Work With Depth of Field
I often receive the question from people as to whether lens aperture has any impact on depth of focus with an autofocus camera. This question really makes no sense--but points rather to a lack of knowledge about depth of field--and more importantly, the relationship of depth of field with focus and depth of focus. The accompanying photo has a large depth of field, meaning that everything from the foreground to the background is sharply in focus. In this article I will try to explain these terms and help you learn use depth of field.
Instructions
1. First, let's talk about focus and depth of focus. In a normal camera viewfinder, like the one pictured, there is at least one center spot. Some cameras might have a center spot and a square. Normally, the center spot is where you would focus on your object manually, or if you have autofocus it would use this area to determine and set the focus. If there is another circle or square, this is normally the center area of the light meter where light readings will be given weight in a center-weighted light meter. Depth of focus, however, is a measurement of how much distance exists behind the lens rear element and film or sensor. It is normally not controlled by the photographer and has nothing to do with depth of field or focus.
2. Depth of field is the degree of which items remain in focus throughout the image. This picture is a great example of why you need to pay attention to depth of field. The California poppy in the foreground is out of focus, yet the one in the center is in focus. This is not a good image because the out-of-focus picture in the foreground is a distraction. The APERTURE in the lens controls depth of field. The smaller the hole or aperture, the broader the depth of filed. A large number (f16) determines a small aperture and a large depth of field. A small number (f4) lets in more light through a larger aperture and produces a narrower depth of field. This has nothing to do with autofocus or focus, but rather how the image area you did not focus on will be reproduced.
3. This image has too broad a depth of field. Everything blends together and the mountain laurel gets lost in the sea of green behind it. A tighter view with a narrower depth of field would have been much better.
4. It may seem natural to always go for the broadest depth of field. Most of us start in photography by taking scenic postcard views and we want the whole scene sharp. However, selection of depth of field is really dependent on the lighting and the subject. In this case, a narrow depth of field blurred out everything but the poppy, making it jump out of the frame. Photographers need to think about depth of field with every picture they take; it is not an option. I have seen portraits ruined by using a narrow depth of field leaving the ears or tip of the nose out of focus. Automatic cameras balance aperture, ISO and shutter speed automatically. You cannot always depend on these automatic settings to give you the best depth of field for a subject.
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