F-Stop is a measure of the size of aperture (opening) in a lens. Some photographers refer to F-Stop as the F-Number or simply as the aperture setting on the camera. It all amounts to the same concept. As a professional you should know that the origin of the letter F in the term F-Stop is frequently attributed to Ansel Adams, who dedicated much time, one-third of the way into the 20th century, to developing optimum aperture settings and therefore is widely recognized for his contribution to photography and developing the acknowledged Aperture Zone System.
History aside, you're possibly wondering what the F-Stop numbers represent. I'll tell you that they represent a geometric progression starting at 1 where each subsequent F-Stop number greater than 1 is multiplied by a factor of approximately 1.414, (which is the power of the square root of two). Grab a calculator, enter 1, multiply it by 1.414 and you'll obviously get 1.414 as an answer. Now multiply your answer by 1.414 again and you'll get 2 as your answer. Do it one more time and your answer will be approximately 2.8. If you continued to do the math, you would swiftly determine the entire F-Stop range of values, I.E. 1, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 and so on until F-Stop 45 or even F-Stop 64.
Great, now that you know what the word F-Stop means and what the numbers associated with it are, you're probably still asking yourself what these numbers actually mean! To explain I will use an analogy of a cat's eyes. (If you have a cat, you've probably noticed this several times). When your cat is outside and it's a bright and sunny day, you'll observe its pupils are long and thin and resemble tight vertical slits, but as your kitty walks into a darker indoors, its pupils widen up to become large round discs very quickly. Now this happens so that your cat can see equally well in the different environments where the intensities of light may vary.
If we relate the above analogy to the school of photography, please imagine that the cat's pupils are akin to that of the shutter in your camera -- being able to open each time you take a photo to a lesser or to a greater extent. Hence to allow plenty light to reach the image sensor in your camera you would select a set an F-Stop setting (pupil size) which would not restrict the amount of light entering your camera. On the other hand if you wanted to limit the quantity of light entering your camera, a high F-Stop setting would be the setting to choose.
Each time you move between F-Stops, you either halve or double the light intensity. To give an example, if you selected an F-Stop of 4.0, and then adjusted it to 5.6, you would then have halved the amount of light entering your camera. Should you have set your F-Stop to 2.8, you would've doubled the light intensity. It's important to remember that a higher F-Stop number means a smaller aperture or opening allowing light into your camera. Conversely a lower F-Stop number means more light entering your camera. To compensate for a one stop lower F-Stop number you would have to double your shutter speed to attain the same exposure. Should you increase your F-Stop by one stop, you would need to halve your shutter speed to keep the playing fields equal.
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