We all know that the aperture value controls the depth of field, but I am shocked how many intermediate photographers have never learned what other factors also impact depth of field (how much is sharp and how much of the photo is blurred). In fact, these other factors have as much impact on depth of field as the aperture does.
The first “other” factor that controls the depth of field is the focal length. Assuming all else is equal, the longer the lens, the shallower the depth of field. This is yet another reason why I advocate so strongly that landscape photographers buy an actual wide-angle lens rather than just the 18mm kit lens. A true wide angle lens has terrific depth of field for wide landscapes.
The second factor is the subject-to-lens distance. The closer you get to your subject, the shallower the depth of field. I always get a laugh when I see flickr comments on a macro shot that say something like, “Wow! Great creative use of depth of field!” I laugh because it is almost impossible NOT to have shallow depth of field for macro (close-up) shots because the lens is so close to the subject. Knowledgeable macro photographers usually put forth every effort possible to INCREASE the depth of field by using a high aperture value.
The last factor is the distance between the subject and the background. If you're taking a photo of a newborn baby, you might as well give up on trying to use depth of field. Newborn babies are always lying down, so nothing you do can make the blanket behind the baby's head blurry. It is so close to the focus point on the baby's face that it is not possible to make it blurry.
What do these three factors teach you? If you're disappointed with your lens not being able to get a sufficiently shallow depth of field, choose the lowest aperture, stand close to the subject, increase the distance between the subject and the background, and zoom in as far as you can. This technique also works on point and shoots when you want to blur out the background!
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