Building Blocks: Aperture & Shutter Speed


Words and pictures by James Gifford.

Confused? When you buy your first DSLR camera, all its functions can seem pretty daunting; a fact which is not helped by an instruction manual which appears to be written in a different language. The temptation is to switch everything to automatic or – if you’re feeling really adventurous – choose the ‘stick-figure’ running man on your mode dial. While you might end up with some reasonable images, an understanding of the concepts of aperture and shutter speed will allow you to choose how to portray the scene, and dramatically improve your photographs.

It’s a Balancing Act The primary goal is to expose your image correctly, (so it is neither too light, nor too dark) by ensuring that the correct amount of light is allowed through the hole onto the sensor of your camera.

There are two ways of adjusting this. You can either adjust the size of the hole (the aperture), or you can adjust how long the hole is open for (the shutter speed). These two functions work in tandem; so, for a given scene, the larger you make the hole (or the wider the aperture), the shorter the time the hole must be open (the faster the shutter speed), to allow the same amount of light in, and to ensure that the image is still correctly exposed.

Thus for any scene, there are several combinations of apertures and shutter speeds which will all result in the image being correctly exposed. (See table). Shutter speeds are expressed as fractions of a second, so 1 000 is 1/1 000th of a second (very fast), 10 = 1/10th of a second (quite slow), 10’’ = 10 seconds (very, very slow). The f-number on your camera’s display represents the aperture. Somewhat counter-intuitively, a low number (f/4) equates to a wide hole, while a high number (f/18) results in a narrow hole.

What’s the point?
Good question – but quite an important one, actually. If you wanted to create a panning effect (as we discussed in the last issue), then you would need to use a slow shutter speed. If, however, you wanted to freeze the action so that you could see tiny droplets of water, then you would select a fast shutter speed. The aperture controls the depth of field – i.e. how much of the picture is in focus. Have you ever wondered how photographers capture that classic wildlife portrait with a completely blurred background? Choosing a wide aperture (low f-number) gives you a shallow depth of field, blurring the background so that just your subject is in focus. In contrast, a narrow aperture (high f-number) gives you a greater depth of field; ideal for landscapes when you want the whole picture in focus.

The Wonders of Modern Technology
If this all seems like a lot to think about, don’t panic – your camera will help. On your mode dial (the one with the running man), there are two shooting modes designed to make things easier. A is Aperture priority – you select the aperture (the f-number) and the camera will automatically calculate the shutter speed to make sure the image is correctly exposed. S or Tv is Shutter priority. Here, you select the shutter speed and the camera will pick the appropriate aperture to expose the image correctly.

Fast, Slow, Shallow, Deep – The Choice is Yours
Look at the scene in front of you and decide what type of image you want to create. If a cheetah is chasing a springbok amidst a cloud of flying dust, select Shutter Priority (S/Tv) and choose a fast shutter speed like 1/1600 to freeze the movement, and the camera will select the appropriate aperture. If you want to record every detail of a mesmerising landscape, switch to Aperture Priority (A) and choose a narrow aperture, say f/18, to maximise the depth of field so that everything is in focus. Just remember that narrowing the aperture will result in a slower shutter speed, so you may need a tripod to avoid camera-shake…

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