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

March 2013 8:00 AM 21 Comments

We got a lot of great things in store for all of you SLR Loungers. One of our upcoming workshops on DVD is a full course on photography. From the basics of Photography 101 on through to professional techniques in Photography 301 and 401. As we push forward in writing the script and filming, we will give you all little glimpses into the content. Be sure to check out our Store and catch up-to-date photography news, gear reviews, and more on our Facebook Page and our Youtube Page.

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!


Founding Partner of Lin and Jirsa Photography, LJP Studios and SLR Lounge. Follow my updates on Facebook and my latest work on Instagram both under username pyejirsa. Also, join me on the SLR Lounge Community Page where I am frequently posting tips, tricks, tutorials and behind the scenes details.


  1. Noaman Khan

    Pye, When is the DVD coming out?

    • Pye

      We just finished the Couples Photography DVD, that will be coming out soon. We have barely started writing the scripts for the Photography 101, 201, 301 series. We do a lot of research to figure out the best method of presentation, so it will probably be a few months before we finish the series. 

    • Noaman Khan

      Can’t wait for it to be out. I recently bought Lightroom 4 Workshop collection and I can’t thank you enough.

    • Pye

      Thanks Noaman appreciate the support buddy =) 

  2. Jeff Bullman

    Yay, according to this article my 70-200 is really an F/.9 when I zoom out. Who knew.

  3. Jeffrey Howarth

    So my 135mm F1.8 is 75mm (77mm thread)?

    • Pye

      Try again, we made a typo in the description, should have been Diameter of Entrance Pupil, not Lens. The graphic shows it correctly, and we have updated the description to be correct.

    • Jeffrey Howarth

      135 divided by “X” = 1.8    X = 75

    • Pye

      Jeff, what lens you using? 

    • Jeffrey Howarth

      Sony SAL-135F18Z 135mm f/1.8 Carl Zeiss

    • Pye

      Nice lens btw =) Yeah, so it is 135mm Focal Length, if you divde 135mm by 1.8 you get the Max Aperture Diameter of 75mm. Or the other way around 135mm/75mm = f/1.8

    • Jeffrey Howarth

      Oh yeah that easier for me to do. Thanks. 

       I kept this lens on for a year doing nothing but head shots. Awesome detail.  Maybe too much :-)

    • Pye

      The way you calculated gives you your Aperture Diameter of 75mm. So 135/75 = 1.8

  4. GG

    This still isn’t quite right, though. Entrance pupil, exit pupil and aperture stop are different things. It seems that the terms enterance pupil and aperture are used interchangeably in this write-up.

    The entrance and exit pupils are the optical images of the actual aperture stop, seen from the front and the rear sides of the lens assembly, respectively (“rear side” is, somewhat arbitrarily, defined as the camera mount side). In practically all cases, there are lens elements in front of the diagphram, which will magnify or diminish its size. Therefore, if you were to disassemble a camera lens and measure the diameter of the aperture stop with a ruler, you’d probably find that the math wouldn’t work out, because the number the focal length should be divided by is the diameter of the optical image of the aperture stop, not the aperture stop itself.

    Of course, how much this matters for practical purposes is arguable — but when the objective of the article is to explain this (quite simple) concept, the terminology must be correct as not to cause unneccessary confusion.

  5. Karrie Porter Bond

    Ah cool! Thanks for an informative article and easy to understand graphics to go along.

  6. Meghan Black

    Which full course DVD is referenced in this article? I notice in the comments above, that it was in the works roughly a year back. Is it in your online store?

  7. prince sharma

    it was really interesting! thanx


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