The 3x crop function on the 600D is a digital zoom, but unlike other digital zooms this function works not by cropping the current image, but by cropping the signal coming from the sensor. This leads to much better image quality, with hardly any loss of sharpness and definition. In fact, the effects of aliasing and moiré are greatly reduced in crop mode compared to the normal mode. Below I'll outline the main benefits of the 3x crop mode on the 600D.
Increase the zoom range of any lens
The most obvious benefit would be the fact that this crop mode allows you to increase the zoom range of any lens by three times, with no light loss. For example, with a normal 17-55mm lens, you suddenly have a range of 17-55 in normal mode, and 51mm (17 x 3) to 165mm (55 x 3) and everything in between! Likewise, a 70-200 lens suddenly has a range from 70 to 600mm! Zooming in without loss of aperture
Another important benefit is that in crop mode, the maximum aperture is maintained. A 135mm f1.8 lens becomes a 405mm f1.8 lens! There's not even a lens on the market that combines this tele-reach with such a large aperture. This allows you to do close-ups in dimly lit situations, and shallow depth of field tele-shots without buying any expensive glass. Macro shots with any lens
Not just faraway subject can be magnified: the3x crop function allows you to do macro shots without needing any specialised macro lenses or macro converters. Each lens will focus three times closer! Take a look at this example:
Geting rid of aliasing and moiré
One of the biggest issues with HDSLRs is the fact that fine patterns cause aliasing and moiré problems (flickering or discolouring of the image). Turns out the 3x crop function can even cure these! Take a look at the example below: in the first (wide-angle) shot you see the camera can't quite cope with the pattern of the fabric on the amplifier. The second shot shows the same amplifier shot using the 3x crop mode. See how much of a difference?
Conclusion
The 3x crop mode on the Canon 600D is a very versatile tool, that allows you to go from wide-angle to tele, from portrait to macro without changing lenses. For reporters and documentary filmmakers this means saving time and concentrating on getting the shots you need, while keeping the gear lightweight and portable. This feature makes the 600D stand out in the Canon line-up of camera's and makes me prefer the 600D above all other models.
Tip: shortcut for the 3x crop mode If you want to switch between normal mode and 3x crop quickly and you have Magic Lantern installed, press the 'disp.'-button and the + or minus button simultaneously.