Your camera has two controls that vary the amount of light entering (the exposure) and creating your photograph

The Shutter

Shutter speeds are expressed as seconds or fractions of a second so a typical scale on a mechanical camera will look like this:

2 1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000 1/2000

When the amount of light let in doubles or halves the difference between the two values is 1 EV (often called a stop).

Digital cameras can set any desired shutter speed but the same principle applies.

When the subject of your photograph is moving a slower shutter speed will allow it to blur as the movement continues during the exposure. Faster shutter speeds may freeze the motion; depending on how fast it is going.

You should also consider that we humans tend not to hold cameras very steady. The subject may be rock solid but if the camera moves during the exposure the picture will still be blurred. This effect is called camera shake and is even more pronounced with telephoto lenses.

The average person can hold a camera steady at around 1/60 or 1/30 sec or 1/15 or 1/8 with a VR (vibration reduction) lens but these speeds reduce dramatically if you zoom in or use a telephoto lens.

The camera may warn you by displaying a red shaking hand when the shutter speed is too low. It is often possible to take photographs at quite low shutter speeds if you hold the camera correctly, brace yourself against something solid or rest the camera on something. At really low shutter speeds you will need to use a tripod.

We can set the shutter speed we want by selecting TV (Time Variable) or S (Shutter Priority) on older cameras. The camera then meters the light to maintain the correct exposure for our selected shutter speed.

Aperture and Depth of Field.

The second exposure control on a camera is the aperture. This is set by an iris which opens and closes behind the lens to form a round hole that lets light onto the sensor. The wider the iris opens the more light it lets in and the width of the aperture is measured on a scale of f stops. This can be a little confusing as the bigger the f stop number the smaller the aperture and the value of the numbers used to denote various stops are generated by a complicated formula.

f2.8 f4 f5.6 f8 f11 f16 f22

f22 is much smaller and lets in much less light than f2.8.

This complex scale isn’t made any easier to understand by the fact that your digital camera can set any value in between these numbers as well so you may look in the viewfinder and see that it has set f4.6 or f12. Don’t panic – all you need to remember is that a bigger number means a smaller hole and a smaller number a wider hole.

If our camera measures the exposure and the brightness of the light required 1/125 @ f8 we could also set 1/250 @ f5.6 or 1/60 @ f11. Each has the same EV.

We can set the aperture we want by selecting AV (Aperture Variable) or A (Aperture Priority) on older cameras. The camera then meters the light and controls the shutter speed to maintain the correct exposure at our selected aperture. Most digital cameras warn you when the shutter speed is slow enough to cause camera shake by displaying a red hand symbol.

Modern digital cameras can set a wide range of shutter speeds, often anything from 1/2000th of a second to 30mins but have a very restricted range of apertures.

Often the maximum aperture of a lens designed for amateur use is only f5.6 which means that if we set a high shutter speed the camera may not be able to ‘open up’ the aperture enough and the photograph is underexposed (too dark). At the other end of the scale if we choose too long a shutter speed the aperture may not be able to close down enough and the picture is overexposed (too light).

Why do we need to know this at all?

The aperture has an effect on the depth of field in our photograph (also referred to as depth of focus). This refers to how much of the image in front of and behind the point at which we focus is in focus. Larger apertures (smaller numbers) give less depth of field than small apertures (big numbers).

So imagine I am on holiday and want to take a picture of someone standing in front of a stately home. The camera auto-focuses on the person in the foreground and the house behind is out of focus. If I set the camera to aperture priority and select a smaller aperture the depth of field will increase and both will appear relatively sharp.

If I want to take a portrait with a nice out of focus background I will set a wide aperture and focus on the person.

There are a couple of things you will need to remember when doing this. The first is that the camera needs a fixed amount of to light create an image. If we cut down the amount of light hitting the sensor by making the aperture smaller the shutter will have to stay open longer to let in enough light.

This may mean that the shutter speed drops below what you can hand-hold the camera steady enough for so keep an eye on the shutter speed the camera is selecting as you make the aperture smaller.

Secondly, the effect of the aperture varies with the focal length of the lens. If we zoom out to a short focal length (wide angle view) the depth of field for any given aperture is a lot more than if we increase the focal length of the lens (zoom in) and, just to be really awkward, decreases the closer we get to something.

To sum up

When we set the camera to P (Program) or Auto the camera will select the best (it thinks) aperture and shutter speed.

We can set the shutter speed we want by selecting TV (Time Variable) or S (Shutter Priority) on older cameras. The camera then meters the light and controls the aperture to maintain the correct exposure for our selected shutter speed.

If we want to photograph a landscape and have the flowers in the foreground and the mountains in the background in sharp focus and where nothing much is moving we would set our camera on a tripod, set the lens to wide angle and select the smallest aperture possible. The shutter speed might be quite long but the camera won’t wobble during the exposure – just watch out for the flowers blowing in the wind!

To create a portrait with a soft, out of focus background we zoom in on the subject’s face so that we are using a long focal length, set a wide aperture which will also give us a high shutter speed so we can handhold the camera, make sure the camera is focused on the eyes and take out picture.

Wide aperture (smaller number) = less depth of field = more light = higher shutter speed

Small aperture (bigger number) = more depth of field = less light = lower shutter speed

And finally, your camera will have a minimum and a maximum aperture. Compact cameras often have maximum apertures of f5.6 or f8 and minimums of around f22 whilst DSLRs may have lenses that range from f2 down to f45.

This gives the DSLR user more control over aperture effects. It also means that DSLRs can often work in lower light levels without a flash; the wider maximum aperture means the photographer can use a higher shutter speed in lower light.

Before you start to wish for wide aperture lenses look at the prices, they are much more expensive to produce than smaller aperture ones.

Taking Better Photographs