ALL ABOUT EXPOSURE CONTROL

Controlling exposure is a fundamental of photography, but if you leave it entirely to the camera, you’re missing out on some vital creative possibilities. Here’s what you need to know to access some powerful pictorial tools.

Translated from Greek, the word photography means ‘drawing with light’ and the essence of creating a photographic image lies with controlling exactly how much light reaches the recording medium, whether it’s a digital imaging sensor or a frame of film. Logically, this is called an ‘exposure’ (i.e. the recording medium is exposed to light) and in any camera, it’s essentially regulated by two controls – apertures and shutter speeds. As explained in the previous article about depth-of-field (see our January/ February 2017 issue), the aperture mechanism – also known as the diaphragm – is located within a lens and comprises a set of blades which move to create openings of different sizes. This allows more, or less, light to pass through to the image sensor, and their sizes (in other words, the diameter) are expressed in values called ‘f-stops’. An f-stop is written as the letter ‘f’ followed by a number… i.e. f4.0. These numbers are determined by a simple equation; the focal length of the lens divided by the diameter of the aperture. This results in a numbering sequence which is the opposite to what would seem logical… so the larger apertures have smaller f-numbers and the smaller apertures have larger f-numbers.

In practice then, a lens might have an aperture range from, say, f2.8 – its very largest or widest aperture – to f22 – which is its smallest aperture setting.

As the diaphragm progressively closes down, each aperture setting – which represents one f-stop of adjustment – actually lets in half the amount of light as the previous one. Stops And Speeds You can think of a ‘stop’ as the unit which applies to any exposure adjustment. Shutter speeds are also adjusted in stops, and the doubling or halving relationship between settings again applies.

While the aperture controls the amount of light coming through the lens, the shutter regulates how long the sensor is exposed to it… i.e. the duration of the exposure. This is done by opening the shutter

for a period of time and then closing it. These timed settings are called the ‘speed’ and they can vary from a mere fraction of a second up to several hours or even longer.

Short – or fast – shutter speeds are typically used in very bright conditions while slow shutters speeds – a.k.a. long exposures – are required when there’s very little available light (i.e. at night). Fast shutter speeds are considered to be those from around 1/60 second up to 1/8000 second while slow speeds extend from 1/30 second to around 30 seconds or perhaps 60 seconds. This is generally the timed speed range available on a digital SLR or mirrorless camera, but there are provisions for making much longer exposures that you time yourself via ‘B’ (for bulb) or ‘T’ (for time) settings.

As with f-stops, the difference between each shutter speed setting – for example 1/30 second and 1/60 second – is one stop, but modern cameras offer intermediate settings representing one-third stop or half stop adjustments such as 1/80 second (which is 1/60 second less one-third of a stop) or 1/400 second (which is 1/250 second less two-thirds of a stop). Remember that the faster shutter speeds progressively reduce the overall exposure with, for example, the 1/500 second setting halving the exposure from 1/250 second. Timing In a digital camera, exposure times can be controlled via a physical shutter or by switching the imaging sensor on and off. The advantages of the sensor-based shutter are that it is completely silent and eliminates any vibrations.

However, there can be some issues when shooting fast-moving subjects so a conventional shutter mechanism is retained.

In D-SLRs and digital mirrorless cameras, this is a focal plane (FP) type shutter, so-called because it’s located in the camera body just in front of the image plane (i.e. the receptor surface of the sensor). An FP shutter employs thin metal (or sometimes composite) blades which run vertically and open and close to control the exposure timing. With very fast settings, this opening will merely be a slit – of varying widths – that scans across the front of the sensor, but from about 1/60 second and slower, the shutter will open completely. The fastest speed at which the shutter is fully opened is also the maximum speed for using flash – and hence known as the flash sync speed – because otherwise part of the shutter would still be in front of the sensor when the flash is fired (so that part of the image will be significantly underexposed).

Some fixed-lens digital cameras and some of the lenses available for digital medium format camera systems employ leaf-type shutters. These are also known as betweenthe-lens shutters or simply lens shutters because, you guessed it, they’re located within a lens rather than in the camera body. Consequently, leaf shutters are circular in shape and work in much the same way as the aperture diaphragm, except they open fully and it’s the time factor that controls the exposure. This design puts some limits on how fast they can be opened and closed, but flash sync is available at any and all speeds (which is attractive to some professionals such as wedding photographers). Exposure Values Now we know that apertures and shutter speeds both control exposures, then obviously they can be worked together which is important because both adjustments also have visual implications.

