Exposure Triangle 3/3: Aperture

June 15, 2018

What is the difference between all types of photography and how do we achieve them using the same DLSR and equipment? My answer to this is one word “knowledge”. Once you have knowledge on the Exposure Triangle you are good to go as you have achieved the first step to having creative control over your images.

Aperture is the last topic on our Exposure Triangle Series

  1. Shutter Speed

  2. ISO

  3. Aperture

As a reminder of what we discussed in the previous two topics comparing the exposure triangle to the human eyes.

The shutter speed is the time duration our eyes is open to let light

enter,while the ISO is the sensitivity of rods and cones behind our eyes to

the light and the aperture, is our eye’s iris that opens and constricts to control the amount of light entering.

 

 

f/1.8 1/200s ISO 200

 

Aperture

Light travels through the lens and into the photosensitive surface(film) and it goes through an opening, this opening is known as the aperture and it opens in different sizes ranging from wide to narrow. Using the human eyes to describe this concept one more time, when we are in a dark room our pupils opens wider for more light to enter and in a bright room they constrict to lessen the amount of light entering i.e. the human eyes adjusts the brightness by controlling the size of our pupils. This is concept behind Aperture only it is done mechanically using diaphragm made of overlapping blades that are designed to vary in size. Unlike the opening and constricting of the pupils, changes in size of aperture not only controls exposure (brightness) it also has creative effects. Aperture is represented by f-stop (f-number).

 

 

 

Sizing of Aperture

This part of the Aperture took me a while to understand, I will make it simple so you won’t have such issues. The wider the aperture the smaller the f-stop (f-number) and the smaller the aperture the bigger the f-stop (f-number). For instance, f22 (bigger f-stop) means the aperture is narrowly open and conversely f1.4; (smaller f-stop) means the aperture is widely open. As long as you have this in mind the whole process becomes easier, saves you the stress of knowing the actual complicated calculations going on. What will be the size of the aperture for f11?

Common Sizes are

  • f/1.8

  • f/2.0

  • f/2.8

  • f/4

  • f/4.5

  • f/5.6

  • f/8

  • f/11

  • f/16

  • f/22

Source: www.creativelive.com

 

Effects: Exposure (Brightness of Image)

Just as we have for the shutter and ISO, the aperture also controls the the exposure of the image. In a scenario where a wide aperture (small f stop) for instance f/4 will be brighter than an image shot with same settings but a narrower aperture for instance f/11.

 

 

f/4 1/100s ISO 200

 

 

 

f/11 1/100s ISO 200

 

When dealing with aperture, dialling the f-stop from f/8 – f/4 does not double the amount of light entering the lens instead it quadruples it. This is because of some mathematics involved and also Inverse Square Law. Recall that when the light entering is doubled or halved that’s a one EV shift / one stop i.e. ±1 EV. In the case of dialling up the aperture from f/8 – f/4 we have a + 2 EV i.e. quadrupling the light passing through the diaphragm. The next article will be on EV (Exposure Value) so I can explain this freely, but for now just take note of these values.

-Dialling down f/2 – f/2.8 halves the light intake (i.e. -1 EV)

-Dialling down f/2 – f/4 quarters the light intake (i.e. - 2 EV)

-Dialling down f/4 – f/5.6 halves the light intake (i.e. - 1 EV)

-Dialling down f/4 – f/8 quarters the light intake (i.e. -  2 EV)

 

-Dialling up from f/8 – f/5.6 doubles the light intake (i.e. +1 EV)

-Dialling up from f/8 – f/4 quadruples the light intake (i.e. +2 EV)

-Dialling up from f/16 – f/11 doubles the light intake (i.e. +1 EV)

-Dialling up from f/16 – f/8 quadruples the light intake (i.e. +2 EV)

 

On your DSLR you will notice for instance between f/8 and f/4 other little increments in f-stops. You can simply just dial up/down till you get a correct exposure (The most important thing to know about these numbers is that, from each number to the next, the aperture decreases to half its size, allowing 50% less light through the lens)

 

I am tempted the give a little explanation of the mathematics behind aperture, you can feel free to skip to the next sub-heading, the main concept has already been covered.

The focal length of your lens can be used to work out the area of opening diagram to better your understanding on how the numbers work. For example, if you use a 85mm lens with an aperture of f/5. Divide the 85 by 5, giving you 17mm diameter which is now the width of aperture.

Area= ∏ * r2

Divide this diameter 17mm by 2 to get the radius 8.5mm. Now the area of the opening is approximately 227mm2 i.e. a f/5 on a 85mm lens the area of opening of 227mm2

 

For further illustrations lets assume aperture of f/10 (Note that you will not find such value in real life), following the calculations above the area will be approximately 56.75mm2 which is quarter of 227mm2. Therefore, we can see that the light entering at f/5 will be quadruple the amount of light entering at f/10.

There is a further calculation on getting an exact +1 EV shift using aperture but will not be discussed in this article.

 

 

 

Source: www.bhphotovideo.com

 

Creative Effects

The main creative effect when dealing with aperture is “depth of field” which is a whole article on its own. You can check it out here.

What you should know is a wider aperture (low f-stop) will have a shallow depth of field (Blurry background and foreground) and a narrower aperture (higher f-stop) will have a deep depth of field (little or no blurry background and foreground).

 

The images below will aid in the making this concept more relatable.

Arranged in an order of f/2, f/4, f/8, f/22. Notice that as the aperture reduces from f/2 to f/22 the depth of field (blur) also follows that order. Note that the ISO and shutter speed were adjusted in this example to compensate for the change in aperture values

 

 f/2

 

 f/4

 f/8

 

How to set the Aperture

Just as we have for both the ISO and Shutter speed. The first step is by switching the Camera mode from “Auto Mode” to “Aperture Priority mode” for a start and tweaking the f stop (aperture) values to

see the corresponding results. Then using “Manual Mode” to gain full control of your exposure triangle.

Now you have the full knowledge on the Exposure Triangle you can decide which of the settings to dial, in the case of aperture it is used mostly for its depth of field effect and rarely for its brightness functions but as we recall a wider aperture gives a brighter image with a shallow depth of field and a narrower aperture gives a dark image with a deep depth of field. Read up on all three parts of the Exposure Triangle and Keep practicing till you can’t get it wrong.

 

 

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