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Understanding Aperture and How It Can Rock Your Photography!

Aperture is the first point on the exposure triangle that we will look at. When you understand what aperture is, and can control it on your camera, you will gain a huge amount more creative control over your photos than you previously had.

What actually is the aperture?

The word aperture simply means an 'opening' in common usage. The same meaning applies in photography, where the opening is a specific mechanism contained within the lens. The aperture is formed by a set of of thin, interlocking metal blades called the iris. These blades can be adjusted to increase/decrease the size of the aperture. The shape of the opening is roughly circular, and it is the area through which light passes as it is focused by the lens.

Changing aperture size

The size of the lens aperture can be changed to control the amount of light that flows into the camera. We can control the size of the aperture using a series of numbers, or f numbers, that are used to measure the size of the hole. The larger the f number the smaller the aperture, and the smaller the f number the larger the aperture.

So, with a large aperture we can admit lots of light into the camera, whilst a small one will admit less light. But that's not where it ends with the aperture. It also determines the size, or depth, of the area that is within sharp focus. This is called the depth of field. A large aperture produces a small depth of field, whilst a small aperture produces a large one (we'll explore this further in a moment).

Technical point, if you're interested:

Do you remember we looked at f ratios earlier in this series of tutorials? The f ratio (or the f number) is calculated by dividing the focal length by the size of the aperture. So, if we have a focal length of 100mm with an aperture size of 50mm the f ratio is 1:2, or f2. So, by changing the size of the aperture we are changing the f ratio/f number of the lens.

Aperture sizes: f numbers

So, we change the size of the aperture by using a sequence of f numbers. These are also called f stops, because there is a scale of aperture sizes that correlates with exposure stops. This scale is as follows:

f1 f1.4 f2 f2.8 f4 f5.6 f8 f11 f16 f22 f32

Each aperture size on this scale is half the size of the previous one, and double the size of the following one. So, as you switch from f/4 to f/5.6 you halve the amount of light entering the camera. Meanwhile, switching from f/11 to f/8 would double the light entering the camera.

Why isn't the scale just simple whole numbers, like 1,2,3,4.... Because the numbers relate to the diameter of the hole. When the total area of a circle is doubled, its diameter only increases by 1.41. Hence the next step up from f1 is f.1.4, and then 2, 2.8, 4 etc. The sequence continues in this pattern.

If you explore the aperture settings in your camera now, you'll probably notice a different scale of f numbers. Each of the main f stops shown above will be there, but there will be additional numbers too. For example, between f/5.6 and f/8 you may have f6.3 and f7.1.

What's that all about? Quite simply, it's there to give us more control in setting exposure. So, each f number increases or decreases brightness by 1/3 of a stop. Reduce the aperture size on your camera, by increasing the f number once, and see how the exposure meter display shows a decrease in exposure by 1/3 of a stop.

The amount of light admitted by a given f number is the same across different lenses. It's a uniform system. So, if you were shooting at f/8 with a shutter speed of 1/60th and ISO of 100 on a 50mm lens, you would require the same shutter speed and ISO when using an aperture of f/8 with a 70-200mm lens.


Aperture size and depth of field

In a previous tutorial we looked at how the basics of a pin hole camera works. We saw that increasing the size of the hole produced a brighter image, but it was also fuzzy and blurred. Meanwhile using a smaller hole gave a duller image that was also nice and sharp.

Thanks to lenses, this frustrating trade-off is not necessary, because we can precisely focus on our subject. But the size of the hole/aperture does still impact sharpness, in terms of depth of field. The smaller the hole, the larger the area surrounding the focal point that remains in sharp focus.

This is actually a great thing! It means we can use the aperture creatively, deciding when we would like to show everything in focus, and when we'd like to isolate the subject from its surroundings. Both options are effective in different scenarios.

For example, it's very effective to use a wide aperture in portrait photography, which throws the background out of focus (shallow depth of field) and draws attention to the face and emotions of the subject alone. Conversely, landscape and nature photography - where it's great to capture as many intricate details throughout the photo as possible (wide depth of field) - often demand a small aperture.

Landscapes: large depth of field

Image by Daniel Zedda

Portraits: Small depth of field

Image by: Bui Linh Ngan

While we're talking about depth of field, it's worth mentioning that it is also influenced by your proximity to the subject. This is common sense, as it works the same with our own eyes. The closer you are to something, the more shallow the depth of field and vice versa.


Depth of field preview button

The aperture setting you have chosen is not actually set until the moment that you press the shutter release button. Lenses remain set at the widest aperture (lowest f number) by default. So as you peer through the viewfinder, this is what you see.

Wouldn't it be handy if you could preview the depth of field of the photo you are about to actually take. You can! DSLR's have a little button on the front, next to the lens, which can be pressed to give a preview of depth of field. Consult your manual or do a Google search of you're not sure where it is on your camera.

