F-Stop in Digital PhotographyThe F-stop setting on a camera is a measure of how open or closed the hole is that lets light through to the sensor. When a F number is small, like 1.4, the hole is quite large. When the F number is large, like 8, the hole is much smaller. That means less light gets through to activate the sensor.
Changing the F-stop, in addition to altering the amount of light going through the hole, also alters the depth of field. This is how much of the photo is in focus. So with a very small f-stop, only a narrow part of the image will be in focus. This can be useful if you're trying to focus a butterfly and want everything else behind the butterfly to be fuzzy. On the other hand, if you're taking a photo of a landscape, you generally want all of the landscape to be in focus. There you would use a large f-stop.
Often a camera will have a range of f-stop it can handle. For example my Canon S-100 can do F8 through F2.0. However, if I zoom out to the 5x zoom range, I can now only do F8 to F6.3. So zooming your image can change the range of f-stop available to you.
To see f-stop changes in action, take a look -
F-Stop Changes in Snow
F-Stop Changes Near Distance
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