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Exposure (Part 2)
Stops / Exposure Value (EV)
You may have already heard people say things like "Open the aperture up one stop" or "the image is one stop underexposed" (see below). If you haven't heard these phrases, trust us you will, so we shall explain what they mean. First of all here are a couple of lists:
Apertures - f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32
Shutter speeds - 1s, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000
These don't represent all possible aperture and shutter speed settings - they are just a set of them. The ones available to you will depend on your camera and lens.
The lists above are progressions of stops. If you open the aperture by one stop, e.g. from f/5.6 to f/4, then you double the area of the 'hole' and therefore the amount of light coming through it. If you increase the shutter speed (make it slower) by one stop, you double the time the shutter is open and therefore the amount of light that hits the sensor.
With shutter speeds this is fairly easy to understand. A shutter speed of 1/250 second leaves the shutter open for twice as long as a shutter speed of 1/500 second; a shutter speed of one second leaves the shutter open for half as long as a shutter speed of two seconds. If you leave the shutter open for twice as long, you double the amount of light that gets through.
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With apertures it is less obvious but suffice to say (see maths right) that an aperture of f/5.6 results in a circle with half the area of a circle with an aperture of f/4 therefore half the light gets through. Each stop in the aperture list above results in a circle with half the area to the one on its left.
Stops are often now called exposure values. For example changing the aperture from f/8 to f/5.6 will increase the exposure by +1 EV. Changing from f/8 to f/16 will change the exposure by -2 EV (ie. reduce the exposure as it is negative).
On your camera you are likely to be able to adjust shutter speeds and apertures in 1/2 or 1/3 stop increments. |
More Maths...
50mm f/4 ; diameter=12.5mm
Area = 123mm˛ approx
50mm f/5.6 ; diameter=8.9mm
Area = 62mm˛ approx
Therefore the area of a circle with an aperture of f/4 is twice that of a circle with an aperture of f/5.6.
Another way is to look at the sequence of apertures above and square them all - you get a sequence that doubles each value. |
Finally we have ISO numbers - 100, 200, 400, 800, 1600. Each of these ISO numbers is one stop more sensitive (twice as sensitive) than the one on its left which means it needs half the amount of light to make an image of equivalent brightness.
Controlling Exposure
Despite the fact that there only three controls to think about, this may all seem quite complex. We have heard the phrase "I only want to take a $%##! picture!" a few times and we can understand that it seems there are too many things to think about. However once you have got the hang of this, and it won't take that long, you will be able to combine these three controls to give you an incredible variety of creative possibilities.
Of course all modern cameras have a wide range of modes that govern what you are allowed to set, and what the camera will set automatically for you based on its built in light meter.
| Mode |
Description |
| 'Auto' or 'P' (Programmed AE) |
The camera will set the aperture and the shutter speed for you to get a correctly exposed image. |
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Aperture Priority
(A or Av) |
Sometimes called a semi-automatic mode. You set the aperture and the camera will select a shutter speed that will give a correct exposure for that aperture. |
Shutter Priority
(S or Tv) |
Another semi-automatic mode. You set the shutter speed and the camera will select an appropriate aperture. |
| Manual (M) |
Full manual exposure mode. You can select the aperture and the shutter speed. Usually the camera will show you by some sort of scale whether the values you have selected will give an over or underexposed image. |
The shutter speed, aperture and ISO all work together to give you the required exposure and hopefully an image that is neither to bright nor too dark. The aim is to use these controls to get the right amount of light to hit the sensor. To try and make it clearer, here's an analogy to explain how these controls link together.
Imagine that your camera sensor is a bath and in order to get a correctly exposed image you need to fill the bath with water (which represents light in our analogy). If you don't put enough water in the bath the picture is too dark (underexposed) if you put too much in and the bath overflows then the picture is too bright (overexposed).
To fill this virtual bath with water you have a tap (bath tap or kitchen sink tap it doesn't matter... it could even be a gold-plated mixer tap with attachments - it's just an analogy). The more you turn on the tap, the more water you allow through; this is the aperture. The longer you leave the tap on, the more water gets into the bath; this is the shutter speed. The shallower the bath, the less water you need to fill it; this is the ISO number. An ISO 1600 bath is shallower than an ISO 800 bath and needs half as much water to fill it up.
If we take a standard ISO 100 bath and want to fill it with water to make our image, we can choose to either have the tap (aperture) quite wide open and have the tap on for a short time (a fast shutter speed) or we can narrow the tap opening (aperture) and leave the tap running for much longer (a longer shutter speed).
Equivalent Exposures
Here's something which may not be obvious to you yet but hopefully should be in a moment. The following exposures are all equivalent in that they let the same amount of light (assuming the same amount of light is available) and you don't change the ISO value (the depth of the bath):
1. 1/1000 at f/4
2. 1/500 at f/5.6
3. 1/250 at f/8
4. 1/125 at f/11
5. 1/60 at f/16
6. 1/30 at f/22
7. 1/15 at f/32
They all have the same exposure value.
This is because f/4 lets in twice as much light as f/5.6, but in our example above the shutter is open for half as long ie. 1/1000 sec vs 1/500 sec. To go with our bath-time analogy above, in exposure 1 we have opened that tap wider than in exposure 2 but left it on for a shorter time. Both result in the bath getting full but in different ways.
You may now be wondering why anyone would want to chose exposures 2, 3 and 4 if exposure 1 does the business just fine. The next section on exposure will introduce you to the creative possibilities provided by these controls.
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