DOF (a.k.a. Depth of Field) Preview button is one of the most under-utilised tool on a DSLR camera. Not many people know about it, not many people know where it is while the rest do not know what is the function of it.
It is located in front of your camera where you hold the camera with your left hand. As you can see, there are ‘built-in flash’ button and ‘lens release’ button. The ‘DOF Preview’ button is placed right below the ‘lens release’ button, conveniently around your thumb’s resting area.
This button allows you to see how your shot will look like in the viewfinder – particularly the Depth of Field. You will be able to see how much of your subject in the image is in focus or how big is the area of focus of the subject in the image. It is particularly useful when you are messing around with your prime lens so you could get good bokeh shot that you always intended to.
When you press the button, the viewfinder will be darker than usual. Be not afraid as this is usually the case as it shows you the actual DOF based on the aperture you set, which in turn will be in final shot. However, DOF Preview performs at its optimum rate during broad daylight, i.e. sufficient amount of light, as it clearly defines the darker and brighter spot. You will not see any difference if you use it at low light situation.
Moreover, there are other uses for this button which could be found from this website.
So, start using DOF and see the difference it makes.