Highlight Tone Priority
The 600D has Canon's Highlight Tone Priority (HTP) mode, a method for getting more highlight detail into your images. This works by amplifying the sensor's output by one stop less than usual, meaning highlight detail is less likely to be discarded. The result is that HTP ISO 200 is, at the Raw level, the same as conventional ISO 100 underexposed by a stop. A modified tone curve then ensures you still get the correct image brightness, and any properly compatible Raw converter should reflect this and be able to render the additional highlight detail.
In line with the proverb, you're not actually getting a free lunch, of course. Using a lower chunk of the sensor's response leaves more space to capture highlight information, but means shadow detail is recorded using the noisier bottom end of that output. Thus, when these tones are pulled up to the correct brightness, they tend to bring a bit more noise with them.
|HTP Off - ISO 200|
|HTP On - ISO 200|
As you can see, in good light (where you're most likely to want the extra highlight capacity), Highlight Tone Priority can be rather useful. The additional shadow noise isn't particularly problematic in such situations, and is more than outweighed by the benefits of the additional information in the highlights. HTP is certainly a useful feature to have access to, which makes it all the more frustrating that it's buried in the Custom Functions sub-menu. We'd strongly recommend adding it as an option to the 'My Menu' list, and using it on bright, sunny days.
Auto Lighting Optimizer
Canon's other mode for coping with high-contrast images it the much more readily accessible 'Auto Lighting Optimizer.' This attempts to make context-specific tweaks to the image in shadow regions, and balance out the contrast. As with previous Canon cameras, we've found it rather difficult to provoke much of a response from the ALO system.
If you were feeling charitable you might see this as being a sign that its effect is subtle and naturalistic. However, in repeatedly comparing the effects of its different settings, we'd tend to see it as being unhelpfully cautious.
|ALO Disable - ISO 200|
|ALO Strong - ISO 200|
Highlight clipping / dynamic range
Even without HTP the Canon's highlight dynamic range is not at all bad. As we're used to seeing, there is scope for recovering some highlight detail that isn't being pulled into the JPEGs.
As always, the amount of detail recoverable depends on a number of factors, including which of the color channels has clipped first. At best you might get as much of a stop of detail back, but the further you have to reach, the more likely it is that you'll experience false color shifts.
|RAW - Metered Exposure, 0EV correction|
|RAW - + 1.0EV Exposure, -1.0EV correction|
|RAW - +1.33EV Exposure, -1.35EV correction|
Here we've taken a region of an image that's very close to being overexposed. We've then intentionally increased the exposure by 1EV and tried to correct for that over-exposure (middle image). As you can see, most of the detail is recovered, despite being over-exposed by 1EV. However, if we over-expose by another 1/3EV (bottom image), it's no longer possible to recover all the detail. Worse still, grey patches are starting to appear in the recovered regions. So, at best, you might recover 1EV of detail from over-exposed regions of an image, if you process from Raw.
Overall Image Quality / Specifics
In general the 600D's images are very good (which was also true of the 7D, 60D and 550D, all of which used the same sensor). Color response is familiar from existing cameras and is more geared towards pleasing than realistic. Dynamic range isn't at all bad and becomes rather good once you've engaged Highlight Tone Priority. Detail capture is excellent though you'll want to consider processing from Raw if you want to show the very finest detail in very large prints.
Raw processing can also be useful for making the most of low-light images - the ability to fine-adjust the noise reduction allows you to choose a balance between noise suppression and detail retention that suits your subject and output requirements. The preset options for JPEG output do a pretty good job, though.
Canon's characteristic overly orange response to artificial light also rears its head as usual, but can be dialed-down with a bit of white balance fine tuning. Beyond that there are few nasty surprises waiting for users.