Editors' note, March 28, 2013: Canon recently discontinued the T4i and has replaced it with the nearly identical EOS Rebel T5i.
People who use Live View with autofocus for stills or video are the biggest winners with this generation of the top model in Canon's EOS Rebel series. The big updates to the T4i (aka the EOS 650D overseas) over its predecessor are the addition of a touch screen and the incorporation of a new hybrid CMOS sensor that includes both contrast autofocus sensors, the type of autofocus used in camcorders and other video AF systems, as well as the traditional phase-detection sensors you find in dSLRs. That said, the new sensor and expensive STM lens that you have to pair with it to take advantage of improved Live View autofocus really serve to bring the T4i into parity with newer technology, such as Sony's fixed-mirror SLT system and mirrorless ILC alternatives.
Overall, the photo and video quality of the T4i still rates as excellent, though at high ISO sensitivities it's slightly surpassed by the T3i; it looks like either the blue channel in the new sensor just isn't as sensitive as the previous one or Canon's processing the heck out of it. JPEGs are fine up through ISO 400 if you're not a pixel peeper; if you are, you'll find even slightly out-of-focus areas start to show some artifacts as low as ISO 200. At high ISO sensitivities the noise reduction is relatively intelligent, with suppression artifacts only in the dark areas and on some high-contrast edges. I suggest you don't use the default Auto Picture Style when shooting JPEG in low light, because it boosts the contrast and you really lose a lot of shadow detail. If you shoot raw, you've got latitude up to about ISO 1600, but you'll end up applying a lot of luminance NR to get rid of the hot pixels.
The T4i is capable of reasonably accurate color -- just not in its default Auto Picture Style setting. Auto pushes the contrast and saturation so that you lose detail in shadow areas and normally bright, saturated colors clip highlight detail. That's why Canon offers four different non-special-effect color settings: Auto, Standard, Neutral, and Faithful. My normal Canon setting is Neutral with sharpening bumped up a couple notches, but at the very least dial it back to Standard, which is still better than Auto.
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The camera's tonal range is fine for a consumer model, but there isn't a lot of latitude in shadows and highlights, despite the fact that the camera offers a +/- 5-stop bracketing range.
Video quality is very good for a consumer dSLR but not outstanding for its price class (if you count the price with the 18-135mm lens), which includes models like the SLT-A65 and Panasonic Lumix DMC-GH2. It's pretty comparable to the A65, though I have to say I like the A65's tonality in bright light a little better (I actually reviewed the SLT-A77V, which uses the same sensor and processing technology). In dim/dark conditions, the T4i does very well, though. There's some color noise in blacks and on thin edges, but it delivers a nice range in the details. I didn't see any rolling shutter but did spot some moiré and aliasing on diagonal edges.
Note: We recently updated our testing methodology to provide slightly more real-world performance, so the results aren't necessarily comparable with previous testing. Until we're finished refining our procedures, we will not be posting comparative performance charts.
Through the viewfinder -- i.e., using phase-detection AF -- the shot lag is roughly the same with both the traditional 18-55mm kit lens and the 18-135mm lens. In bright conditions, time to focus, expose and shoot was a solid 0.3 second for both lenses, though the 18-135mm rounds up to 0.3 second and the 18-55mm rounds down to that speed. In dim conditions, they're both 0.7 second. In contrast AF mode (i.e., Live View), the STM clearly surpasses the 18-55mm: 0.8 second in bright light and 0.9 in dim versus roughly 2.2 seconds for the 18-55mm. (This is the only test I ran for the 18-55mm lens)
Sequential shooting is similarly fast: 0.3 second for two shots of JPEG or raw, rising to 0.8 second with flash enabled. Burst performance results were interesting, in a disappoining way. For a nonmoving subject but with center-point AF active, the T4i clocks about 5.4fps on average for JPEGs and 5fps for raw (once you've exceeded the 6-frame buffer limitation for raw). However, with moving subjects -- in practice, that is -- it's far slower. In some cases it seemed to drop to roughly 2fps based on the file time stamps. It may measure a bit faster, but it's definitely not 5fps, even with the 95MB/sec SanDisk Extreme Pro card we use for testing. Accuracy, on the other hand, is very good, with roughly 75 percent of shots locking well enough for typical nonprofessional uses for panning, approaching, and random movements.
The AF system seems to work fairly well with wide apertures on fast lenses, including the 50mm f1.2. But despite updates to its automatic operation, it's still making unintelligent AF choices, such as always opting to focus on the closest element in the scene.