If the camera is just a dark box, then the Aperture is the hole that lets the light in. Bigger holes let more light in, and smaller holes let less light in.

The aperture numbers are called f-numbers. Confusingly, bigger holes have smaller numbers like f1.4, f2.8 and f3.2. Smaller holes have bigger numbers like f11, f16 and f22. This is slightly counter-intuitive so I’d just like to make it very clear at the beginning.
The Aperture numbers are standard; f1.4, f1.8, f2, f2.8, f3.2, f4, f5.6, f6.3, f7.1, ….
Yes, you’ll learn these Aperture numbers by practice, but for now just remember that bigger numbers mean smaller holes that let less light into the camera, and smaller numbers mean bigger holes that let more light in.
The difference between the standard aperture numbers is called a Stop. Each ‘Stop’ lets in either double or half as much light as the one next to it. f8 is one Stop darker than f5.6 and lets in half as much light. f11 is one Stop brighter than f8 and lets in twice as much light.
These Stops are standardised units across all cameras measuring how much light is let in; so an Aperture value of f8 on your lens will let in the same amount of light as an Aperture of f8 on an entirely different lens, even though the physical size of the hole itself is likely to be different.
The Shutter Speeds also have standard values with a Stop between them. They are usually encountered as fractions of a second; 1/250 second or 1/60s, for example. Most photographs really are taken in less time than the blink of an eye. You can read more in the chapter on SHUTTER SPEED.
Aperture Priority mode (A or Av) allows you to take control of your camera by deciding which Aperture it will choose; that is, which size hole will be used when you take a picture. It is the most useful of the EXPOSURE MODES.
Your camera will still calculate how much light should be let into the camera so it will take control of your Shutter Speed. Interestingly, it won’t actually choose the standard Shutter Speed values such as 1/60s; it will be more precise; 1/53s, for example.
You can take control of your EXPOSURE by using your EXPOSURE COMPENSATION button and dial; one of the most useful controls on your camera. In Aperture Priority mode, reducing your exposure by one Stop (underexposing by -1 from the camera’s calculated guess) will force your camera to change the Shutter Speed to compensate. The other EXPOSURE MODES work in different ways.
But Aperture isn’t just about how much light is let into your camera. Your choice of Aperture determines what is sharp in your photographs; and how sharp your photographs can be.
The largest Aperture values (so smaller numbers like f1.4) use more of the lens’ glass; this results in lower contrast and a smudging of fine detail. Most lenses are much better when the Aperture is ‘Stopped down’ by a couple of Stops; my Nikon 85mm f1.4 lens has a maximum Aperture of f1.4, but is much sharper and has higher contrast at f2.8.
The smallest Apertures (larger numbers) are affected by the nature of light itself; the light waves refract noticeably when passing through a tiny hole. Think of waves spreading out in semi-circles after they’ve passed through a narrow entry to a harbour to imagine the effect. This means that with smaller Apertures like f22, your photographs will show less contrast and fine details will probably be blurred.
The different CAMERA FORMATS make a difference; large format cameras used by landscape photographers project images onto a much larger area of film so they can use Apertures of f22 or f36 without losing sharpness. Compact cameras use tiny sensors so suffer loss of definition due to refraction much sooner; therefore they are usually limited to a minimum Aperture of f8.
A Medium Format digital camera will be sharper at Apertures of f16 or f22 than a digital 35mm SLR. And a full frame dSLR won’t have as much diffraction at f16 as a camera with an APSc sized sensor.
There is a happy medium that we will want to use to get the sharpness that our lenses and cameras are capable of. The actual sharpest Aperture depends on the lens, but is usually between f3.2 and f8
Try this with your camera so that you can see it in practice. Photograph fine detail (a book with small print from across the room) at the maximum Aperture available on your lens (the lowest number), then f8, then the minimum aperture (the highest number), and compare the results. Make sure your ISO is set to 100. Zoom in to check the pictures.
