What is Aperture? – Understanding the Basics

Among the three factors affecting the exposure, Aperture is the most important thing to understand and master. Because, while being the main feature to control the exposure, it also has its own effect for the photograph, ‘depth of field’. Before reading this article, I highly recommend reading the detailed guide to ‘Exposure‘.

What is Aperture?

Aperture is an opening situated on the path of light rays in the lens, through which the light passes towards the sensor (or film). Its size can be regulated, hence the amount of light rays (intensity) passes through can be regulated. Which means it can control the exposure. So obviously, larger apertures (more light rays) gives higher exposures and smaller apertures (fewer light rays) gives lower exposures.

Above images are taken with three aperture sizes (smallest to largest from top to bottom). It can be clearly seen how exposure is increased when the aperture is getting larger.

Normally, a wheel is assigned for aperture, and by rotating it, its size can be changed.

Mechanism of Aperture

The easiest way to understand how aperture works is to think of how the ‘iris’ and the ‘pupil’ of our eye works (hope you remember it).

The opening in the middle of the eye is called the pupil. Iris is the group of tissues situated around the pupil that regulates the diameter of the pupil. When more light is required to see the subject, the muscles in iris pull themselves out radially to enlarge the size of the pupil. Then more light is allowed to enter the eye. On the other way, when less light is required, iris muscles loosen themselves up and reduce the size of the pupil.

This exact concept is used to create the aperture mechanism of a camera. The pupil has become the aperture and the function of the iris is executed by the diaphragm. A diaphragm is a set of movable blades situated in a circle creating an opening. The size of the opening can be adjusted by moving the blades. They are also called ‘Iris Diaphragms’, of course due to their functionality same as iris.

The number of blades and shape of them differs from lens to lens. When talking about the shape, basically there are two types, straight and rounded. Rounded blades create more circular aperture than straight ones. And the number of blades also affects the shape. Higher the number of blades, more circular the shape. Most modern high-end lenses have diaphragms with a higher number of rounded blades.

You might be wondering why the number of blades of the diaphragm and shape of them is a concern. Does the shape of Aperture matter? Of course, it does matter up to some extend. We will come to that later.

History

Even though the diaphragm is fully electronic and totally smooth in functioning today, back in the early days it was not an easy task. Earliest diaphragms were interchangeable ones. To change the aperture, the photographer had to remove the existing diaphragm and put another diaphragm with a different aperture. Guess you can imagine how hard was the life of a photographer back then.

The above image is of a rotating diaphragm developed by Carl Zeiss. It was a slightly developed stage than the above mentioned interchangeable diaphragms. To change the aperture, the wheel has to be rotated externally. And it only had five values.

But the things kept evolving and new technologies emerged day by day. With the passage of time, number of types of diaphragms came to use with various mechanisms and shapes. It is said that there had even been square shaped diaphragms also.

Another important thing to understand is, the diaphragm gives the desired aperture only at the moment of the shutter is released. Until the shutter is released, the diaphragm remains fully opened in order to achieve the focus accurately. When the shutter button is pressed, diaphragm blades move in and give the desired aperture, and then again the blades move out to leave the diaphragm fully opened.

f-number

We have talked about the purpose, mechanism and the history of the aperture (or diaphragm). Now we have come to the tricky part. How the aperture is measured? Of course, almost everyone who has a camera in hand would say, ‘f-number’ is the measurement of aperture. But what does the f-number really means? It is clear that it does not represent the diameter or the area of the aperture. Because f-number has an inverse relationship with aperture diameter. When the aperture gets larger, f-number gets lower.

The terms f-stop and f-value have the same meaning as f-number. To understand f-number, we have to get familiar with two terms first. That is ‘Absolute Aperture’ and ‘Relative Aperture’.

Absolute Aperture means the actual size (diameter) of the aperture.

Relative Aperture means the light gathering power of a lens (and in any other optical devices such as the telescope, microscope etc.). In other terms, it means the intensity of light that is being sent to the sensor (or film). It is obvious that the diameter of the aperture (absolute aperture) is a factor to decide that. But that’s not all.

There is another factor that affects the intensity of light sent to the sensor. That is the focal length of the lens. You can understand that the magnification of the image increases when the focal length increases. And the intensity of the light decrease when the image gets magnified (because the same amount of light has spread over a larger area). Therefore, there is an inverse relationship between the intensity of the image and the focal length.

Taking all these factors into account, relative aperture (light gathering power) is defined as the ratio of aperture diameter to the focal length of the lens. Other optical devices have separate definitions for the same.

For example, if the diameter of the aperture is 30mm and the focal length is 120mm, relative aperture is equal to 0.25 or 1/4. If the aperture diameter is 40mm and the focal length is 80mm, relative aperture is 0.5 or 1/2. So, you can understand that the relative aperture increases when aperture diameter increases compared to the focal length, which leads to higher exposures.

Now let’s get back to the topic, what is ‘f-number’? The f-number is defined as the multiplicative inverse of the relative aperture. In other words, the relative aperture equals to focal length divided by aperture diameter.

In the above examples, if the relative aperture is 1/4, f-number is f/4, if the relative aperture is 1/2, f-number would be f/2. That means, higher the f-number, lower the relative aperture, lower the exposure.

