A Very Detailed Guide to the Exposure Triangle | Aperture

Home » A Very Detailed Guide to the Exposure Triangle | Aperture
Monday 25th February 2019

Taken at F2 with a Helios 44 lens on an APSC camera // the image is in soft focus

In part 1 of this three part guide to the exposure triangle I explained what shutter speed is, how it can help you freeze or blur motion, and how to avoid blurry photos resulting from holding your camera in your hands. After playing around in shutter priority mode for a while, I hope that you have a better idea of how shutter speed shapes your images. In part 2, I’m now going to explain what aperture is in photography and how you can use it to photograph anything from portraits with beautifully soft backgrounds to detailed landscape photographs in the mountains.

A detailed Guide to the Exposure Triangle

What is aperture?

I think the best way to explain aperture is visually, so imagine a simple garden hosepipe. Assuming that pressure remains constant, the diameter or size of the cross section of your pipe controls how much water can pass through in a given time. The greater the diameter and therefore surface area of the pipe and opening, the greater the volume of water that passes through the pipe in a given time.

The diameter of your pipe will also influence the nature of the water stream it produces. A wide or broad pipe with a big opening creates a soft stream of water, useful for washing your dog or watering the plants without destroying their leaves and petals or creating holes in the topsoil. A narrow pipe will produce a hard, focused stream of water that can be useful for pressure hosing mud off your bike or moss off your terrace or patio.

Let’s move back to photography. While diameter describes how wide your pipe is, aperture describes how open your lens is and, along with shutter speed and ISO, how much light reaches your sensor during your exposure.

Most lenses can be adjusted to a range of settings between ‘wide open’ (for example F3.5 on a standard kit zoom) and ‘stopped down’ (for example F16 or F22). The opening is controlled by aperture blades which click open and closed as you press a button or move a dial. You can also buy ‘bright’ or ‘fast’ lenses that allow even more light in, for example lenses with a maximum aperture of F1.8, F1.4 or F1.2. These are often prime lenses (you zoom with your feet) and they vary in both quality and price, with many 50mm F1.8 lenses being a good balance between the two for beginners.

Soft Focus & Aperture

Zuiko OM 55mm F1.2 // note how soft the image is, even in the in-focus areas (the bouquet and glasses)

Just like diameter with our hosepipe example, aperture also controls how focused the light is and how soft or detailed your final image is. Please note, the softness of an image, as rendered by shooting at a wide aperture, is different from the depth of field of an image. I will explain both.

An image shot at F1.2 will be much softer in detail in the in-focus parts of the image than an image at F16 will be in the in-focus range. Knowing this can be useful for portrait photography, as we don’t always want every line and wrinkle to be rendered in clinical detail. In other settings, for example landscape photography or medical photography, capturing every part of the image in sharp, precise detail is exactly what you may want or need.

Historically, soft lenses have been considered to be inferior optically than an equivalent lens that renders detail in greater clarity. Most modern lenses are quite well balanced, and will render sharp detail in the in-focus range. A good modern lens can be very sharp ‘wide open’ at F1.8 or F1.4 and stopped down at F16 or F22. The same can’t be said of my beloved Helios 44M with its swirly bokeh and soft details! I love using aperture to create soft, dreamy, almost painterly images. It is an attribute of the lens, and if you are curious to try it then I encourage you to experiment with old, film-era lenses. I have written a post with more details about how to make your images look like film, or soft and dreamy, and you can find it here.

Depth of Field

You might have heard the phrase ‘depth of field’ but not be too sure what it means. Depth of field refers to how much of your image is in focus. Picture a person stood in front of a lake, with trees on the opposite bank behind them, and in the distance a mountain range. If you were to place your camera on a tripod and snap a photo at every aperture range from ‘wide open’ at F1.8 to ‘stopped down’ at F22, you would find that more and more of the background came into sharp focus.

Using a shallow depth of field (F2) allowed me to focus on the little boy and his gong in the CNY procession in London in 2015

At F1.8 you might find that only your subject’s eyes are in focus. At F4, perhaps the swans on the lake behind them are in focus too. At F8, your subject, the swans on the lake and the trees on the opposite banks are in sharp detail, and at F22 everything from your subject’s eyes to the climbers walking on the mountain ridge in the distance will be in focus.

Shooting at a narrower aperture (F8) helped me make sure the whole of Mont-Saint-Michel was in focus, together with much of the foreground
A narrow aperture keeps the water in the foreground, the locks in the middle distance and the castle on the horizon in sharp focus

As you can see, aperture greatly influences how soft or detailed your image is, as well as how much of the foreground, middle and background is in focus (your depth of field).

I should mention at this stage that a lens will behave very differently on a crop sensor camera to on a full frame camera. For example, if you own a 50mm F1.8 and put it on your XXD or XXXD range crop sensor Canon, the depth of field will be deeper than if you were to mount the same lens on an XD range full frame Canon camera. This is just basic physics; the larger the sensor, the shallower the depth of field at the same aperture. This is why it can be quite tricky to achieve a very shallow depth of field on a mobile phone or on a Micro Four Thirds camera. It’s not impossible though. Let me explain.

How to emphasise depth of field on a crop sensor camera

A trick which you can use to emphasise depth of field, either to make it very shallow or to make it very deep, is how close you stand to your subject. Simply put, the closer you are to your subject, the shallower the depth of field will be, regardless of whether you are shooting wide open or stopped down. If your camera is a crop sensor camera, you can use this piece of knowledge to make your images look like they were taken on a camera with a larger sensor. If your camera is a full frame camera and you find that your depth of field is too shallow, then allow a little more space between your camera and your subject to bring more of your image into sharp focus.

I was very close to the leaves, and I used a wide apertrue (F1.4) which contributed to the very shallow depth of field you can see in this image

How to get started with manual aperture selection

Now that you have a better idea of what aperture is and how different aperture settings can help you create very different images on a variety of cameras, it’s time to get some practice.

Just as you did when you were first learning about shutter speed in part 1, I encourage you to use another semi-automatic mode on your DSLR or mirrorless camera. Aperture priority mode allows you to select the aperture, safe in the knowledge that the camera will handle shutter speed and ISO for you. Make sure that ISO is set to ‘auto’.

It’s best if you can practice in good light, for example outside on a bright day. Start by setting your aperture to the widest setting you have available. On most kit lenses, this will be F3.5. An ideal composition for learning aperture is something where you have lots of layers. If you can go to a local park where you have lots of space, trees and plants to photograph, you will be able to find a suitable composition for trying out a range of settings. Try taking the same photo at F3.5, F5, F8 and F16. If you have a very bright lens, for example a 50mm F1.8 then depth of field between the brightest (f1.8) and darkest (F16 or F22) will be even more pronounced.

A note on a shutter speed

As one of the aims of experimenting with the different modes on your camera is to gain an understanding of the exposure triangle as a whole, it’s important to keep an eye on how your ISO and shutter speed are changed as you stop down or open up your aperture. Keep an eye on shutter speed especially. You don’t want it to go too slow as you may introduce blur into the frame from your hands. Refer back to part 1 of this tutorial for a very easy rule of thumb to avoid blurry photos.

I hope you have found this tutorial helpful and that you now feel more confident in your understanding of both shutter speed and aperture in photography. In the final part of this tutorial, I will explain what ISO is as well as how you can start to bring all three sides of the exposure triangle together to set your exposure manually and take full creative control of your photography

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