Depth Of Field
You might be asking yourself exactly why any of this matters. What difference does it make if the lens opening is small or large? How does this affect anything? The answer to that is something called “depth of field.” Remember our mention of "different appearances" in the final image? This is one of those effects.
Depth of field is the area that remains in focus in front of and behind the distance you have set your lens to focus at. A lens only focuses on one set distance. You can see this distance on the focus ring of your lens. The aperture you set affects this depth of field:
The lower the f/stop, the larger the opening in the lens, and the LESS depth of field you have.
- The higher the f/stop, the smaller the opening in the lens, and the MORE depth of field you have.
This is very crucial to making photos look the way you want them to. By controlling your aperture through Aperture Priority mode, you control what parts of your image are in or out of focus.
It might be counter-intuitive, but you don’t necessarily want everything to always be in focus in a photo. It depends on the subject matter. You generally want the main subject matter of your photo to be in sharp focus, and you want other items that may distract from your subject to be out of focus.
For example, if you are taking a portrait photo, you want to have the background be less in focus. If the background is in focus, it distracts from the portrait. In this case you want to open up the lens as wide as possible to get narrow depth of field, and keeping only the person in focus and the center of attention.
Conversely, if you are taking a landscape photo, you often want to have much more of the image in focus, as the entire location is what you want to present as the focal point of the image. In this case, you stop down the lens to a smaller aperture to increase your depth of field, giving you more areas of the image that is in focus.
Different lens types will have different depth of fields. A wide-angle lens will usually have more depth of field than a telephoto lens, and some macro lenses have very small depth of fields, as little as a few millimeters. Depth of field is one of the reasons you would choose one lens over another in a given situation. Generally, wide-angle lenses are good for landscape photos, and telephoto lenses are better for portraits.
Between the two extremes of having very shallow depth of field and very large depth of field, there are other choices that might look better to you as well; all dependent on how you want your image to look. Each situation presents different challenges to you in creating the most compelling image.
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