The Exposure Triangle – Aperture

For me, aperture is the most important setting in day-to-day photography. The reason is that aperture is the setting that has the greatest impact on what you see in a photo. By controlling the aperture, you decide what parts of the scene should be in focus.

Hips on green velvet, by Marius Waldal

What is aperture?

The aperture is the opening – the hole if you like – in the lens. The opening is adjusted by a diaphragm that controls the size of the hole, stopping various amounts of the light entering the lens from passing through to the sensor/film (the focal plane). Thus, the various aperture sizes refer to “f-stops”.

What many inexperienced photographers have problems grasping is the concept of the f-numbers, or more precisely, the seemingly unintuitive “reverse scale”: higher f-number means smaller opening. The f-number scale is actually dimensionless, meaning that f/4 or f/16 does not refer to a specific size, like 4 and 16 millimeters. If you like techie-stuff, read why in the box to the right. If not, just accept this fact and move on!

What happens when you adjust the aperture is that you increase or decrease the amount of light that is allowed to reach the sensor. This means that wider apertures allows for faster shutter times. Large-aperture lenses is therefore often referred to as fast lenses.

Aperture controls depth of field

While larger aperture makes your lens faster, it also reduces your depth of field (DOF). In some situations, this can be limiting. But most of the time, this is something you can actively choose to use to your advantage.

Small thoughts, by Marius Waldal

Another example is macro or close-up photography. You want the viewer’s attention only on your subject, or maybe just a part of it.

The parts of the photo that are not in focus, is called bokeh. Shallow DOF in a photo may create large amounts of bokeh. In many situations, this can be used actively to “paint” a background for your subject, like in the top photo where the green bushes in the background are so blurred out that they create a soft green colored background.

In other situations, you want as much depth in your photo as possible, e.g. you want most or all of the scene to be sharply focused. The most notable type of scene is of course landscapes. To achieve this you would use a small aperture, like f/16 or even f/22.

The corners of the triangle affect each other

Getting a good grip on all the corners of the triangle is essential, as adjustments to one have impact on the others.

Adjusting the aperture one stop wider or smaller doubles or halves the amount of light reaching the sensor, respectively. Following the same pattern, adjusting the shutter speed one step up or down has the same effect. Let’s say your light meter says your settings will produce a correct exposure. You realize that your aperture is so large that your depth of field will be too shallow for the scene you want to capture. By stopping your aperture down two stops, you need to increase your shutter speed equally two stops to achieve the same exposure. These two settings follow like hand in glove.

But this metaphor is actually not very good, as there is a third element here; the ISO. Your ISO setting has the same effect on exposure “correctness”, meaning that if you stop down your aperture 1 stop, you can go from ISO 100 to ISO 200 to achieve the same exposure.

Shooting in aperture priority mode

Normally when I’m out shooting, I first decide what aperture will fit the scene. If the lighting conditions are not complex (as with strong reflections, large dense shadows etc), I sometimes go for aperture priority mode (A or Av on the mode wheel, depending on your camera). This is faster, as I don’t have to constantly adjust the shutter speed as I move around exploring the scene from different angles. The camera measures the light and determines the shutter speed automatically. This will give me a consistent scene and depth of field and save me some time (and also ensure I don’t freeze my fingers off during the winter. Today, it’s minus 16 degrees (celcius) outside!).

Hettie, Nettie and Lettie, by Marius Waldal

Notice that in the above photo, only the middle duck (Nettie) is in focus. The other two are slightly out of focus, while the background is completely thrown out, revealing only a hint of what is there. When photographing these ducks I had to react quickly to movements, so I chose to use aperture priority mode to reduce the hassle whenever I found a nice scene. As you can guess, the ducks didn’t hold this lineup for a long time…

Although I normally shoot in full manual mode, I’ll definitely recommend those of you that are taking the step from fully automatic to manual modes to use this as a starter. It will give you less to think about, and at the same time all the benefits of shooting in RAW (you are shooting RAW, right? If not, stay tuned for an upcoming article on RAW!).

Large-aperture (fast) lenses

Most “budget”-type zoom lenses are relatively slow, with a maximum aperture around f/4.0. For Canon, if you step up to the “L” lenses, they start at f/2.8. Primary lenses are often even faster.

If you need a lens to take photos hand-held in low-light (like in a wedding, I would recommend a 50mm primary. Canon has three of these:

Canon EF 50mm f/1.8 II – This lens is very fast, and extremely cheap. If you want a fast lens but don’t have the money for it, this gives you a lot of speed for a low price.

My recommendation, though, would be this lens:
Canon EF 50mm f/1.4 USM – This is a very sharp lens with good quality, and with a price that can be justified by many. I use this myself, and I love it.

Canon has an L-series 50mm as well, for those of you with serious needs (or seriously fat wallets):
Canon EF 50mm f/1.2 L USM – This is a high quality lens aimed at professionals, and priced accordingly.

Conclusion

When adjusting your exposure, the aperture is the setting that will more often than not have an “artistic” impact on your composition. Using large aperture will create areas of bokeh in your photo. It is also the most important setting for portrait photography, to separate your subject from the background.

Nevertheless, the two other settings on the exposure triangle, shutter speed and ISO, are settings that you need to know innately to be able to adjust correctly and fast for any given scene. So stay tuned for articles on these settings in the near future!

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1. Take pictures in aperture-priority mode