Photography News | Issue 37 | absolutephoto.com
Technique 59
NEWSERIES
Camera School Here we lift the lid on all things camera related, showing how to get better results from your DSLR and providing all the info you don’t find in the manual. So, stick with us and you’ll soon wield your camera like a pro. This month, how three factors – shutter speed, aperture and ISO – combine to make an exposure
Words & pictures by Kingsley Singleton
How much light do I need for an exposure? This calls for an analogy, and one of the best out there compares an exposure to the filling of a glass with water. Imagine the sensor is the glass and the water is the light. The glass needs to be filled by just the right amount to create a good exposure. Too little and there won’t be enough to make a good exposure (too dark); too much and the quantity will be too high to make a good exposure (too light). Now, you can tip water into the glass slowly or fast (the shutter speed), and using a broad or narrow stream (the aperture). Either way, when you get to the top of the glass you’ve got enough light. The size of the glass – the amount of light you need – is linked to the ISO setting. In short, it doesn’t matter how you fill the glass, either with a lot of water quickly, or a little over a longer period, the resulting brightness will be the same. Up and down, hand in hand In practice, this means that, if you open the aperture by a stop and make the shutter speed twice as slow (also a stop), you’ll record more light. But if you open the aperture by a stop and make the shutter speed twice as fast, no change will be visible
The exposure of a picture – how light or dark it is – is governed by three factors: the speed the shutter opens and closes, the aperture of the lens and the ISO setting of the sensor. Varying any of these factors individually can make a picture lighter or darker, or they can be used in concert. Which factors you can change depends on the exposure mode. In Auto you can’t change any; in manual (M) you can change all of them. In semi-automatic modes, like aperture- priority (Aor Av), you can set aperture and ISO, and in shutter- priority (S or Tv), you can set shutter speed and ISO. Stop right there Understanding how these factors are linked is vital if you’re going to take control of exposure. The way to unlock it is to realise each is consistently measured against the others in ‘stops’ or ‘f/stops’; each stop represents a doubling or halving of the light that you’re letting onto the camera sensor. So, if you double or halve the shutter speed you’re changing the amount of light recorded by one stop. And if you open or close the aperture by a stop, you’re doubling or halving the amount of light reaching the sensor. And if you double or halve the ISO setting you’re also amplifying the light recorded by a stop.
in the light level as the same amount of light will be recorded. Similarly, if you close the aperture by a stop and double the length of the shutter speed, the same amount of light will be recorded. But if you close the aperture and make the shutter speed twice as fast you’ll record a lot less light. Don’t worry, it’s only maths When it comes to picking actual exposure settings, it’s easy to work out the stops for shutter speed and aperture as they’re numbers; 1/25sec becomes 1/50sec, and ISO 400 becomes ISO 200, etc. With the f/stops which determine the aperture of the lens it’s a bit more tricky reconcile their numbers denoting a halving or doubling of light, so they’re best learnt as a sequence (f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and so on) – we’ll go into why they’re called what they in a later instalment. Secondary effects Although we (correctly) tend to think of the aperture, shutter speed and ISOas affecting how light or dark the picture is, each has a secondary effect. Juggling them determines the overall look of the image as well as its brightness, but the reason you juggle them is to make the most of these secondary effects.
The exposure triangle and secondary effects Each side of the triangle represents 8 stops of exposure latitude. Move one stop on one side and you would need to move one stop on one of the other sides to produce the same brightness of exposure. So, move from f/8 to f/11 and you would need to move from 1/30sec to 1/15sec; move from 1/1000sec to 1/250sec and you would need to move from ISO 200 to ISO 800. Each of these movements also has a secondary effect as outlined in the boxes.
ISO 12800
SHALLOW
CLEAN
f/2.8
news Photography
f/16
Secondary effects of aperture (depth-of-field) Widening or narrowing the aperture means you record a larger or smaller volume of light within the time the shutter is open. It also affects the depth-of-field of the image. At wide apertures, like f/2.8, less of the scene will be in sharp focus than at narrower apertures like f/16.
ISO 100
DEEP
(SLOW) SHUTTER SPEED (FAST)
GRAINY
Secondary effects of ISO (digital noise) Raising or lowering the ISO amplifies the signal from the sensor, so more or less light is required to produce a ‘good’ exposure. As a byproduct of raising ISO, interference appears in the image as noise. This is virtually invisible up to a point, but beyond a certain level it begins to degrade image sharpness and colour.
1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000
SHARP
BLURRED
MOVEMENT
1/60sec
20secs
Secondary effects of shutter speed (movement) Raising or lowering the shutter speed means you record the light for a longer or shorter period. This also affects how moving subjects (or movement of the camera itself) is rendered. Slow shutter speeds show more movement, which will be recorded as a motion blur, while fast shutter speeds show less movement keeping things clearer.
Nextmonth How the metering mode you choose affects your exposure setting, and who needs a ‘good’ exposure anyway?
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