digital visualisation of a camera. Image supplied by GraphicStock

Camera Basics #1 – Aperture

A box with a hole in it.

Take away all the fancy electronics and the lenses and your camera is basically a box with a hole in it. To that we add something for recording the light that comes through that hole, and some glass to focus that light in various ways. But essentially, it’s a box with a hole. Remember those science lessons where they talked about pinhole cameras, while you daydreamed about what you were going to do as soon as the end-of-day bell rang? Yep, we’re talking about the same thing, with bells and whistles. Well not actual bells and whistles, you might get some pretty odd looks.

A camera?

In this first installment, let’s talk about that hole. It’s there to let in the light in a controlled way. Whatever the medium for recording that light, film or electronic sensor, control of the amount of light that reaches it is essential. Even if you’re relatively new to photography, you’ve probably heard of the terms “underexposed” and “overexposed”. This refers to having too little light and too much light reach the recording medium, respectively. Within reasonable limits, this can be used creatively, but the effect of way too much light is a white photo, while the effect of way too little is an image too dark to make out any detail – to all intents and purposes, a black photo.

So if the hole is there to let light in, in a controlled way, how do we control it? And what’s this got to do with aperture? We refer to the size of the hole as aperture, or aperture size. But mostly aperture. Like every other group of people with something in common, we like to give fancy names to things, I guess. Though I suspect it was actually some scientist who came up with it. Men in white coats, huh. Anyway, to control the light getting in, we can do one of three things. We can alter the amount of time the aperture is open for, we can alter the size of the hole, or we can make our recording medium react faster (or simulate the same effect through amplifying the light) to the light. The first of these is  controlled via a shutter, and we shall discuss it in Camera Basics #2. The third relates to ISO (aka film speed), and we shall discuss that in Camera Basics #3. The second, the size of the hole, is what we’re talking about here. It’s important to note that there is a relationship between all 3.

Here we see a circle representing a wider aperture (left) and one representing a small one (right). All other things being equal, more light gets into the wider aperture.

When people talk about aperture, they usually refer to it as “f something” or more accurately “f/something”. No, this isn’t because they’re swearing at their cameras. Think of the “f” as being the hole fully open. Because we’re talking “f/something” or “f divided by something”, the larger the number for that something, the smaller the hole.  So half a hole (or f/2) really is smaller than a hole… As well as altering the amount of light reaching the recording medium, altering the size of the aperture has a side effect. There’s this thing called depth of field, which is probably best demonstrated by showing two images which have been taken with different aperture sizes.

Photograph of trumpet player
In this photograph, the background – in this case the man too – is greatly blurred, leaving focus on the horn of the trumpet he’s playing. This was shot at f/2.8, a wide aperture.
Photograph of Lucy Futcher
This photograph was taken with a small aperture (f/22) – see how the background, while slightly blurred, is still sharp enough to distinguish features. The model, Lucy Futcher, is rendered sharply from back to front.

We can see the big differences between these two images in terms of their sharpness. Depth of field is used to describe how much of the image is at an acceptable level of sharpness. So f/2.8 has a shallow depth of field while f/22 has a deep depth of field. Depth of field for a particular aperture is relative to the focal length. “Focal length?!? Huh???” you exclaim…Focal length is measured in mm, and could be described as how much the lens magnifies the image it sees. The zoominess, if you like. A magnifying glass. Without going into too much detail, it’s something to bear in mind. If you’re shooting wide-angle, for example 24mm, your depth of field is greater than at 200mm (telefocus).

So why would you use a wide aperture or a small one?

Creative reasons is one possibility – to intentionally blur out details that you don’t want to have in focus, like backgrounds. This is one of those things which makes people think “professional photo” if done right, when they are looking at indoor or outdoor shots. Not that there’s anything wrong with photos which show more detail, of course. Most likely it’s the association of lots of depth of field with compact camera photos with built in flash. The reason for having the backgrounds blurred is that we tend to focus on what’s in focus, rather than what isn’t. In the trumpet player photograph, I wanted to draw attention to the horn of the trumpet, rather than the player, so I focussed the camera on the former. Yes, you can do all this in Photoshop with a photograph that’s got good front to back sharpness, but that can take time and careful effort to do convincingly. Better to get it how you want it up front.

Another reason is the control of light we talked about. On a bright sunny day, we might need to overpower the light of the sun and use our own lighting effectively instead. In order to do this, we need to let in less of the ambient light (the light that’s made by the sun) and more of our own (made by our portable studio flashes). So we shoot at a small aperture like f/22, and use our flashes on full power. We could additionally use our neutral density filter (think sunglasses for a camera) to darken the light before it gets to our camera, thus allowing us to use wider apertures – because there’s less light reaching the camera, we’re opening the aperture wider to get the required amount of light in.

Let’s take the flip side of this. If we were in a nightclub or at a live music performance, there isn’t usually a huge amount of light available. I don’t want to talk much about shutter speed at this stage, but suffice it to say, that if we are shooting a live music performance, we might well have to shoot without flash – and we need to freeze the movement of the performers to avoid blurred images. The only way to do this is to use fast shutter speeds. Unfortunately this limits the amount of light reaching the camera. So we need to open the aperture pretty wide (f/2.8 is typical) to compensate, as well as raising the speed at which the recording media reacts to light. From the camera’s point of view, even at major venues, we’re talking DARK CITY. Even if we are allowed to use flash in a relatively dark situation (e.g. an indoors location like at a party), the chances are we need to blend our camera with the available light. This is a whole topic in it’s own right, for later discussion.

There’s something that’s worth mentioning about wide apertures in regards to people who are on the move. Because of the relatively shallow depth of field that you get when zooming in on them, you’ll find that it’s harder to get all your photos in focus. Not only do you have to cope with people moving unpredictably throwing out the camera’s ability to actually get a lock using the autofocus system, you also face the probability that they’ll move just as you click the shutter. Frustrating, but all part of the fun 😉

Okay, that’s about it for now. Stay tuned for the next blog, Camera Basics #2 – Shutter Speed.

Have fun.

Alex

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