What is an f-stop? Photography 101

With optics, nothing is simple.


When answering the question: “What is an f-stop in digital photography?” getting your head around the concept can be daunting. This is because when talking about lenses, one cannot avoid talking about the complicated and intricate science of optics. But we are photographers, and I will only get into some of the details so you can grasp what I will eventually be simplifying.


Therefore, for the sake of being thorough in this blog post, I will go about telling what you really need to know. To get you guys up to speed, I’m going to show you a skippable list. You can decide what you need to know to better your photography skills. I will be giving you:

  • A technical definition
  • A Photography friendly definition
  • The essentials of what you really need to know

A technical definition of f-stop



The F-stop is also known as the f-number or f-ratio. In photography, the f-number of a camera lens is the ratio of the system’s focal length or capacity to bend light to the diameter of the entrance pupil of the lens. It is also known as the focal ratio and is very important in photography because it is a way of measuring the absolute lens speed; increasing the f-number is referred to as stopping down.

Here is the f-number equation

The f-number is the reciprocal of the relative aperture

N = f/D

various graphs showing what focal length is
Distance between a lens and the point of convergence of light is the focal range.

Where in f is the focal length, and D is the entrance pupil diameter. For instance, if you have a lens with a 35mm focal length and the pupil opening (or aperture) is 27mm, your f-number will be roughly 1.3



weeds dancing in a fall sunset
It is important to learn everything that defines lenses. Every image you take is based on light that passes through this one thing. It is the most important part of the photographic experience.

A Photography friendly definition of F-stop


All lenses are defined by their focal length (measured in millimeters) and their maximum iris (aperture) opening or F-stop. Because of the ratio equation that we just mentioned, remember that the lower your F-number is, the more light your lens will be able to capture.


Woman in a poncho glancing to the side

What is most important to remember for photographers is that f-Stop affects two crucial aspects of your photography – depth of field and lens light. Also, remember that all lenses offer different f-stop adjustments on the aperture or f-ring on the lens. Where lenses differ will always be with their max f-stop. Lenses with a lower max f-stop, in my experience, are always more expensive.


Depth-of-field and f-stop


A lens that is a 50mm f/4.5 will not be as luminous as a lens that is a 50mm f/1.2. 


For all intents and purposes, depth-of-field is how much behind and in front of an in-focus point, objects are in focus. If an F-stop is low, depth-of-field will be shallow. Shallow depth-of-field means that objects between the focus point and the lens will mostly be out of focus. The same goes for objects past the focus point. Those objects will also fall out of focus.

  • A lens with an f of 2.0 will have shallow depth-of-field
  • A lens with an f of 11 will have a high depth of field. i.e., everything before and after the focus point of your subject will all be in focus.
Girl posing and holding her sunglasses

What photographers need to know about F-stop


Depth-of-field: Good for some and bad for others


So the real question is this. What f-stop opening is more desirable for photographers? Both high depth of field and low depth of field can be desirable. It all depends on what type of photography you would like to get into. However, having a lens that has maximum light capability is always better.

Camera lens front view with accessories on the side
F-stop was "open" wide and allowed me to draw attention to the lens breand and markings and take away from the accessories that come with the lens.

For instance: Portrait and wedding photographers like to have a subject in focus with a bit of a dreamy background to give a sort of fantasy or dreamy effect. It is sometimes referred to as bokeh.

man and woman in a wedding setting
The blurry fantasy effect called bokeh can also allow photographers to make beautiful backgrounds out of otherwise bland setting. This couple is essentially in front of a condo complex. Bokeh allows you to focus in on a subject. A much needed effect for wedding photographers.

Sports photographers and wildlife photographers capture action and many times need a margin of error in their focus. A higher f-number allows for more in the frame to be in-focus even when you are not totally of the mark.

f-stop and the light tradeoff


Because lenses with low f-numbers make the iris (aperture) bigger, more light is exposed to the camera film or camera image sensor. This allows you to sacrifice less on your ISO sensitivity and shutter speed. There will be more on the dance between these 3 settings (ISO, Shutter speed, and F-stop) in another blog post. But, for now, know that getting a very “open” lens lets you make fewer sacrifices on other crucial camera settings.





It must be noted that photographers all refer to f as actually BEING aperture. This is not completely exact. However, this reference serves the purpose of photographers and, in my humble opinion, can be made by a photographer who doesn’t want to lose credibility.


F-stop is a lens-specific setting. But this setting is so important in the trilogy of camera settings. Both adjustments of opening up “f” or closing down “f” have benefits.  Settings like ISO and shutter speed are one-sided in their adjustments. I other blog posts, I will look at ISO and shutter speed, the more artistic of the technical camera adjustments.


What do you think?


Have you guys learned anything from this post? Are there other photography topics you would like me to explain? I love getting feedback from readers! Comment below.

Other links that might interest you

Viltrox 56mm f/1.4 review
Viltrox 85mm f/1.8 review
10 beginner photography mistakes
Phot gear lust
Jacques Gaines looking in a camera pointing upward

About the author

I am a photographer, videographer, and copywriter living in Quebec City, Canada. I also have a YouTube channel and an Instagram account dedicated to creation and creativity via my main loves of photography, music, and writing.

To get in touch with me and discuss your collaboration, service needed, or advice, either go to the contact page and write to me via the contact form at the foot of this page.

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