Shutter speed
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In photography, shutter speed or exposure time is the length of time that the film or digital sensor inside the camera is exposed to light when taking a photograph.
The amount of light that reaches the film or image sensor is proportional to the exposure time. 1⁄500 of a second will let half as much light in as 1⁄250.
A spoon falling in water, taken at 1⁄2000 s
The shutter speed dial of a Nikkormat EL
Slow shutter speed combined with panning the camera can achieve a motion blur for moving objects.
An extended exposure can also allow photographers to catch brief flashes of light, as seen here. Exposure time 15 seconds.
F-number
Videos
An f-number is a measure of the light-gathering ability of an optical system such as a camera lens. It is calculated by dividing the system's focal length by the diameter of the entrance pupil. The f-number is also known as the focal ratio, f-ratio, or f-stop, and it is key in determining the depth of field, diffraction, and exposure of a photograph. The f-number is dimensionless and is usually expressed using a lower-case hooked f with the format f/N, where N is the f-number.
A 35 mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring. This lens has an aperture range of f/2 to f/22.
Comparison of f/32 (top-left half) and f/5 (bottom-right half)
Shallow focus with a wide open lens