I used the flash taking some pictures outside during the day. I wanted to get rid of the shadows on the people. Unfortunately, I ended up overexposing everything.
How do you use the fill flash outside during the day? Or, should you not use the flash?
I just got a new lens. The tamron 17-50mm 2.8 for my T2i.
I've attached the picture that was overexposed. I had it set to Av and the exposure level is set right in the middle. It was set to f2.8.
I assume it is because the shutter speed was too fast for the flash? Is the built-in flash on the T2i fast enough for a photo shot at 2.8 in sunlight?
You ended up creating a "high key" image (which actually is a kind of artistic effect on it's own as long as you can see the critical details).
Your shot is over-exposed, but this had nothing to do with flash.
When you shoot outside with the shot lit by the sun, your exposure is predictable because the sun puts out a very consistent level of light. There's a baseline rule with a catchy-name to make it easy to remember called the "Sunny 16" rule.
The rule says this: If you are shooting in full sun, you can set the aperture to f/16 and set the shutter speed to the "inverse" of the ISO speed. So at ISO 100, the shutter speed would be 1/100th and this will create a correct exposure. It turns out you can also use any "equivalent" exposure. E.g. you can open the shutter up by a stop (so that'd be f/11) which collects twice as much light, but then set a shorter shutter exposure by a stop (1/200th) to compensate. You still get the same amount of light.
In YOUR photo, you used ISO 100 and f/2.8. At f/2.8 you have increased the amount of light gathered by the lens by 5 full stops. That means to balance the exposure, you'd need to change the shutter speed ALSO by five full stops (which would be 1/3200th). If indeed this was taken at 1/200th, then your shot is over-exposed by 4 full stops (making it 16 times brighter than it should be.)
At that point, the flash is basically irrelevant.
When using a flash, you have to remember that there's a shutter speed limit called the flash "sync" speed.
When a flash kicks out it's burst of light, the duration of that light is VERY short. It may only be a few thousandths of a second. But since the camera shutter is mechanical, the shutter can't actually move that fast. It takes time to fully open the shutter, followed by more time to fully close the shutter. On YOUR camera, it takes about 1/400th of a second for the shutter to fully OPEN, followed by another 1/400th of a second for the shutter to fully CLOSE. That's 2/400ths or 1/200th of a second for the entire process. If the flash fires when the shutter is partially open, the only the part of the sensor which is exposed will get the benefit of the flash. THAT is why there's a max speed for using flash (on your camera that speed is 1/200th.)
If you're now wondering how it's possible for a camera to take a photo at a fast shutter speed (say 1/2000th) when the shutter has a max speed of 1/200th, it's because there are actually TWO shutters on the camera (they refer to these as "curtains") and they "chase" each other. One begins opening, exposing just a slit and then the second curtain begins following it... exposing only a small gap which sweeps across the face of the sensor.
The flash can be put in "high speed sync" mode IF the flash supports this feature (not all flashes do.) In this mode, the flash pulses rapidly while the curtain shutter moves across exposing that slit. Each "slit" gets an equal pulse of flash ... it's all done by computer to be precise and the image looks great. BUT... there is a catch. Since the flash has to pulse many times and normally a flash needs time to recycle (re-charge the capacitors to get ready to fire again), each pulse has to be at a small fraction of the total power that the flash could normally provide if it were just flashing once. This really cuts down on how much power you get and how far away your flash can effectively work.
The answer to the question you really want to ask is: How do you take a photo during the day using a flash as a "fill" to reduce harsh shadows?
Easy:
1) Set the camera to exposure settings which will ensure the shutter isn't FASTER than that 1/200th speed (the max "flash sync" speed). E.g. use Tv and set it to 1/200th.
2) Set the "Flash Exposure Compensation" setting (this has an icon of a lightning bolt with a +/- next to it) to "-1". This tells the flash to deliberately fire at a power level which is 1 full stop weaker than it would normally use (each "full" stop is 1/2 the light if going weaker or double the light if going stronger.) I find that "-1" creates a fairly naturally look shot that wont "look" like flash, but will adequately weaken shadows. You do want some shadows -- no shadows means no 3-dimensional look to your shots and they end up looking "flat". You just want the shadows to be gentle... not harsh.
You can tweak the flash exposure compensation setting to taste... I usually work in the -2/3rds to -1 1/3rd ... and I start at -1.
You wanted a wider aperture for narrower depth of field. This gets just slightly trickier because if open the aperture you have to speed up the shutter and if you go faster than 1/200th of the shutter you have to enable "high speed sync" mode (and you also must have a flash which supports that mode). Then the flash has to operate at a fraction of full power so the subject needs to be close. You can cluster a gang of flashes all operating at high-speed sync mode to overcome that obstacle but you can see how complex this is getting.
OR
You can use a neutral density filter.
Your shot was over-exposed by 4 stops. If you used a circular polarizer, that will cut about 2 stops of light (technically these are variable but usually pretty close to 2 stops). You could then use either a 2-stop ND filter or a 3-stop ND filter (I prefer the 3 stop). These filters screw onto the threads at the front of your lens (which means you have to buy the filters with the correct diameter for your lens.) That'd give you a total of about 5 stops of light. NOW you can shoot at f/2.8, NOT have to deal with high-speed sync, set a shutter speed of 1/100th, and set the flash compensation to -1 and you should get a result you really like.