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Photography Basics – Aperture [Part V]

Photography Basics – Aperture [Part V]

In Part I, II and III we dealt with basics of Exposure, how to achieve optimum exposure for various scenarios, and how to achieve optimum exposure using Filters, respectively.

Part-IV was a pre-cursor to understanding Aperture, Shutter Speed and ISO.

Let us now dive deep into understanding the most important aspect of Photographic Exposure, the Aperture.


Aperture is the opening in the lens that controls the amount of light that falls onto the sensor to make an exposure.

The opening in the lens can be controlled using the standard set of values for the Aperture. We looked at the hypothetical example in Part-IV and understood the following key concept:

Each Aperture value is half the previous value and double the next value.

Below figure illustrates the concept.

DSLR Photography Basics: Understanding Exposure, Aperture, Shutter Speed, ISO


Aperture is represented in f-stops, where f stands for focal length. It is the ratio of the len’s focal length (f) to the diameter of the entrance pupil of the lens.

N (Aperture) = f (focal length of the lens) / D (Diameter of the entrance pupil)

Entrance pupil is a window opening at farther end of the lens that fits to the Camera body. The light that enters the lens travels through the various optical elements to finally pass through this window or entrance pupil before reaching the Camera Sensor.

This entrance pupil or the window opening can be controlled by using an adjustable diaphragm in the lens, thereby controlling the exposure.

For instance, a 300mm with an entrance pupil diameter of 75mm will have an aperture value of f/4. We can rephrase this sentence like this: A 300mm f/4 lens will have a maximum entrance pupil diameter of 75mm.

300mm (f) /75mm (D) = 4


300mm (f) / 4 = 75mm (D) 

An aperture stop or an f-stop is usually denoted as f/aperture value [we will deal with denominator values in a little while]. If we look at the above equation, an increase in the denominator value leads to decrease in the entrance pupil diameter.

300mm (f) / 5.6 = 54mm (D)

300mm (f) / 8 = 38mm (D)

In summary:

  • Bigger denominator values decreases the diameter of the entrance pupil resulting in less light to fall onto the Camera sensor
  • Smaller denominator values increases the diameter of the entrance pupil resulting in more light to fall onto the Camera sensor

Let us look at a pictorial representation of the same (Note the relative sizes of the circles).

DSLR Photography Basics: Understanding Exposure, Aperture, Shutter Speed, ISO

An f-stop functions similar to that of a car door. You might have observed that the car door opens in steps. It opens upto some predetermined point and then stops. If you open it further, it opens free upto next predetermined point and then stops again, and so on.

F-stop works very similar to this concept by dictating how much a lens should be open or closed before it stops at a predetermined point. These predetermined points are standardized set of values that every manufacturer follows.

Why do you need f-stops? 

Simple reason is to have control over the exposure. It enables you to precisely control how much light should enter the sensor to make a proper exposure. Since every scene requires a different exposure, it is necessary to have different f-stops in order to control the incoming light.

Here are the standard f-stops:

f/1, f/1.4, f/2, f/2.8, f/4, f5.6, f/8, f/11, f/16, f/22, f/32, etc

Each of these denominator numbers like 1, 1.4, 2, 2.8 and so on are the rounded values of the geometric sequence of √2.

For instance, (√2)0 ­= 1

(√2)1 = 1.4

(√2)2 = 2

(√2)3 = 2.8

These are called full stops, which lets-in:

  • double the light compared to its next stop (bigger denominator number), and
  • half the light compared to its previous stop (smaller denominator number)

For example:

  • f/8 allows double the light than f/11 and half the light than f/5.6
  • f/5.6 allows double the light than f/8 and half the light than f/4

Below picture, which is similar to the one used in Part IV, illustrates the concept. Note that the size of the circle (or diameter) decreases as the denominator value becomes larger.

DSLR Photography Basics: Understanding Exposure, Aperture, Shutter Speed, ISO

Note that smaller the f-number bigger the size of the entrance pupil and vice versa.

DSLR Photography Basics: Aperture f-stops. Understanding Exposure, Aperture, Shutter Speed, ISO

Fractional F-stops

Manufacturers also give fractional f-stops to have precise control over the exposure. The intermediate stops are usually in steps of ⅓ stops.

