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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're 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're looking through it, you're not necessarily seeing the true depth of field that you will see in your final image. If you have dialed in a very deep depth of field, you won't see how deep the depth of field is simply by looking through the viewfinder. To help you pre-visualize your depth of field, your camera includes a Depth of Field Preview button. When you press it, the iris is closed down so that you can see the actual depth of field that will occur in your image.
The Depth of Field Preview button is this one right here. It's kind of recessed against the body, so you've really got to be sure you give it a good hard shove when you are ready to push it. When the iris closes down when you press the Depth of Field Preview button, your viewfinder will possibly get very dark because there is not as much light coming into the camera. This is why the iris was opened in the first place. This can also make it more difficult to actually see the depth of field in your image. But if you wait a moment and give your eyes time to adjust to the darker view and if you can find a way to maybe cup your hand around the viewfinder and your eye, then your eye should adjust and you should be able to get a clearer view of your scene with truer depth of field. One more thing.
The image in your viewfinder is much smaller than the image that you'll most likely view on your monitor or on a print. So it's going to be harder for you to tell fine sharpness in your viewfinder. Depth of Field Preview doesn't give you a perfect way to gauge very fine depth of field effects, but it should let you see if certain large things in your scene are in focus or not.
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