Bear in mind that if you won’t see much difference on the computer when the pictures are a normal size; this technique is for when you want ultimate sharpness such as when you are creating photographs for printing and display. If you are photographing people, you might even want to choose the maximum Aperture (the lowest number) to blur detail slightly and soften the appearance of pores and imperfections.
Changing the size of the hole actually alters the DEPTH OF FIELD too. You can actually take photographs without a glass lens. The Ancient Greeks discovered that a tiny hole in a dark room projected an image of the outside world upside-down on the wall. Painters used a portable version, a Camera Obscura, as an aid for sketching landscapes.
With a pinhole (literally a tiny hole in a sheet of material like tin foil), you have a way of creating images in the same way. A pinhole is an incredibly small Aperture. This results in very long Shutter Speeds because the hole needs to be open for longer to gather the required amount of light, but everything in your picture will be in focus – sharp – regardless of close or far away it is from the camera.
Pinhole photography isn’t practical, and the results will never be as sharp as using a glass lens to focus the light from your scene. I remember experimenting with them excitedly at 3am but the results were lacklustre; too soft. It did re-inforce that the smaller the Aperture (the higher the number), the more is in focus; and the bigger the Aperture (the lower the number), the shallower the Depth of Field. You’ll come across this term a lot, so let’s explain it.
On your camera, there might be a small symbol; a circle with a line through it. It looks sort of like a rather frugal kebab. This line is to tell you where the film or digital sensor is. I have never found cause to use it; but that’s what it is. Your film or digital sensor is a two dimensional flat rectangle, and light is projected onto it by your lens.
Because it’s flat, it can’t capture three dimensional information. You can focus your lens at a specific distance from the film or sensor; represented by the little line. Essentially, your Depth of Field is how much is in focus in front and behind your point of focus.
Depth of Field extends about a third of the distance in front of your chosen focal distance; and two thirds behind. So there will be more in focus behind your chosen subject than in front of it. The out of focus areas are peculiar to photography but painters have begun to copy the effect. It is a useful way to separate your subject from a busy background.
Why does it happen? Depth of Field is related to rather charmingly named Circles of Confusion. Holistic Photography is fascinated by science’ practical effects; not its numbers and equations. It is enough to imagine that your point of focus is literally that, a point in space. In front and behind of it are cones with the pointy ends touching; sort of like an hour-glass. If you’ve played about with a magnifying glass and the sun, you’ll understand this.
Suppose we focus our lens on a yellow light. When we bring it into focus, it will be sharp; the meeting point of the two cones will be on the light. As we change the focus, we will see the light as a blurry circle of varying size; a cross-section of the cone This is just dealing with one light as a point-source; a small dot in space.
The world is more complicated, and not only will you probably have more lights of different shapes, colours and sizes, you’ll also have the other objects in your scene reflecting light. Each produces its own cone and they all overlap. Smaller Apertures like f22 create narrower cones, so the distance at which things look like they’re sharp is larger; greater (or deeper) Depth of Field. Larger Apertures like f2.8 create cones shaped more like piles of sand than London’s pointy Shard building; so less (or shallower) Depth of Field. Most people have trouble understanding this; it’s quite a strange concept.
What we want to take from this is that when you change your Aperture or your focal distance (how far from your camera you’ve chosen to focus), you also change how the other elements in your scene appear. If I focus my lens on a model’s eyes, both the harbour lights in the background and her earring may be out of focus; but because her earring is closer to my point of focus, its cone of light is closer too; so the Circle of Confusion created by the earring will be smaller. Because the harbour lights are farther away, they will create large blurred Circles of Confusion that will appear dimmer because they’re spread over a greater area.
The out-of-focus larger Circles of Confusion are called Bokeh; a Japanese word. Overlapping soft colours and subtle shapes created by the blurry background can look amazing and a lot of photographers get rather excited about Bokeh. Large Apertures such as f0.95 or f1.2 have a very shallow Depth of Field and create large Circles of Confusion at normal distances; enabling beautiful Bokeh effects if used properly. Bokeh is affected by the lens itself, and all lenses aren’t created equal.
