Photography News | Issue 43 | absolutephoto.com
Technique 55
PART8 Camera School Here we lift the lid on all things camera related, showing how to get better results from your CSC or DSLR, and providing all the info you won’t find in the manual. So, stick with us and you’ll soon be wielding your camera like a pro. This month, how ambient light and aperture affect the shutter speed you can use, and how to deal with it
Words & pictures by Kingsley Singleton
As one of three factors that make up an exposure, shutter speed affects not only the brightness of the picture, but also how anything moving within it is captured. A fast shutter speed freezes anything moving; a slow one shows moving subjects as blur. It’s useful for adapting to different subjects or applying creative effects, such as flowing water. Faster and slower speeds It’s logical then that you’d oftenwant to use the fastest or slowest speeds available, allowing themost pronouncedshutter speedeffects. Try this in practise though, and you can run into problems. For example, in shutter-priority mode (S or Tv), if you set a fast shutter speed, say 1/4000sec or 1/8000sec you may notice the aperture setting blinking or reading ‘Lo’. This means the aperture can’t open enough to compensate for the fast shutter speed; because the fast shutter speed only allows a tiny amount of light to hit the sensor, the aperture needs to widen, letting more light in. Conversely, if you set a very slow shutter speed, such as more than a second, the aperture reading may blink or read ‘Hi’. This means that the aperture can’t close enough to compensate for the very slow shutter speed being used; when the shutter is open for a long time, lots of light hits the sensor, so the aperture needs to close to restrict it. In either case, the resulting picture will either be under or overexposed, or the camera may refuse to shoot at all. Stop me if you think that you’ve heard this one before Why can’t the aperture setting open or close enough to compensate in these cases? Well, the range has physical limits beyond which it can’t increase or decrease in size. But here’s the important thing: the aperture range, in terms of stops (the doubling or halving of the amount of light recorded by the sensor) is smaller than the range of shutter speeds.
1/2000sec, f/4, ISO800
8secs, f/16, ISO 100
Above Depending on whether you want to freeze motion or show it as blur, you can set the shutter speed fast or slow. But the fastest or slowest speeds aren’t always possible in the available light. In those cases, you need to take steps, including raising or lowering the ISO.
In practice, a shutter speed range of 1/4000sec to 30sec, which is typical on many DSLRs, covers 19 stops. Compare this to the aperture range of even a fast lens, running from f/2.8 to f/22, and you’ll only get seven stops. This means that, when working in shutter-priority mode, there are far more selectable shutter speeds than apertures to cover them – and eventually you’ll top out. Available light The amount of light in the scene has a big influence on which shutter speeds you’ll be able to use. In bright conditions, you can use faster speeds, but slower shutter speeds will be restricted. In dim conditions the opposite happens: there’s limited access to the faster settings, but the longer shutter speeds will be available at the other end of the scale. So, while in bright daylight you might be able to shoot from 1/1000sec to 1/15sec (covering apertures from f/2.8 to f/22, when
the light is dimmer, the same aperture range will give you 1/60sec to 1sec.
theminimumaperture scale at f/22 and 1/8sec, you could dial an ISO two stops lower than you’d been using and shoot at 1/2sec. Hitting the limits The ISO range has its limits, too. Modern cameras feature very high ISO settings, allowing fast shutter speeds even in low light, but beyond a certain level the amount of digital noise in the image may be a problem. And at the other end of the scale, most DSLRs’ ISO scales can’t drop below 100, although settings of 64 or 50 are becoming more common. If you need to go below the lowest ISO setting and the smallest aperture to get the shutter speed you want, you’ll need to fit an neutral density (ND) filter to the lens. This will block a certain amount of light, and this blocking power is helpfully rated in stops, just like the exposure variables of aperture, shutter speed and ISO. We’ll go intomore detail onND filters and how to use them next month.
ISO to the rescue Short of waiting for the amount of available light to increase or decrease, you can’t do much to change this. So instead there’s the ISO setting. The ISO setting is a measure of how much the available light is amplified by the sensor, and it works in stops, just like the other two exposure variables. Therefore, if you need a slower shutter speed than is currently possible, you can try to lower the ISO setting. And if you need faster shutter speeds, you can raise the ISO setting. In practise this would mean that if you’d reached the end of the aperture scale at 1/250sec, and needed to go higher, raising the ISO by two stops would give you a speed of 1/1000sec at the same aperture. At the opposite end of the scale, if you wanted to use a slower speed but had reached
Shutter range of camera
30sec 15sec
8sec
4sec
2sec
1sec 1/2sec 1/4sec 1/8sec 1/15sec 1/30sec 1/60sec 1/125sec 1/1250sec 1/500sec 1/1000sec 1/2000sec 1/4000sec
Aperture range of lens
f/22
f/16
f/11
f/8
f/5.6
f/4
f/2.8
Aperture range -2 stops ISO
f/22
f/16
f/11
f/8
f/5.6
f/4
f/2.8
Aperture range +2 stops ISO
f/22
f/16
f/11
f/8
f/5.6
f/4
f/2.8
Above Notice how the camera’s shutter speed range covers many more exposure stops than the aperture range of a typical lens. This is why it’s sometimes difficult to access the speeds you want without over or underexposing – the aperture can’t open or close enough to accommodate them. You can use faster or slower speed by altering the ISO setting.
NEXTMONTH When you want to use the slowest shutter speeds or widest apertures in bright light, and you’ve lowered the ISO as much as possible, the next step is to add neutral density filters. Find out all about them next issue.
Powered by FlippingBook