What are apertures, f-stops, How does depth of field work, and how does lens measurement factor into the equation?

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What are apertures, f-stops, How does depth of field work, and how does lens measurement factor into the equation?

In: Technology

An ideal lens focuses light from a single plane (called the focal plane) onto its sensor. However, that’s not super useful, as we often want to take pictures of things that are thick. As it turns out, there is a region around the focal plane where the image is still well focused. This is called the “field” of the photo, and the “depth of field” (DOF) measures the thickness of this region from the point nearest the camera that is well focused to the farthest point that is well focused.

As it turns out, actual lenses are not ideal lenses. This matters when it comes to DOF. At small apertures, much less light enters the lens, and it all enters through the middle part of the lens. The result is a larger DOF. In fact, you can make pictures with no lens at all using a pinhole camera. The aperture is so small that the DOF is essentially infinite. Since the amount of light that comes through is similarly small, you need a very bright scene.

Since aperture effects both amount of light and DOF, it’s not exactly a DOF control. As less light comes through, more integration time (or exposure time if you’re still thinking of a film camera) is required to get an image.

f-number (or f-stop) is a ratio of aperture to focal length. This is a camera-specific idea, but the exposure time for similar f-stops is similar. This was a more interesting parameter when light meters were separate from cameras. Almost all modern cameras use through-the-lens metering and automatic (or at least semi-automatic programs) to select appropriate f-stops and exposure times.