Opanda EXIF software tells me that your sample shot was done with T3i and 18-135mm II lens at ISO 400, 1/60 shutter speed, f4 aperture with the lens zoomed to its widest 18mm and the camera doing all the exposure settings in Program mode. Also, the flash fired.
All the recommendations about focusing technique are well and good. I'd also use a single AF point, manually selected. Probably the center one. Also I'd use One Shot as the most accurate of the viewfinder-based AF focusing modes. Alternatively, Live View focus could have been used, which is slower but can be even more accurate. It also can help with manual focus, if you do that instead, because you can magnify the Live View image up to 10X on most cameras that have it.
In this case, I'd take several shots, refocusing each one. The reason is the lighting, which appears to be common fluorescent, which is notorious for "messing with" focus and metering accuracy. The reason is that fluorescent lamps cycle on and off at a high rate... 120 Hz in the US, 100 Hz in some othre parts of the world. This is so fast the human eye doesn't notice. But our cameras can and do see it... and can be fooled by it.
However I think the main problem is that your are using an f4 aperture. Lenses are typically not at their best and sharpest at or very near their largest aperture. Stop down to f5.6 or f8 and I think you'll see a lot of improvement. To do so, you'll have to get out of Program mode and use Av or aperture value exposure mode instead. This still gives you the same auto exposure and flash performance, but will allow you to select the aperture, after which the camera will automatically set the appropriate shutter speed. You may need to set a higher ISO to get adequate shutter speed.
Stopping the lens down also would give you more depth of field to work with. That can be very important because another concern is "field curvature". When your subjects are in a straight line and you focus on the one in the very middle, chances are that natural lens field curvature effects toward the edges will cause subjects there to be softer. This is more pronounced the wider angle the lens is... Ideally you would instead line up the people in a slight arc, where the ones on either end are a little closer to you. However, you'd be guessing or basing your posing on experience... because it's hard to see in the viewfinder. An easier solution is stopping down to increase depth of field, which will to some extend offset the field curvature problem.
Using a wide angle lens present other possible problems, though. One is that there is inevitable optical distortions with wide lenses, one of which causes subjects nearer the edges to be exaggerated and look larger than they are. You can see some of this occuring in the ceiling light panels (which also show "perspective" distortion). But I'd also bet that the ladies at either end of the row don't have that large an arm, as the one nearest the edge of the image appears in the photo. This is called "anamorphic" distortion and is an optical effect that's unavoidable with wide angle lenses. It's one reason most portraiture is done with standard to short telephotos, instead. The best solution is to back up farther and zoom to the 30mm or 50mm range... i.e. to avoid using wide angles for group shots like this. But to do so will require adequate working space and a stronger flash.
Which brings us to yet another consideration.... when shooting a Canon camera and flash in any of the auto-exposure modes, the camera will set exposure according to the ambient light, and then the flash will be fired as FILL.... about 1-2/3 stops underexposure automatically. If you want the flash to be the main or primary light source, you need to set the camera to M or Manual mode. So long as the flash is set to ETTL, the camera then will ignore ambient light for exposure calculations and will fire the flash as strongly as needed for illumination... In other words, you still get auto-exposure even though the camera is set to M. However, now it is being done with FULL flash.
Only problem is, if you are using the camera's builit in flash, it may not have enough power for this wide a shot at this distance. Built-in flashes are notoriously wimpy. If using an external 430EX or 580EX or similar flash, you would have ample light for the purpose, as well as better/more even wide angle coverage (using the flash's built in wide angle diffusion panel.)
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