Do notice the way the f-number is written. It is customary to write as f/N (with a hooked f), where N is the f-number (Ex. f/8, f/2.8). The hooked f represents the focal length. f / N forms a mathematical expression of the aperture diameter, that is why f-number is written that way. Not clear? Let’s see.

If f-number is given in ‘N’, focal length is given in f and diameter of the aperture is given in D, the above equation can be written as,

N = f / D

If the equation is solved for D,

D = f / N

That is why the f-number is written as f / N to give a mathematical expression of the aperture diameter.

Depth of Field

As mentioned in the very beginning of this article, other than controlling exposure, the aperture has its own effect on the photograph. That is the depth of field. What exactly the depth of field means?

Depth of field is the distance between the nearest and the furthest objects giving a focused image.

In simple terms, it is the range in the photograph, where the objects are focused and sharp. When an object is focused on the frame, some distance in the foreground and some distance in the background also give sharp images. That total distance is called the depth of field. Other areas give blurry images. That out-of-focus blurriness is called ‘bokeh effect’.

How Does the Aperture Affect Depth of Field?

When the aperture is wide open (low f numbers), depth of field is low. It is usually called ‘shallow depth of field’. On the other hand, when the aperture is small (higher f numbers), depth of field is high and it is usually called ‘wide depth of field’. To get the things clear, check out the below demonstration. It has done with numbered cards with the same size, placed with the same intervals on a desk.

The above photograph has a very shallow depth of field. The focus is on number 1, and both 2s and 3s are out-of-focus and blurred. 3s are blurrier than 2s, because 3s are situated on larger distance from the focus point than 2s. This was shot at f/1.8.

Here, the f-number has increased to f/10. Notice that, now 2s are sharper than the above photograph and almost equal sharpness of 1. 3s are still out-of-focused, but not as much as the above one. The focus point is still on 1.

Now, the f-number is f/16 and focus point is still on 1. Here, 3s have also focused and become sharp (not perfectly though).

All the above images were taken using the same camera, same lens and at the same distance. But what caused the difference? Clearly, it is the aperture size. Now you must be clear, when the aperture size decreases (f-number increases) depth of field increases. One thing to be noted is, the aperture is not the only thing that affects the depth of field. Altogether there are four factors that affect the depth of field. We will discuss all of them in a separate article.

Application of Depth of Field

Depth of field is a great tool which can be used in photography, especially in artistic photographs. Shallow depth field is often used portrait photography, because a out-of-focus blurry background can keep the attention on the subject strongly. That is why professional portrait photographers always keen to buy f/1.2 or f/1.4 (maximum aperture) lenses. This principle is commonly followed in wedding photography, wedding pre-shoots, baby photography, maternity photography and in any other where a person or a set of persons is the subject. Shallow depth of field is often used in product photography also for the same reason.

On the other hand, a wider depth of field is mostly used in wide angle shots like landscapes, street photography, architecture photography etc. When the whole frame is equally important, you don’t need to focus on one point and de-focus the other areas. So, by using a higher f-number, the whole frame can be taken equally focused and sharp.

Shape of Aperture

I mentioned earlier, that number of blades and the shape of them in the diaphragm matter up to some extent. It has a relation with highlights in out-of-focus areas. See the below image.

You would definitely have seen this in photographs. This is how small highlights (usually light bulbs or shiny reflectors) seen in out-of-focus areas of the image. Ever wondered how those spots got the same unusual shape? Definitely, it cannot be the actual shape of all of them. Of course, it isn’t, it is the shape of the aperture of the lens, at the moment.

By looking at the out-of-focus highlight spots at an image, some details can be said about the diaphragm of the lens that took the image. Sides of the polygon is equal to the number of blades, because each blade creates one side of the polygon. And it also can be observed that whether the blades are straight or curved.

In the above photograph, the highlight spot shape is hexagonal. It means that diaphragm consists of six blades. And the sides of hexagon have a slight curvature, and obviously, the blades are curved.

This one is taken with my 50mm lens. It is the same one, of which the diaphragm is shown above. Its diaphragm has seven blades, so the shape of light spots are ‘heptagonal’.

Even though the polygonal shapes of highlights have its own artistic expression, it is not the case all the time. Most of the time smooth bokeh is preferred rather than rigid polygonal shapes. Naturally, a photograph is more beautiful and eye-catching, when the out-of-focus areas are soft. Otherwise, it can disturb the scene and the viewer could be distracted from the subject.

In order to achieve smooth bokeh, the shape of the aperture has to be more circular. As mentioned earlier also, to achieve that, the number of blades in the diaphragm has to be increased and their shape has to be curved. That is why most high-end lenses have more blades (9 or more) and they are curved. But the thing is they are also pricey. Another thing to mention is, in some lenses, aperture shape becomes a perfect circle when it is fully opened. Because when the aperture is fully opened, the blades completely go into the lens barrel in some lenses and then lens barrel itself become the aperture, which is a perfect circle.

There is another situation where the shape of the aperture can be seen in the photograph. It is the lens flare.

To sum up the things, the aperture is the main feature to control the exposure. And also it has its own effect, depth of field, which could add an expressive value to a photograph. Aperture has to be mastered by each and every photographer, to both expose the image correctly and to give the correct depth of field to the image that matches with its message.

If you have any queries, please feel free to leave a comment below.

References:

https://en.wikipedia.org/wiki/F-number

https://en.wikipedia.org/wiki/Aperture