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Your DOF (Depth of Field) is what dictates what is in focus.
Sometimes you see photos where the main subject is in focus but the background is blurry.These means that there is a real shallow depth of field. Depth of field is a rather complicated concept that can be learned in detail on Wikipedia.
(Photo by James Batt Photography)
Essentially, truly precise focus can only be possible at one distance at a time. At any other distance you will get a blurred spot shaped like the aperture of the camera (circular).
We shall call this the ‘circle of confusion’ Factors that influence the depth of field include the focal length of the camera, the f-number, the format size, and the circle of confusion. Talk about confusion! Lets just take a step back and realize the general concepts here….
For our purposes, just know that the aperture controls the depth of field. Reducing the aperture diameter (increasing the f-number) increases the DOF (depth of field). Decreasing the aperture size reduces the circles of confusion which cause the ‘confusing’ blurriness. A smaller aperture means less blur for objects not directly in the plane of focus. Thus, if you have a larger aperture, you will have larger circles of confusion and more blurriness for items not in the plane of focus.
DOF is also determined by magnification at the film / sensor plate and the f-number. By increasing the magnification while keeping a constant f-number, you will decrease the DOF and cause more blurriness. This magnification can be accomplished either by physically moving the camera closer to the subject, or by using a lens of greater focal length.
Smaller Aperture ==> Less blur BECAUSE Larger f-number ==> Larger DOF).
Larger Aperture ==> More blur BECAUSE Smaller f-number ==> Smaller DOF).
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