What Does F-Stop Mean?

In photography, an F-stop is the ratio of the focal length of the lens to the diameter of the aperture. The number is expressed as a fraction. F-stop is a dimensionless number and gives an important measure of the lens speed.


An F-stop is also known as a focal ratio or f-number.

Techopedia Explains F-Stop

F-stop is commonly notated by a hooked f (f). It is measured with the help of a scale called the f-stop scale. The larger the opening of the aperture, the smaller the f-stop, and vice-versa. A low f-stop number would be ideal for dimly lit areas, as the aperture would be opened to its largest size. F-stop is sensitive to light fall-off, meaning the closer the subject is to the camera flash, the whiter the image would be, and the farther the image from the camera flash, the darker the image would be. Significant light fall-off is shown by wide-angle lenses at the corners for large apertures.

F-stop is an important measure, as it has tremendous impact on image qualities. It gives the measure of the amount of light that is allowed through the lens of the cameras as well, and thus impacts the image sharpness. Each f-stop allows twice as much light as the previous one. The higher the f-stop, the higher the increase in the depth of the field, meaning a lower f-stop would have the objects at one distance in focus, with the remaining image out of focus. This property is used in creating aesthetically pleasing videos and pictures.

In digital photography, usage of f-stop has a big impact on the final picture. In order to boost the sharpness of the image, one or two wide open f-stops are used. Depth of the image can be increased or decreased by stopping down or opening up the f-stop of the digital camera. It is often advised to avoid smaller f-stops in digital photography, as it could increase imperfections in the picture.


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Margaret Rouse is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical, business audience. Over the past twenty years her explanations have appeared on TechTarget websites and she's been cited as an authority in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine and Discovery Magazine.Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages. If you have a suggestion for a new definition or how to improve a technical explanation, please email Margaret or contact her…