Apertures affect the depth-offield – the zones in front of and behind the focusing point that are still sharp – and the shutter speed will affect how movement is recorded in the image. Put simply, a fast shutter speed ‘freezes’ movement while a slower speed will create varying degrees of blurring (simply because the subject moves while the shutter is open). There are obviously creative implications here, so the combination of aperture and shutter speed that you select does more than simply control the exposure. It’s for this reason that being in control of these settings – rather than simply leaving it to the camera – enables you to achieve the visual outcomes that you want or, indeed, which are better matched to the subject or scene.

This is where understanding exposure values comes in handy. At any given light level – as measured by the camera’s exposure meter – the combination of aperture and shutter speed which gives a ‘correct’ exposure is collectively called the exposure value (or EV for short).

But remember that apertures and speeds work together so, if you reduce one by a stop, but then increase the other by a stop, the EV remains exactly the same (i.e. you’ve halved one and doubled the other). You can do this across the full ranges of apertures and speeds available on your camera so the following combinations actually all deliver exactly the same exposure value. You’ll again note that the relationship within any exposure value is simple; if you reduce one by a stop, you have to increase the other by a stop in order to maintain the same amount of exposure. So, for example, if you’re shooting high-speed sports action, you might use 1/2000 second at f2.8, but if you’re shooting close-ups and need maximum depth-of-field, you might use f32 at 1/15 second.

At any given light level, then, there is always a choice of aperture and shutter speed combinations that you can use to obtain the required exposure, and your selection may often be dictated by the nature of the subject itself. Sensitivity There is a third key adjustment which is related to exposure and that’s the sensitivity setting which is expressed in ISO values (primarily to it can be related back to the old film speeds). An imaging sensor’s sensitivity is determined primarily by the size of the pixels, but also the design

of the microlenses which help concentrate the light rays over each photodiode.

Like a film’s speed it is a fixed value – say ISO 200, for example – but digital cameras also allow for higher sensitivities to be set. This is achieved via electronically amplifying the signal from the sensor, but this process also amplifies the interference component which creates ‘noise’. The continued refinement of noise reduction processing is delivering ever better high ISO performance from digital cameras, but there are still some image quality issues caused by the processing’s ‘side effects’ (often called artefacts) which essentially limit the reproduction size when making prints. One option is to shoot RAW files – which aren’t corrected in-camera like JPEGs – and then perform the noise reduction processing later using imageediting software.

Like apertures and shutter speeds, ISO settings are adjusted in stops with, again, the values either doubling or halving with a one-stop adjustment. So, for example, ISO 400 is twice as sensitive as ISO 200, and ISO 100 is half as sensitive as ISO 200. However, again many cameras allow for these adjustments to be made in either one-third stop or half stop increments for finer control. You might select a higher ISO setting, perhaps 800 or 1600, when shooting sports action because it will then allow you to use faster shutter speeds with smaller apertures than a slower ISO setting (at a given exposure value). However, as just noted, the low sensitivity settings will deliver optimum sharpness and detailing, but in low-light situations, you will then need to use very slow shutter speeds and/or larger apertures to obtain the correct exposure (and, consequently, may have to put the camera on a tripod).

The good news is that high-ISO performance – even with smallersized sensors (remember that sensitivity is related to pixel size) – is now good enough to allow shooting at ISO settings that were never available with film. If you’re only making smallish prints (i.e. no bigger than A4) or posting the images online, there’s even more scope for shooting at very high ISO settings such as 12,800, 25,600 or even 51,200. Some stratospheric high ISO settings are now available on higher-end D-SLRs and mirrorless cameras, but the reality remains that noise is still an issue so their ‘real world’ applications are limited. Taking Control What happens if you change, say, the aperture, but then don’t adjust the shutter speed to compensate? If you’ve selected a smaller aperture, then the image will be underexposed. If you’ve selected a larger aperture, the image will be overexposed. The same outcomes will happen if you set either a slower shutter speed without adjusting the aperture to compensate. However, this isn’t always a bad thing because you may, in fact, deliberately want to either under- or overexpose an image, either for technical reasons or for creative effect. An image where highlights – the lighter tones – predominate is described as being high-key, while one where the shadows – or darker tones – are dominant is called low-key.

Again, only by manually setting the apertures and shutter speeds do you have this level of control, although there are alternative options which will be covered in the next article.

With experience, you’ll learn to recognise when a lighting situation or the type of scene demands some intervention on your behalf to achieve either a correct exposure or a desired creative outcome. Next we’ll look at how incamera light meters actually work – and so why you can’t always rely on a camera’s automatic exposure control system – and the ways you can take charge to create the images you really want.