above is just a summary of the major points setting up exposure, metering methods, focusing system and techniques, frame rates, and white balance.
Today's cameras have a ton of automation... some of which can be helpful and useful, but a lot of which is more of a crutch instead of learning to fully use the camera, can get in the way of making great images. Have fun learning how to get the best out of your 50D!