Here are the standard fractional f-stops in steps of ⅓ stops:

f/1, f/1.1, f/1.2, f/1.4, f/1.6, f/1.8, f/2, f/2.2, f/2.5, f/2.8, f/3.2, f/3.5, f/4, f/4.5, f/5.0, f/5.6, f/6.3, f/7.1, f/8, f/9, f/10, f/11, f/13, f/14, f/16, f/18, f/20, f/22

For instance, consider the intermediate stops between f/5.6 and f/8 which are f/6.3 and f/7.1.

  • f/6.3 is 1/3rd stop smaller than f/5.6 and 2/3rd stop bigger than f/8
  • f/7.1 is 2/3rd stop smaller than f/5.6 and 1/3rd stop bigger than f/8

DSLR Photography Basics: Aperture fractional f-stops. Understanding Exposure, Aperture, Shutter Speed, ISO

F-Stop notation

Reading an f-stop can be confusing for many. Let us understand the notation in a simpler manner.

  • Bigger the f-number smaller will be the diameter of the entrance pupil resulting in lesser light to pass through (f/8, f/11, f/22 are usually considered as smaller apertures)
  • Smaller the f-number larger will be the diameter of the entrance pupil resulting in more light to pass through (f/2.8, f/4, f/5.6 are usually considered as larger apertures)

In case of any confusion, just take the focal length of the lens you are using and divide it by the standard f-stop denominators like 2.8, 4, 5.6, etc. The resulting value will indicate the diameter of the entrance pupil.

If you remember the below diagram you will never be confused again!

DSLR Photography Basics: Understanding Exposure, Aperture, Shutter Speed, ISOI hope you will never get confused in the future.

Let me know if you had any difficulty in understanding the basic concept of aperture? Do you have similar examples which you would like to share with us?

In the next article we discuss about The Role of Aperture in achieving the required Depth of Field (DOF)

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Prathap is a professional nature photographer and founder of Nature Photography Simplified blog. He aims to simplify every photography concept to help beginners and amateur photographers.

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This Post Has 40 Comments
  1. That is the simplest way to explain, Prathap.
    Happy to go through the theory lesson once again after a gap of almost 20 years.
    Thank you very much,

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  2. hello,

    i hv been going thru ur articles… and hv been able to capture razor sharp images if humming bird with “light in the eye”… i m using 70-300 of nikkor in D 60 body….

    after going thru ur article on DOF and ETTP… i ve a query… nikkor makes lense of 400 mm with f/2.8… would u please clarify how such high focal length lense with that high aperture would deliver sharp images when situation demands?

    i mean a telephoto lens will yield shallow DOF… and a low aperture number would add on that….

    looking forward to ur reply.


    keep up the good work.


      1. Dear Prathap,

        Thanks for replying.

        After going through your article on DOF, I have a question.

        Nikon/ Canon makes lenses of 400 mm / f2.8. How come such higher focal length along with higher aperture number would provide substantial DOF of animals/ birds?

        I mean the DOF at 400 mm and f/2.8 mm would be less than a few millimeter i guess?\

        Hope I have been able to convey my query.



        1. Dear Mayukh, if I understand you correctly, your question is how can you get increased DOF resulting in sharper images with higher focal length and larger aperture, right?
          It is true that the DOF will be in millimeters in longer focal length lens like 400mm f/2.8. However, DOF also depends on camera-to-subject distance, subject-to-background distance and various other factors. Every lens possess something called Minimum Focusing Distance at which the lens gives minimum DOF or shallower DOF. As and when the distance increases, generally the DOF increases.
          Since animals/birds are usually pretty far from us, we end up getting a greater DOF depending on background distance and sensor type used.
          It translates to, how big is your subject in the frame? which generally depends on the size of the subject, camera-to-subject distance.
          Bigger the subject in the frame, shallower the DOF you will generally achieve with f/2.8 setting. Smaller the subject in the frame, deeper the DOF you will get with f/2.8.
          Above all, if you happen to use smaller aperture values (like f/8, f/11, etc) for 400mm lens, you will get increased DOF, resulting in sharper images. Everything depends on the scene and how much of sharpness you like.
          I will cover the Depth of Field concept in depth in an article. Hopefully it will be more clear.

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  3. hi prathap,

    your explanations are explicit and really helpful. I have one question, what happens to the aperture in case of zoom lenses? is the dia of pupil remain fixed while it is zoomin or zoomout( I.e. focal length is changing) or, it is go on changing with change of focal length?

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