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F number:
According to wikipedia: In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture[1]) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter.
Basically, it's a measurement of how big or small the aperture is inside the lens. Its use is to regulate the amount of light that hits the sensor or film. It can also be used to adjust the depth of field. A wider aperture (lower number) will increase the depth of field, while a more narrow aperture (high number) will decrease the depth of field. Low number = less in focus, High number = more in focus.
Noise:
Noise is the amount of grain in a photograph. Noise is caused by high ISO. Some people like a bit of noise so it's not something you should necessarily avoid.
ISO:
ISO is a measurement of the sensitivity to light. The sensor is more sensitive to light at high ISO numbers, and less sensitive to light at lower numbers. At higher ISO numbers the sensor also has more noise. A high ISO setting will allow you to use faster shutter speeds at the cost of increased noise.
In photography there is a triangle of settings that you must manage in order to capture the photo you want. ISO, Aperture, and Shutter. Understanding how each of these influence the image is the key to creating the effect you want.
A high ISO will give you more light for faster shutter speeds but also more noise. A fast shutter speed will freeze the action but limit the amount of light hitting the sensor. A low f number will give you more light but decrease how much is in focus.
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