What is Aperture

Table of contents:

What is Aperture in Photography?

Aperture is the circular opening in a lens through which light reaches the sensor. We can widen or narrow the diameter of this opening to control the incoming light.

Table of contents:

As human has eyes to see this world, so a camera sees through the aperture of a lens. Our eyes receive light which is converted to neural signals and sent to the brain where an image is formed and identified. In a camera, light passes through the aperture-hole and reaches the sensor/film to form an image.

Different sizes of aperture Aperture of a lens, large to small f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22

Not shown here, the maximum aperture of a lens may be larger than f/1.4, say f/1.2, f/0.95..., or smaller than f/22, say f/32, f/45...

The aperture-hole of a lens is made up of a number of blades. Those blades create a roughly circular hole by moving themselves when we change aperture setting in a camera. Light reaches the film or sensor through this hole. A large aperture-hole allows more light to reach the sensor; a small aperture-hole allows less light.

Aperture is a ratio of focal length and diameter

Aperture is one of the three variables to control the overall exposure or brightness of a photograph. It is an element of lens, and expressed as f/numbers such as f/2, f/5.6 etc.(the 'slash' is omitted sometimes ). When aperture-hole is large in diameter, it is represented by a small f/number like f/1.4, f/2 and allows more light to enter. On the other hand, a small aperture-hole is represented by a large f/number, like f/22, f/32 etc., which allows less light. This happens because aperture is the ratio of focal length and the diameter, i.e.,

 aperture = focal length ÷ diameter
⇒ diameter = focal length ÷ aperture
∴ focal length = aperture × diameter

Focal length is the distance between the optical center of a lens and the sensor or film, when the subject is in focus.

Every lens has a maximum (or widest or largest) and a minimum (or narrowest or smallest) aperture. For example, the AF-S DX NIKKOR 35mm f/1.8 G lens has a largest aperture of f/1.8, and a smallest aperture of f/22. In general, we should avoid the maximum or the minimum apertures while shooting, as they may show some unwanted effects in a photograph.

For example, for the above-mentioned lens, at apertures f/1.8, f/2 some optical aberrations may creep in the image. To avoid that we should stop-down the aperture 2 to 3 stops than the largest aperture (stopping down an aperture is to apply a smaller aperture).

Why small F/number represents large aperture

Why does a small f/number represents a large aperture and allow more light, and a large f/number represents a small aperture allow less light?

A large aperture is represented by a small f/number, while a small aperture is represented by a large f/number.

If we attach a 35mm f/1.4 lens to a camera, the distance between the lens and the sensor will be 35mm when the subject is in focus. The focal length of this lens is constant(35mm), it being a prime lens. The diameter of the opening of the aperture will vary depending upon the f/number. At f/1.4, diameter of the aperture-opening will be :

 diameter = focal length ÷ aperture
⇒ diameter = ( 35 ÷ 1.4 )mm = 25mm

This will be the largest opening (diameter) at the smallest f/number of f/1.4.
Again, this lens has the smallest aperture of f/16. So, at f/16, diameter of the aperture-opening will be:

diameter = ( 35 ÷ 16 )mm = 2.18mm

This will be the smallest opeing at the largest f/number of f/16. With the above format of calculation, we can get the f/numbers and respective diameters of a 35mm f/1.4 prime lens in the following table.

With largest diameter 25mm, highest amount of light will be allowed to reach the sensor. At that time the aperture is f/1.4 (lowest f/number). The opposite is happening at f/16 when diameter is 2.18mm.

So, a small f/number represents a large aperture and allows more light.

Apertures one stop apart

The following aperture numbers are said to be one stop away from its previous aperture. This means each of the apertures allows double the light than its previous value :

f/45, f/32, f/22, f/16, f/11, f/8, f/5.6, f/4, f/2.8, f/2, f/1.4, f/1

When you divide an f/number by √2 or 1.4, the resulting f/number will allow double the light than the previous f/number. The difference is just one stop. Please check the page:  a short note on what is one stop exposure in photography.

