All You Need To Know About Aperture


Aperture is one of the elements of the exposure triangle. It is one of the controls that we learn very early in our exploration of photography. At a base level, it’s a pretty simple and basic control. However, if you step back and think about it just a little, aperture is quite a complex and interesting subject. 

Today we are going to delve a little deeper into the wonderful world of the aperture, starting with the basics then journeying through some of its stranger nuances. 

The Basics Of Aperture

The aperture on your lens is a diaphragm that opens and closes. This, in turn, allows more or less light through to your camera’s sensor – or film. We can control the aperture ourselves using Aperture Priority mode or by shooting manually. In Aperture Priority, we set the aperture ourselves and the camera will set a suitable shutter speed. 

Photographers have several terms to define the state of an aperture. A large, big or wide aperture means that the diaphragm is open, close to, or at its maximum expansion. A small or narrow aperture means it is close to or at its minimum size.

A large, or wide aperture. Paul Skorupskas
A small or narrow aperture. Fidel Fernando

Aperture is measured on a scale called f-stops and we will look a little deeper at that later. Typically lenses are around f/1.8 to f/4 at their widest aperture and f/16 to f/22 at their narrowest. The aperture can be changed in full stops, half stops or in some lenses 1/3rd stops.

What Is The Effect Of Aperture On An Image?

Most of you will know and understand the term Depth of Field. In simple terms, this is the amount of the image that is in focus in front of and behind the actual point of focus. If we use a wide aperture, that area is quite small, giving us a blurred background, if we are focused on a close subject. 

If we use a narrow or small aperture the distance that is in focus becomes much greater. Even if the subject we focus on is close, the background will be partly or fully in focus. The effect varies with the focal length of the lens and the position of the subject relative to the background. 

A wider aperture will give us a shallower depth of field. Ratiu Bia

A wider lens will have more depth of field for an equivalent aperture than a telephoto lens This is why portraits are most often taken with a moderate telephoto and a wide aperture. Also the closer the subject is to the camera, the more the background will be out of focus. One of the “side effects” of depth of field is Bokeh and we will look at that in a moment. 

A smaller aperture leads to deeper depth of field. Mark Harpur

Why The f Stop Scale?

With the basics out of the way, let’s delve into some of the more interesting aspects of aperture and we will start with the way it’s measured. Ever stepped back to wonder at the strange, seemingly random nature of the f stop numbers? f1.8, f/2, f2.8…f/11 f/16 there seems no rhyme nor reason to them.

In fact, there is a logic to it, you just need to think of it in terms of fractions and not decimal points. The actual f-number is derived from simple algebraic calculation N=f/D where N is the aperture, f is the focal length and D diameter of the entrance pupil, in the case of a lens, the diaphragm of the lens.

There is a logic to the f stop scale. Adam Birkett

The aperture scale of a modern lens increases by the power of the square root of 2. Hence a one-stop decrease from f/1.4 is f/1.8 and stop further is f/2. As eccentric as it seems there is a logic to it, albeit one we do not necessarily need to know.

Understanding Diffraction. 

One thing we do need to know about f stops is diffraction. This is another relatively complex subject but one to be aware of as it can affect our image quality. 

When light passes through an aperture, the light waves start to spread out. The smaller that aperture, the wider they spread out. That, in turn, can cause light waves to interfere with each other leading to a small but sometimes perceivable loss of image quality.

The diffraction limit or point where diffraction can become an issue is defined by sensor size, pixel density and aperture. A camera with a very high pixel count and pixel density will start to get diffraction at wider apertures than a camera with a lower number of pixels and pixel density. 

For cameras with very small sensors and high megapixels, diffraction might start to occur at f/4 or f5.6 while a full-frame camera with a similar number of pixels might not see diffraction until f/11 or f/16. 

Just What Is Bokeh?

Another strange term related to aperture is Bokeh. We hear it a lot these days particularly from Youtube reviewers who go on about a lens’ creamy Bokeh. The word comes from the Japanese word for blur and in photographic terms defines the quality of the out of focus regions and not the amount of blur itself. By its nature, it’s an entirely subjective term but it is related to depth of field. The shallower the depth of field, the more blur you will have. In turn, that means your Bokeh will be more defined. 

Bokeh is a highly subjective quality. Aaron Burden

The term creamy Bokeh describes a background where the detail is so undefined and blurred as to look “creamy”  Apart from a wide, fast aperture, one other thing that can define the quality of your Bokeh is the number of aperture blades your lens has. 

It’s All About The Blades.

The aperture in your lens is a complex and delicate piece of kit. It is controlled by a series of very thin, overlapping blades that can open and close to control the amount of light passing. Those blades can have straight edges or more rounded edges and this, in turn, can affect the way the Bokeh appears. The number of blades in the lens also has an effect on the way the out of focus regions look. 

Image quality can be defined by the curvature and number of aperture blades. Agence Olloweb

The rounder the actual diaphragm is, the more pleasing the Bokeh will be. That means lenses with the right number of more curved blades will give the best results. That “right number” is generally regarded to be around 9. 

As you can see from above, aperture is so much more interesting than just the amount of light that reaches your sensor. Next time you are out shooting, give a thought to some of the varied things that are occurring when you are setting your f stop.

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Jason has more than 35 years of experience as a professional photographer, videographer and stock shooter. You can get to know him better here.

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