Here's a helpful link to help you get a feel for "depth of field" generated by the focal length of your choice at any given aperture and how that works out in the number of feet that will be in focus when you open the shutter. This is how I learned.
http://www.dofmaster.com/dofjs.html
For example: Your 50mm f/1.4 mounted on 20D with aperture set at f/1.4* with your subjects 6 feet out = about 1/4 of a foot of sharp DOF.
Call it 4 inches.
Now you have to split that 4 inches. If your focus point is dead on you get maybe an inch and a half in front and two and a half inches behind your focus point sharp as can be. What does that translate into? The nose will be sharp. Eyes a little less so. The ears will look a bit soft.
Now another variable. Subject distance from the lens and to a lesser degree, how far off the center point they are.
What a lot of people don't appreciate is that depth of field distance starts at your lens and goes out left, right AND center.
In other words, lets say you have your 3 kids arranged in front of you side by side and you focus on the middle ones nose using the above settings. You should be OK right? everyone is within a few inches of each other as far as how far out "in front of the camera" they are. Good to go right?
Not always true, The ones on each side are actually a few degrees off center and as a result, just a
little further from the lens.
How much further? Not much. Maybe an inch or 2. It's enough to throw the ones on both sides a little out of focus if you are shooting at f/1.4 which you may be doing to get a fast shutter indoors.
Now same lens stopped down to f/8 and move them out to 10 feet and you get about 3.75 feet of sharp focus.
Starting from about 8 1/2 feet out to just about 12 feet. Put them right in the middle on the 10 foot line and everything in between is looking good.
Now swap out to a wide zoom set to 20mm with your subject at 10 feet at f/8 and it's a much different story. Now everything from 4 1/2 feet out to
infinity will be sharp.
You can actually get the same 4ft DOF on a 20mm at
f/1.4 as a 50mm set to* f/8.
Moral? As a general rule wide angle lenses = higher DOF. Long telephoto zooms = much less.
Same shot at f/8 with an 85 mm lens? Now you drop back down to 1.25 feet DOF. Just like that.
You have to stop all the way down to f/22 to even get close to 4 feet of DOF. At a range of 10 feet.
Distance from the lens now has to be considered. If you open the distance to 25 feet you can now drop all the way back to F/4.
At a 10 foot distance you are asking the laws of physics to give you a sharp image
over a much greater percentage of the frame than you are when you back out to 25 feet.
If you need a certain f stop setting to get everything you want sharp and see your shutter speed dropping too low with available light you boost ISO and/or add flash and /or put it on a tripod until you find the best images based on your environment coming out of the camera.
Or you might swap your lens out to a wider lens or push your subjects out a bit from the camera and crop the final image.
I know, I know, decisions, decisions.
Play around with this and you will quickly grasp the math and principles involved.
Hope this helps!
Darryl