Effect of aperture

Generally, the size of the aperture-diameter effects the follwing aspects of photography :

In addition to the above, there are some optical aberrations like chromatic aberration, coma etc. which happen due to unavoidable imperfection of a lens. Lens making is a complex process; no lens is perfect. Some of those optical aberrations may sometimes be avoided by using a narrow aperture.

Please take time to observe the following animation. When aperture is altered, it effects different aspects of a photograph.

Four effects of aperture Effects of Aperture diameter of a lens f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 maximum minimum Brightness shallow deep Depth-of-field Low High Diffraction Indicator Low High Star-burst effect 1.4 2 2.8 4 5.6 8 11 16 22

Effect of aperture on exposure

Changing aperture is one of the ways to set exposure before clicking the shutter finally. Large apertures like f/1.4, f/1.8 etc. allow more light, while narrow apertures like f/22, f/32 etc. allow less light.

Large apertures allow maximum light, small apertures allow minimum light.

Both the following two images were shot from a heavy tripod fixed on the ground and time difference was very minimum, (say 2 seconds). Those are raw images without any post-processing. Shutter-speed was 30 seconds, ISO was 64 at 24mm equivalent focal length. However, the aperture-setting of the first image was f/8, and the same of the second image was f/16. The result is in front of you. Since f/8 is a larger aperture than f/16 (two stops larger), the first image is brighter than the latter, other variables remaining unchanged.

Effect of aperture, Vidyasagar Setu, Kolkata

Vidyasagar Setu, Kolkata, West Bengal, India 2015 * Camera: Nikon d810, lens: Tamron 24-70mm f/2.8

Why did I shoot the second image when I got proper exposure with the first one? Of course not because I wanted to show you this example! Five years ago , I did not think of writing articles on photography. The reason will be explained after we learn about diffraction and starburst aspects of photography in this page.

Effect of aperture on depth-of-field:

Depth of field is a very important aspect of photography. It is the acceptably sharp front-to-back area of a photograph. The deatils about depth-of field may be read in in this and this page.

Effect of aperture on diffraction and starburst/sunburst:

In photography, diffraction and starburst/sunburst effects depend on the aperture settings of lens.
Always try to keep diffraction to minimum level by avoiding very small apertures.
Starburst/sunburst effect also depends on number of blades in a lens.

Here is a table with detailed explanation of the advantages, disadvantages and applications of different apertures.

Aperture F/NumberEffect
Large ... f/0.95 ... f/1.4 ... f/2.8 ...   Maximum light received
  Very thin depth of field
  Creamy bokeh
  Suitable for portrait, astro, low-light
  Unwanted elements may be blurred out
  Less ISO required, so less noise
  Lens aberration may be visible
  Focus-accuracy is difficult
  Unsharp image may result in
  Excessive light may need ND filters
Medium... f/4.0 ... f/5.6 ... f/8.0 ...  Medium light received
  Acceptable sharpness
  Safe w.r.t. lens aberration, diffraction etc.
  Suitable for street, documentary
  Sweet spot of some lenses lie in this range
Small... f/9.0 ... f/11 ... f/13 ...  Less light reaches the sensor
  Large depth of field
  Acceptable sharpness from front to rare
  Suitable for landscape, cityscape & architecture
  Sunstar/starburst effects can be achieved
  ISO may have to be increased
  Diffraction may creep in
Very small... f/15 ... f/16 ... f/22 ...  Very very less light received
  Very large DOF; useful in macro
  Sunstar/starburst effects can be achieved
  Sharpness falls due to excessive diffraction
  High ISO is required
  Avoid the range ...f/22... or smaller

And here is another brief table:

Effect Large aperture Small aperture
depth of fieldsmalllarge
lens aberrationstrongless
sharpnesssoft *sharp **

Sharpness w.r.t. apertures

Even with a large aperture, an image may achieve sharpness but for a small plane only. If the focus point is mistakenly placed somewhere else, an unwanted result will be produced where subject may become blurred.

Narrow or small apertures provide sharpness, but at extremely small aperture a photograph will lose sharpness due to diffraction.

Now we are familiar with the term "aperture"; in the next page let us see how aperture effects depth of field of a photograph.

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