I have had this question asked to me many times and it has been on my list of things to talk about for quite some time. There are a great deal of Digital SLR (DSLR) photographers out there who have probably never used it, or even havn’t even noticed it!
Basically, the depth of field (DOF) preview button is for visually checking how much depth of field you have in the photo (I have a depth of field tutorial here if you are unsure). If you in aperture priority mode, because you want to use a certain aperture – an example might be that you are taking a photo of something and want to try and get a certain part in focus and another out of focus (OOF) – so most people would snap off a couple of shots like this and look at the results of the photo on the display screen to see whether the shot worked out. There is a much easier way to do this, by pressing the depth of field preview button and looking through the viewfinder you can see exactly what will be in focus and what will be blurry. It will look a little darker but hold on, we’ll get to that!
Why when I press the DOF preview button and I look through viewfinder does everything go dark?
When the DOF preview button is not pressed the aperture will stay as wide as the lens will allow, so if you click the button nothing will happen. As soon as you stop down (make the aperture smaller) the ‘eye’ gets smaller to restrict the light entering, so if you set your camera to Av (Aperture priority) mode and select f/11 and NOW press the DOF preview button, you can see that it darkens and you can actually hear the noise of the aperture getting smaller. If you have another look in the viewfinder (although a little dark) you can see EXACTLY the DOF that will be in the final picture!
So in summary:
The DOF preview button is simply to view the ACTUAL depth of field that will be in the final photo – BEFORE you take it -based on the size of the aperture that you have set. When you hold the button, the camera physically adjusts the apeture into exactly the right size as you asked it.
Go and test it, set your camera to Av (Aperture Priority mode), choose a F stop such as f/11 and walk around your house looking through the viewfinder and pressing the button!