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Numerators of the F Ratio

The f-ratio, short for focal ratio and also called an f-number, f-stop, or relative aperture, expresses the diameter of an aperture (opening or pupil) in relation to the focal length. The focal length is the distance from the plane of the lens or aperture to the point where the image focuses, and is the numerator of the f-ratio.

Equation
- The equation for the f-ratio is N = f / D, where N is the f-ratio, f is the focal length, and D is the diameter of the aperture. From this equation, the numerator is the focal length.
Stops
- In photography, a smaller aperture leads to less light exposure. When the denominator of the f-ratio, the aperture diameter, D, decreases by a factor of sqrt(2) (approximately equal to 1.414), the light intensity halves. Decreasing the denominator has the effect of increasing the total f-ratio.
Fractional Stops
- In the past, photographers selected stops, and hence f-ratios, by setting an aperture scale, with the aperture getting smaller as the scale progressed from f/2 to f/8. Modern aperture settings allow for intermediate f-ratios in between the standard fractional stops.
Sunny 16 Rule
- One implementation of the f-ratio is the "Sunny 16" rule, which states that on a sunny day, a photographer gets the correct exposure by using an aperture of f/16. This represents a very small aperture in relation to the focal length, since on a sunny day, a small aperture prevents the abundant light from overexposing the film.

Photography