Depth-of-field preview

Just like your eye, your camera has an aperture in it that can open and close to…let in more or less light.…When the aperture in your camera is more open, you get shallower depth of field.…when it's more closed, you get deeper depth of field.…This is all explained in detail in Foundations of Photography: Exposure.…Obviously, as the aperture closes, less light gets into the camera.…So, by default, the camera always leaves its aperture wide-open so that when you…look through the viewfinder, you see a nice bright image.…Even if you've dialed in a very small aperture, when you look through that…viewfinder, you are looking through a wide-open aperture to ensure that you can…see your scene clearly in the viewfinder.…

When you finally press the shutter button, the camera closes its iris down to…your chosen aperture setting.…Because the aperture in your camera is always wide-open when you are looking…through it, you're not necessarily seeing the true depth of field that you will…see in your final image.…