(Last Updated On: July 30, 2019)

Aperture F Number (f-stop)

We’ve already taught that the aperture is measured in f-stops and that the smaller the number, the bigger the opening. The reason for this is because each measurement is actually the fraction of f/(number) IE f/1.4, f/2, f/2.8, f/4, f/5.6, etc. The f in Aperture F Number simply stands for the lens’ focal length. That way no matter what the size of the lens, the f-number would be proportional or universally applied across all lenses. In other words, when using the aperture and shutter speed, using one lens will generate about the same exposure as using the same setting on a different lens.

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Aperture F Number illustration

As you can see in the illustration, the opening can have the same size of f/4, while having different physical measurements.

So the point of the f-number is just to convey the illuminance of the aperture opening. It’s done as a fraction of f because the physical measurement may change from lens to lens but the light it puts out will be the same, if the f-stop setting is the same.



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  2. The “F” number of a lens is the ratio of the lens focal length to the lens diameter. Therefore, if a lens has a 50mm focal length and a diameter of 10mm, the F number for that combination is 5. If a 50mm lens had an opening of 25mm, that combination would have an F number of 2. Lenses are normally listed as their maximum F number, in other words the largest opening for the focal length.

    Zoom lenses usually show two F numbers; one for the maximum F number for the shortest focal length, and the maximum F number for the longest Focal length. Since the maximum lens opening is the same for both, you find the Maximum F for the longer focal length is smaller than for the shortest Focal length. For example, an 18mm to 70mm zoom might be labeled as F3.8 to 5.6.

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