This is actually a mixed lighting problem.
Light obeys a law of physics called the "inverse-square" law. Photographers called this "light fall-off". The same law governs radio waves, gravity, and other things that behave similarly. The law says that as the distance from the light source increases, the intensity of the light at that distance decreases. The intensity will be 1/distance^2. Each time the distance changes by √2 (approximately 1.4x) the light will be halved. If the distance changes by double then the light will be quartered.
In other words, if you have the right amount of light on a subject 5' away, then the light on a subject 7' away will be exactly half as much (because 5 x 1.4 = 7) and the light on a subject 10' away will be 1/4 as much. At 14' it would be 1/8th as much and at 20' away it will be 1/16th as much. If we use the the difference between 5' vs. 7' then it's 1/5^2 (1/25) vs. 1/7^2 (1/49) and you can see that it's almost exactly half as much light at 7' vs. 5'. BTW... the reason it's not a perfect 1/2 is because 1.4 is a round-off. The actual value is √2 (square root of two) which is 1.414213562373095... but we can safely round-off and just use 1.4 to make the math easier.
Notice how early on a difference of just 2' (e.g. the difference between 5' and 7') was enough to halve the amount of light. But farther away it takes a greater change (e.g. 14' to 20' is a difference of 6') to have the same 50% drop in the light.
This means that powerful lights positioned at a greater distance can more evenly illuminate subjects than lights positioned at close distances.
There is a technique to work around the problem. We call it "dragging the shutter".
The idea is that the lights in the room are pumping out a somewhat consistent amount of light. But the flash only fires a momentary burst of light. When you use a flash, you only need to make sure that the shutter speed is either at or slower than the camera's "flash sync" speed. This is the speed which allows the mechanical shutter in your camera enough time to completely open, THEN fire the flash, then close the shutter again.
If, however, that shutter were to stay open LONGER than necessary for flash, the camera would not continue to collect any more light from the flash... but it WOULD continue to collect more light from ambient sources. The result is that your subject is typically nicely illuminated by the flash, but the background gets "filled in" by the ambient light and the exposure will have a better balance of light rather than the over-exposed foreground and underexposed background.
In the shots below, you used "Program" mode and flash was used in 3 out of 4 of these images. Usually the camera selected a shutter speed indicated at 1/64 in the EXIF data.
INSTEAD you would set the camera to Tv mode and dial in a shutter speed of about 1/30th (you might even try 1/15th since you have an image stabilizing lens - you must learn good camera holding posture (lots of YouTube videos on this) and learn to be VERY steady). This will force the camera to leave the shutter open longer than necessary for a flash shot alone... but it allows it to collect more ambient light. The over-exposed foregrounds will not get more light because the flash is responsible for those over-exposed areas and the flash will have fired and gone dark (no more light is being dumped on the nearby subjects) but the BACKGROUNDs will get more light so they will not appear so dark.
The EF-S 17-55mm f/2.8 IS USM lens would also be helpful here. That lens (though expensive) is able to provide a constant aperture of f/2.8 at any focal length. f/2.8 collects twice as much light as f/4 and it collect four times as much light as f/5.6. In your first and last shots, the camera used f/4. The f/2.8 lens would have collected double the light, required half as much flash power, reduced the over-exposure in the foreground and allowed you to collect more light in the background.
One warning to note... if you are trying to "freeze" action like that last shot, the slower shutter speed will allow more more motion blur. The flash will tend to "freeze" the motion at the moment of the flash. You'll want to put your camera into "2nd curtain" flash mode.
Normally a flash uses "1st curtain" mode. This means the flash fires right after the shutter OPENS. In 2nd curtain mode the flash fires just BEFORE the shutter CLOSES. e.g. if you were taking a shot that was 1 full second long (just to make it easier to understand the point)... in 1st curtain mode the shutter would open, it would IMMEDIATELY flash would fire, the camera would wait the rest of the second duration, and then the shutter would close. In 2nd curtain mode the shutter would open, the camera would WAIT the better part of a second THEN fire the flash and immediately close the shutter after the flashs fired.
It might take some imagination but imaging if you were rolling a ball across the floor when doing this. In 1st curtain mode you'd get a well-illuimated ball and then a dimly illuminated blur... in front of the ball. It would create the mental illusion in the picture that the ball was rolling backward. In 2nd curtain mode you'd get a dimly illuiminated blur of the ball and THEN the brightly illuminated still position of the ball at the moment of flash. This gives the illusion of a motion-blur trailing "behind" the ball as you might mentally expect.
If you use an external flash, you'll want something that properly supports both 1st curtain and 2nd curtain modes. E.g. a Canon Speedlite 430EX II would be a nice unit.
Tim Campbell
5D III, 5D IV, 60Da