The only problem with this is that the viewfinder will darken considerably if you have chosen a small aperture, since less light is entering. But it still provides a very useful preview.


Controlling aperture on your camera

You can control aperture by either shooting in Manual mode or Aperture Priority mode. Aperture Priority is the most useful shooting mode in DSLR's because it enables you to choose an aperture size, whilst automatically setting shutter speed to produce a balanced exposure.

Thee position of aperture controls varies between digital cameras. Many DSLR's feature physical buttons and dials which can be easily used, even as you look through the viewfinder. Compact and bridge cameras usually require you to access the menus in order to change the aperture control.

The dial at the bottom right of this picture is the one I use to control aperture on my Nikon DSLR:

When you are shooting in Aperture Priority mode, be sure to keep an eye on the shutter speed. When hand-holding a camera, speeds below 1/50th often tend to result in images that are not acceptably sharp (we'll look more at shutter speed in the next tutorial).


The trade-off with shutter speed and ISO

As we know, aperture is just one point on the exposure triangle. Shutter speed and ISO are explored in more depth in other tutorials. But suffice to say that when you change the aperture setting, shutter speed, or sometimes ISO, will in turn need to be changed to maintain the same exposure value for the photo.

For example, if you make the aperture bigger by one stop (from f4 to f2.8), increasing the light hitting the sensor, you will have to increase the shutter speed by the equivalent, to compensate by reducing the time for which the light is admitted.

If you want to reduce the aperture size to increase depth of field, but the light is dim and you cannot slow the shutter speed without producing camera shake, increase the ISO (which increases the speed an image is formed by the sensor).


How does aperture relate to lens speed?

Lens speed simply refers to the maximum available aperture size in a given lens. The larger this is, the faster the shutter speeds that can be used for taking pictures - hence 'speed'.

Lenses differ in their maximum aperture settings - for some it may be f/4, for others it may be f/1.4. If a lens has a maximum aperture that is faster than most others available in its category, then it can be described as a fast lens.

But speed is certainly not the only benefit of a fast lens. The larger aperture of fast lenses makes it possible to create a really shallow depth of field. This produces a lovely soft, hazy blur, which sometimes appears as bright orbs (bokeh).


Choosing a fast lens

The speed of a lens (its lowest possible f ratio, i.e. f1.4) is one of the most important considerations when choosing a lens to buy. Fast prime lenses, which have a fixed focal length, can be purchased much more affordably than fast zoom lenses, as a general rule. In other words, you can bag yourself an f1.8 50mm prime for a lot less than you would pay for an f1.8 24-100mm zoom lens for example. Add to that the superior glass quality of many prime lenses, and you'll soon realize the advantage of them.

Nikon Prime Lenses Canon Prime Lenses

The larger the focal length of a lens becomes, the harder it is for the the f ratio to remain low. So telephoto lenses are usually not as fast as many smaller focal length lenses. Professional sports photographers pay vast sums of money for long lenses with large maximum apertures.


The problems with shooting at extreme apertures

Sometimes shooting at the widest possible aperture setting of a lens can result in impaired sharpness. As a result, it's often worth pulling back slightly fron this maximum setting. For example, shooting at f2.8 instead of f1.4.

In fact, most lenses deliver their best optical performance about 2 stops down from wide open - somewhere in the region of f/8. Test some sample shots with your lenses to see where they perform best.

There can also be a problem when shooting with really narrow apertures. The intention of using a narrow aperture is to create a large depth of field, but when the aperture is too narrow something called diffraction can occur, which actually reduces sharpness.

Diffraction is where light comes into contact with the edges of the aperture (the blades of the iris) and is slightly bent (refracted). With a wide aperture, only a small proportion of the light is affected. But when the aperture is small, a greater proportion is affectd and the result is more obvious.

So, when shooting landscape pictures, be aware that you do not have to stop the aperture down as far as possible. Indeed, diffraction is often more of a problem with wide angle lenses (used for landscape photgraphy) so this is important to remember if you enjoy photographing landscapes.


Jargon re-cap

So you've taken on quite a bit of new jargon in this tutorial! Here's a quick re-cap of some key terms:

Iris: the interlocking blades that form the aperture.

F number/stop: the number that denotes aperture size.

Stop up/stop down: increase aperture size/decrease aperture size.

Shallow depth of field: a small area in sharp focus.

Shoot wide open: select the maximum aperture setting.

Prime lens: a fixed focal length lens.

Fast lens: a lens with a large maximum aperture enabling fast shutter speeds.

Diffraction: Loss of sharpness due to a very narrow aperture, cancelling out the benefit of a large depth of field.



So that's the end of our introduction to aperture. You now know what the aperture is, how it is adjusted using f numbers, how it impacts depth of field, what is meant by lens speed and the dangers of shooting at the extremes of aperture. Please feel free to leave any questions or feedback in the comments. Next up - shutter speed...

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