Depth of field preview function (or DOF preview) is typically used to check Depth of Field (duh ?). What it means is that it helps the photographer evaluate which points of the scene will be in sharp focus, which will not, and if the aperture is set correctly to produce a desired creative effect as intended by the photographer.
However, during this summer, I have found 2 more practical uses for the DOF Preview function:
Actively use DOF Preview function for focusing
I have seen most of photography books and websites advertising to use DOF Preview, merely as a preview once when aperture and focus points are set. I say, take a step forward and actively use DOF Preview for adjusting both your aperture and focal point, until you are happy with result. For that to work, you will have to switch to manual focusing, set your aperture, press the DOF preview (to step down the aperture to the predefined value), and then (while DOF preview is pressed) use the focusing ring on your lens until your main focal point is in sharp focus.
We would recommend you to use that feature together with Live View, when your digital camera is already on a tripod, as in that case you can directly see the photo you are getting on your LCD display. Just set your aperture, switch camera to manual focusing, enable live view and while you are holding DOF preview, tweak the focus until you are content with the result.
Use DOF Preview function to help adjusting ND Grad filter
ND Graduated filters are a very useful piece of equipment for shooting landscapes, as they help even out exposure between the sky and the ground. That allows the camera to capture bigger dynamic range, resulting in nicer photos (where the sky is not blown out, or the ground is not too dark).
However putting ND Grad filter in place can be a tricky task, since the effects of the ND grad filter can be very subtle, and hard to see in a viewfinder (especially with the soft grads where the gradation line is not so well defined as it is in hard grads). It becomes easy to drag the gradation line too far down or up, thus killing the overall purpose of using it in the first place.
DOF Preview function helps, combined with higher aperture, as the gradation line of the filter becomes more pronounced in the viewfinder, and thus allowing easier positioning of the ND Grad filter. We also recommend to use it in conjunction with a tripod and live-view for maximum results.
Got another cool suggestion on how to use DOF Preview ? Let us know in the comments.