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Exposure, shutter speed and aperture

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These are the three most important photographic terms. The first two, exposure and shutter speed, are reasonably easy to understand. The effect of varying the lens aperture setting, however, can take a bit of grappling with.

Also, understanding the photographic use of the words ‘speed’, ‘fast’ and ‘slow’ can take some getting used to.

Exposure

This is simply how much light the film or image sensor is exposed to. Film is rated according to its ‘speed’. Film speed is a measure of how much light the film must be exposed to in order to form an accurate image. The slower the film speed, the more light it needs to receive before being correctly exposed. The faster the film speed, the less exposure is required. Digital image sensors tend to have a "native" speed at which they record their best image detail. The sensor speed can be uprated but image quality is degraded, often very little at first, but as increasingly higher speeds are used so the images become increasingly "noisy" as the signal to noise ratio changes.

Using ISO (International Standards Organisation) numbers the relationship between film speeds is directly linear. That is, ISO50 film is half the speed of ISO100, while ISO200 is twice the speed of ISO100, and so on over all film speeds. So ISO800 film is 8× the speed of ISO100 and needs to be exposed to only ¹/₈^th of the light of ISO100 to form an accurate image.

Films with ratings over ISO200 are often referred to as ‘fast’, while those below ISO100 are called ‘slow’. When changing to a film with a high ISO rating, we are using ‘faster’ film, while if we decide to use a lower ISO rating, we are using ‘slower’ film. While digital sensors are infinitely variable in their speed, camera manufacturers have stuck with the ISO method of measurement and digital cameras tend to use the same speed setttings as film cameras. Large digital sensors (not those in compact cameras) produce much "smoother" images at speeds up to ISO800 than film, so many digital SLR and medium format users are quite happy to shoot at ISO400 when they would very seldom have used 400 speeed film.

Because light levels vary throughout the day, with the weather or according to the type of lighting used, we need to be able to measure the light and control how much of it reaches the film or sensor. If we couldn’t do this, accurate exposure would be guesswork.

We measure the prevailing light with a light meter that is bulti into the camera (except in the case of large format view cameras and a few other models) and we use that light meter reading as a guide to accurate exposure. We also use two controls on the camera and lens to adjust exactly how that exposure will be recorded.

These exposure controls are shutter speed and lens aperture.

To understand how the shutter speed and aperture settings work as exposure controls it is important to realise that there is a crucial difference between how film or sensor reacts to light compared to our eye/brain combination.

In dark conditions the human eye adjusts by opening its iris to let in more light. Even so, in dark conditions we can make out very little detail and no colour. It doesn’t matter how long you stare at something, you’ll never see any more detail.

Film is different. Film can accumulate light and the longer it is exposed to light, the more light it accumulates. So, in very dark conditions, film can still form an accurate image provided it is exposed for a long time. Digital sensors can do this too, though often at the expense of increased image noise.

Shutter speed

The shutter is a curtain that blocks light from entering the camera and exposing an image. It remains closed until the moment of exposure, when it opens for a predetermined and variable time, then rapidly closes.

The shutter speed is the amount of time that the shutter is open. All the while the shutter is open, the film or sensor is exposed to light. The longer the shutter is open, the more exposure the received; the shorter the time the shutter is open, the less the exposure to light. Shutter speed could instead be called open shutter duration or shutter time.

Like film speed, shutter speed is a simple linear measure. It is just the passing of time. One second is half the shutter speed of two seconds, but twice as long as half a second and so on. So ¹/₅₀₀^th of a second is twice as long as ¹/₁₀₀₀^th of a second, but half as long as ¹/₂₅₀^th of a second.

The exposure is affected accordingly. If the film or sensor receives a certain amount of light when the shutter is open for one second, it will receive only half as much if the shutter is open for half. Open the shutter for two seconds and the film or sensor will receive twice as much exposure to light than with a one second exposure.

Lens aperture

The aperture is controlled by an iris-like diaphragm within the lens. It is made up of very thin overlapping metal blades that can be adjusted so that more or less light passes through the lens. If we close the aperture down (stop down), the opening it leaves for light to enter is small; if we enlarge the aperture (open up), the opening becomes larger and more light can pass through in a given period of time.

We could control exposure using shutter speed alone; adjusting it up or down (making it faster [briefer] or slower [longer]) to match the prevailing light levels. Indeed, if the lens has a fixed aperture, that is the only way to alter the exposure, but most lenses have an adjustable aperture.

Here is where it starts to seem complicated. It isn’t really, it just needs rather more thinking than film speed or shutter speed to work out what’s going on.

Aperture settings work just like film speed and shutter speed settings. As we move up or down the scale we use a setting that lets in half or twice as much light as the original setting. However, when we adjust the aperture, we’re altering the size of a roughly circular hole. The relationship between the size of that hole (aperture) and the amount of light it lets through is very simple. If we double the size of the aperture, we get twice as much light; if we halve the size of the aperture, half as much light passes through in the same period of time.

The aperture is just a circular hole, but if we’re going to double or halve the size of that hole, we need to know how to do it accurately. The mathematical explanation of this is given later, but for now all you need to know is that the aperture scale (in full divisions or stops) runs as follows:

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc.

Each number along the scale produce an aperture half the size of the preceding one and lets through half the amount of light if exposed for the same amount of time. So if we expose the film or sensor for one second at f/32 it will receive half the amount of light that it would at one second at f/22 but only ¹/₁₀₂₄^th the amount of light that it would if exposed for one second at f/1.

