Canon EOS 650D/Rebel T4i In-Depth Review
Canon's SLRs have long enjoyed a strong reputation for fast phase-detection autofocus in through-the-viewfinder shooting. And that tradition carries on in the EOS 650D. With a 9 point cross-type AF array borrowed from the EOS 60D, focus is reassuringly brisk in a wide range of lighting conditions. Focus hunting can occur but only when photographing extremely low contrast subjects. And in very dim light we found ourselves wishing for a dedicated AF illuminator, found on entry-level peers like the Nikon D3200, for example. With the 650D you must pop up the flash for focus assistance. Overall though, there is little fault to be found with the 650D's phase-detection AF.
The adoption of live view and video modes as standard camera features has, however, highlighted the poor autofocus performance of DSLRs (compared to the best mirrorless models) when using contrast-detection AF in live view. With the EOS 650D, Canon unveils its 'Hybrid CMOS AF' system that makes use of both phase-detection and contrast-detect methods to acquire focus in live view mode. According to Canon, this hybrid AF system is primarily designed as a video tool in the 650D. Yet its implementation here carries obvious implications for Canon's first mirrorless camera, the EOS M, which is built around the same sensor and AF system.
The 650D's on-imager phase-detection system means that the autofocus system is able to make use of the company's extensive understanding of phase-detection subject tracking while in live view. It also means that, if you're focusing near the center of the image (see the diagram below), the camera will always know in which direction to begin searching for focus. Systems based solely on contrast-detection can sometimes get this wrong, with the result that the camera re-focuses all the way out of focus before coming back again, resulting in a video that suddenly drops totally out of focus, before recovering.
|The camera's hybrid AF system uses a combination of contrast-detection AF and on-imager phase-detection AF. The contrast detection can be used across much of the image area (marked by the red dotted line), with phase-detection limited to the central region (bounded by the green dotted line).
For reference, the spread of the conventional phase-detection points is also marked and these are used if the 'Quick AF' option is used while in live view mode.
In live view mode the phase-detection system is used for identifying where the subject currently is, with contrast detection then kicking-in to fine-tune focus. While this does not completely eliminate 'focus wobble' as the camera confirms focus, in theory it should considerably reduce focus hunting - at least for subjects located near the center of the scene - thanks to the combination of the two AF technologies. This hybrid AF technology is available with all EF-compatible lenses. Yet it should be noted that it is only with Canon's STM lenses like the recent EF 40mm f/2.8 and EF-S 18-135mm f/3.5-5.6 (designed for more efficient contrast-detect AF performance) that you'll be able to take full advantage of the hybrid AF system.
The question of course is, 'Does it work?'. The good news is that we have seen a noticeable improvement in live view AF performance. Using an STM lens with focus initially set to infinity and then using AF to acquire focus on a foreground high-contrast subject, the 650D locks focus in almost half the time of its predecessor, the EOS 600D. And the 650D maintains an edge in AF speed (albeit a smaller one) even when using the non-STM EF-S 18-55mm f/3.5-5.6 IS II kit lens.
We're disappointed to say, however, that this improvement still lags significantly behind the AF performance of the better mirrorless cameras on the market, as you can see in the video clips below. Canon's approach is a (welcome) step forward, but the fact remains that even with this new hybrid AF system, the 650D takes far longer to acquire focus in live view mode (and thus video mode) compared to through-the-viewfinder-shooting which uses conventional phase-detection AF.
In the video clip below, we compare Canon's new Hybrid AF system with the camera maker's traditional phase detection autofocus. We simulate the AF speed of through-the-viewfinder shooting by enabling live view's 'Quick mode' AF. This uses Canon's conventional phase-detect AF, requiring that the mirror be lowered for focus acquisition. Both AF tests begin with the lens set to focus at infinity.
From this side by side comparison it is clear that the new 'hybrid' AF system is a far slower method of acquiring focus than using phase detection. What this means in practical terms is that despite the compositional benefits of using live view and the 650D's well-designed touchscreeen controls, shooting through the viewfinder remains the preferable option when trying to use autofocus to catch that 'decisive moment'.
650D Hybrid AF versus Pansonic DMC-G5
It's important to put Canon's new 'hybrid' AF performance into perspective, with the question being how it compares to that of a mirrorless camera. This becomes especially relevant given that Canon's recently announced entry into the mirrorless market, the EOS M, is based largely on 650D technology. In the video below, we've chosen to compare the 650D mounted with Canon's EF-S 18-135mm f/3.5-5.6 IS STM lens against the Panasonic Lumix DMC-G5 with its X Vario PZ 14-42mm f/3.5-5.6 ASPH OIS kit lens, whose contrast-detection AF performance ranks among the top of current mirrorless models.
In the video below, focus is initiated at the same time on both cameras by pressing the touchscreen. The lenses for both cameras are initially set to focus at infinity.
When focusing on any of the three objects in the scene, you can see just how much slower the 650D is to acquire focus. It lags far behind the contrast-detect AF system of the G5. When shooting stills, this can be mitigated by using the 650D's Quick mode AF - if you're willing to live with a reduced AF area - but does not bode so well for recording video, where of course, you do not have that option.
Autofocus accuracy and tracking
The traditional phase-detection AF on the EOS 650D is very fast and impressively accurate. Even in low light conditions, the camera successfully locks focus on all but the lowest contrast of subjects.
|The 650D's AF system works reliably and swiftly, even in very low light. Here the camera locked focus immediately in a scene that required an ISO of 12800 with an exposure of 1/60 sec. at f/2.8.|
The EOS 650D can shoot at 5fps and as we'd expect, Canon's phase detection AF is quite good at tracking focus on subjects moving at a continuous rate. In our real world usage of the camera with the new 18-135 STM lens we photographed cyclists moving towards the camera at relatively fast clip. In shooting consecutive bursts we were able to achieve acceptably sharp results in almost half the total number of exposures, even in challenging backlit lighting conditions. This type of performance is well suited for soccer moms and others who shoot recreational sports.
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