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In this article, we are going to talk about the F-Ratio. What’s the F-Ratio you ask? Well, the F-Ratio is that little f-number written on your lens next to the focal length. For example, on Canon EF 50mm f/1.4 the f/1.4 number is the F-Ratio. But what exactly is the F-Ratio and how is it determined?

F-Ratio in the Real World

Many of you will already know what the F-Ratio means in the real world. From an in practice standpoint, the F-Ratio is simply telling you the maximum amount of light that a lens can allow in, as well as the potential rack-focus strength or bokeh in objects that are out of focus.

For example, a lens with a maximum aperture of f/2.8 will allow double the amount of light as a lens with a maximum aperture of f/4.0 as shown below.

All Images Copyright Lin and Jirsa Photography, post produced using the Lightroom Presets.


In addition, the lens with an aperture of f/2.8 will provide for a stronger out-of-focus effect in the background objects behind the subject in focus as shown below.


How is the F-Ratio Determined

Now you are saying, “yeah yeah, I know what it does, but how is it determined?” Well, it’s quite simple actually.

Simply take the Focal Length of the lens and divide it by the Diameter of the Entrance Pupil (maximum aperture opening) and viola! You have the F-Ratio. The example below shows this calculation on a Canon EF 50mm f/1.8 II.


That’s it people. So, now you know that the F-Ratio is simply a measurement of how large the actual lens glass is in relation to the lens focal length. Hope you enjoyed!