(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.

QUICK OVERVIEW: Online Courses about Aperture and other techniques

A beginner's introduction into the world of blur and depth of field.
Improve your photography by mastering the aperture mode and learning all there is to know about the aperture mode.
Master your camera & photography with aperture,shutter speed,iso,white balance & more digital photography fundamentals
A progressive guide from beginner to advanced on how to choose your next lens
Improve your travel images with any digital camera using simple camera settings & creative techniques
Designed to provide you with all the wedding photography tips and tools you need to start, run, and grow a successful business.
Learn how to perfect baby photos with easy, realistic Photoshop adjustments.
Conducting an engagement photography shoot requires a delicate mix of technical and interpersonal skills – but not an abundance of expensive, demanding equipment.
In this comprehensive series you'll learn Sue's inspiring approach to styling, posing, marketing, selling and so much more!
These classes will help you express what you see in the world in new ways, and teach you techniques that you can apply to all kinds of photography.

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.


  1. I am purely delighted with the detailed information available on this website that almost throws light to every dark corner of my knowledge bank when it comes to Photography.

  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.


Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.