One of the things I have noticed about most SLRs is that if you have them in "auto" mode or even "professional" mode they tend to default to the largest aperture (smallest number i.e. f4-5.6). I don't know whose idea this was, but in my case that is wrong and I think in most cases it would be safer if the default was toward the smaller aperture. Here is why and here is why you may want to adjust your aperture often if you are shooting with an SLR.

The larger the aperture (f2-f5.6) the shallower the depth of field; which means that the subject is in focus but the things surrounding and behind the subject are blurry. You want shallow depth of field in many instances such as a portrait or a photo of an object where you want that person or object to stand out from everything else. Like the photo of the mean looking Emu above.
But if you are shooting a scene, which I shoot most of the time, you want as much as possible in focus. So, if all your vacation shots of the Grand Canyon are in focus in the middle of the photo and out of focus along the edges... this is why. For these shots you need a small aperture (f11-f22), not the f2-5.6 your camera is defaulting to in the "auto" or "professional" modes. There are two ways you can fix this. You can either make sure you select the appropriate "scene" mode on your camera every time you want

everything in focus or you can set your camera to the "aperture" mode and adjust the aperture manually while shooting. If you select the "scene" mode option you will need to be sure and change it back to "auto" or one of the other modes such as "portrait" when you are taking a photo of a person or a bird and so forth.
I use the "aperture" mode because it gives me quicker and more precise control. The "aperture" mode is still adjusting all the other settings for you automatically such as shutter speed to make sure you have a correct exposure. The only thing that is manual is the "aperture". It can be changed quickly, depending on your camera, with a little dial next to your thumb and you can see the setting in your eye piece so you never have to take your eye off the scene.
Either way, just remember that your SLR is going to float toward the most shallow depth of field and if you want that clear shot of the Grand Canyon then you are going to need check the aperture and probably make a change.
There is one note of caution here. This is written assuming you are taking photos in day light situations. If taking the picture in low light your camera will adjust to a slower shutter speed to let in more light. This means you may need to use a tripod to avoid camera shake. More on that in my next post.