Using a combination of shutter speed and aperture

Just as we could use shutter speed alone to alter exposure, we could also use aperture alone. If the shutter speed is fixed we can open up or stop down the lens to give the film more or less exposure.

But we can do better than that. We can use a combination of shutter speed and aperture settings to achieve all sorts of results. All that matters is that the film or sensor is exposed to the right amount of light to form an image.

Exposure Value Chart

Shutter Speed

This exposure value (EV) chart shows that many different combinations of lens aperture and camera shutter speed can yield the same overall exposure.

Each of the numbers in the main body of the chart indicates a certain intensity of light. This is the exposure value. The higher the number the brighter the prevailing light level. A sunny day may be exposure value 13, 14 or 15; a dull cloudy day will be more like exposure value 8, 9 or 10. This chart is for ISO 100 speed.

As an example, consider the light level of a normal sunny day around midday. With ISO 100 film this would usually result in an exposure of around ¹/₁₂₅^th of a second at f/11. If we look for that combination on the chart we see that it corresponds to exposure value 14. Now if we look for other aperture and shutter speed combinations that also read as exposure value 14 we can see combinations such as ¹/₆₀^th at f/16, ¹/₂₅₀^th at f/8, ¹/₅₀₀^th at f/5.6 and so on.

Any of those EV14 combinations would expose the film or sensor to the same amount of light but the resultant pictures could be very different from one another. If we shot at ¹/₆₀^th at f/16 we would have a picture with good depth of field but any moving objects could be blurred. That would be a good choice for a landscape but not a moving cyclist. For the cyclist we’d be better to choose ¹/₁₀₀₀^th at f/4 to freeze most of the movement, although depth of field would suffer.

The chart below shows all the combinations from the chart above that result in EV 14.

The chart could be extended indefinitely in either direction, i.e to ¹/₈^th at f/44 and smaller apertures or ¹/₄₀₀₀^th at f/2 and faster shutter speeds. It’s exactly like the multiples of a number, which say in the case of 100, are 1 × 100, 2 × 50, 4 × 25, 5 × 20, 10 × 10, 20 × 5, 25 x4, 50 × 2 and 100 × 1. They all equal 100, just as all the exposures in our example equal EV 14.

Notice that in every case as the shutter speed halves, say ¹/₁₂₅^th to ¹/₂₅₀^th, the aperture size doubles, from f/11 to f/8 in this example. Remember shutter speed refers to time, so it’s just like looking at your watch; 1 second is twice as long as ½ a second, which is twice as long as ¼ of a second and so on. The numbers for the aperture settings are not so easy to understand but they too follow a doubling or halving sequence. They are really fractions, just like the shutter speeds, but only the lower part of the fraction, the denominator, is shown. It’s the focal length of the lens over the aperture, say in the case of a 100mm lens, ¹⁰⁰/₁, ¹⁰⁰/_1.4, ¹⁰⁰/₂, ¹⁰⁰/_2.8, and so on.

These charts have been complied using what are known as full stop settings. That is when each succeeding setting or ‘stop’ is double or half the one immediately preceding or following it. However, the change between settings is actually continuous and there’s nothing to prohibit using half-stops, quarter stops or any other division of a stop. For example ¹/₉₀^th of a second at f/13 is also EV14, as is ¹/₇₅₀^th of a second at f/4.5., just as 8 × 12.5 equals 100 as does 12.5 × 8. The key thing is that the reciprocity between aperture and shutter speed remains the same regardless.

The mathematical reasons behind the progression of f-numbers

Remember back to your school days. How do you find the area of a circle? The area of a circle equals pi × r² or pi (3.1459) × the radius of the circle squared. That formula tells us that to halve the area of a circle we have to reduce its diameter by approximately ¹/_1.4 times. Knowing that, we can accurately alter the area of the aperture and adjust the amount of light it lets through. The settings resulting from these adjustments are called f-stops or f-numbers. Why f? Because they refer to the focal length of the lens.

Assuming we have a lens with a 100mm focal length. If the aperture of the lens is 100mm wide, we are at f/1. That is, a 100mm focal length divided by a 100mm aperture. 100/100 = 1. At f/1, a 100mm lens will have an aperture 100mm wide, which equates to an area of pi × 50² or 7865mm².

If we want to halve the size of the aperture, to 3932mm², and let in half the light we need to reduce it by a factor of ¹/_1.4. That means that the size of the aperture will now be 100 × ¹/_1.4 or approximately 71mm in diameter. A circle with a diameter of 71mm has an area of pi × 35.5² or 3965mm², which is roughly the 3932mm² we were looking for. Everything is approximated.

For the next step down — half the light transmission of the 71mm aperture or a quarter of the 100mm aperture — we need an aperture with a diameter of 71 × ¹/_1.4 or 100 × ½. That’s 50mm diameter, which is an area of 1966mm², which is ¼ of the 7865mm² of f/1 and ½ of the 3932 of f/1.4.

The actual size of the aperture is not important. We never talk about using a 100mm lens at its 71.43mm or its 50mm aperture setting. Instead, we talk of using it at f/1.4 or f/2; in other words focal length (f) × ¹/_1.4 or focal length × ½.

As we progress along the aperture scale from maximum aperture (f/1), each succeeding full stop lets through half the light (a aperture with half the area) of the preceding one. The number used for each setting refers to the size of the aperture in relation to f/1.

So we have a sequence of 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc. Note that even though the number doubles at every second interval, it lets through ¼ of the light.

Copyright Geoff Bryant