I’m letting you know about these (appropriately named?) Circles of Confusion because they’ve helped me understand Depth of Field. The important learning to take away is that how sharp something looks depends on the size of the Circles of Confusion. Therefore, if you print a photograph very large, its Depth of Field will appear to be much shallower.
If you know that your photographs are only going to be viewed at small sizes on a computer screen, then you can ‘get away’ with less Depth of Field. This means larger Apertures, which means more light let into the camera, which means faster Shutter Speeds. If you plan on printing your photographs very large, the reverse applies.
Depth of Field is also affected by the FOCAL LENGTH of your lens. Low numbers on your lens such as 10mm or 21mm are Wide-Angle lenses. They will show a large area of your scene like looking through binoculars backwards. High numbers like 200mm or 400mm are Telephoto Lenses. They will magnify your scene and capture only a small area like a telescope.
Because Telephoto lenses magnify your scene, the Circle of Confusion look much bigger. Therefore, the apparent Depth of Field is much shallower with Telephoto lenses than with Wide Angle lenses. The Circles of Confusion with Wide Angle lenses are small, so they appear to have enormous Depth of Field; most of the scene seems to be in focus. Your artistic choice of lens will be informed by this difference between the two extremes of FOCAL LENGTH.
The most overlooked way to control your Depth of Field is your focal distance from the camera. If you focus on a subject close to your camera, the cones of light will create larger Circles of Confusion in the background. A Macro lens allows you to focus on subjects that are very close to your camera; this allows you to take photographs of very small things such as coins, magnifying them to fill your frame.
When you focus very close, your Depth of Field, even at small Apertures of f16, is very shallow; often mere millimetres. We’re used to this effect now, so photographs of cityscapes with a very shallow Depth of Field look like they’re miniatures.
One practical application of this knowledge is to change your settings to create the shallowest Depth of Field possible with your lens. Set your lens to its longest focal length (zoom in) and set the maximum Aperture in Aperture Priority Mode (the smallest number; yes, you’ll lose a bit of sharpness as we talked about earlier).
Now, choose MANUAL FOCUS. This allows you to control how far into the scene (from the camera’s film or sensor) your lens will focus. This will be a range from several centimetres (the closest focal distance) to infinity (the horizon, the stars and so on.)
Choose the closest Focal Distance (the opposite end to the infinity sign). Now, you can manually move your camera, with the aid of a tripod if necessary, until your subject is in focus. This technique will give you the maximum magnification and shallowest Depth of Field with that lens.
Knowing about these cones of focus and the Circles of Confusion that determine our Depth of Field, we can understand another creative effect. The Aperture of a lens varies in size using several ’blades’.
Lens makers attempt to make each different Aperture setting as circular as possible. Using more blades increases their ability to do this; think of the shape of an octagon compared to a pentagon for example.
They do this because the Circles of Confusion are actually not circular; their shape is determined by the hole that they enter the camera through; which as we know is the Aperture. More circular Apertures results in more circular Circles of Confusion. This gives us smoother, more attractive Bokeh (out of focus areas).
But we can change the shape of the hole the light enters through too. The front of your lens is circular to let the maximum amount of light in. But we can alter this shape with something as simple as a bit of card with a design cut out of it. Because this will change the shape of the hole, it will change the shape of the Circles of Confusion.
In practice, this means that you can alter the shape of your Bokeh (the out of focus areas of your image) in a way that would take ages in Photoshop. If you photographed a street light out-of-focus in the background and it was a blurry circle before, now you can change its shape. It’s easily overused as an effect, but will get you thinking about your Aperture and Bokeh.
The easiest way to do this is to use a UV filter or other clear filter as a holder. Then, use an image-editing program to create circles of the same size diameter as your filter. I use 77mm filters so my circles are 77mm in diameter. You may find it easier to download some templates for this from the Internet. Then, print them on black card at 100% (not resized) and carefully cut them out.
Photograph point light sources like reflections of the sun in shiny surfaces and street lights for the best effect; the more the are out of focus, the larger the shape, so it’s best to use a large Aperture like f2.8, focus on objects close to the camera and have the lights in the background. Experiment and see what effects you can create.