What’s a Lens F-Ratio and How is it Determined?

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.

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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.

F-Stops & T-Stops? Which One Matters To You & Why

While an F Number may suggest how much light may pass through the lens, it’s not entirely an accurate measure of how much really gets all the way through to the sensor due to light absorbance and reflection etc..

So while, for example, a lens’ aperture may be open to 1.4, the actual measure of light hitting the sensor may, in fact, be equivalent to 1.7. That actual number, 1.7, is the T-Stop, and some lenses, typically cinema lenses, will be rated as such.

T -Stops (or Transmission Stops)

A T-stop is the measure of light that actually arrives at the sensor. Why is there a difference? Light doesn’t cleanly arrive at the sensor as some of it is reflected, and absorbed by part of the lens etc, and the T-stop accounts for this. So a T-Stop is, in effect, a reflection of the real speed of the lens.

It’s important to understand however, that if you have a lens set to f/1.4 and it has a T value of 1.6, the T value has no bearing on depth of field. The fact that the F-stop is based on a physical measurement means it is constant.