Photography News Issue 45

Photography News | Issue 45 | absolutephoto.com Technique 47 Camera School PART 10 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 don’t find in the manual. So, stick with us and you’ll soon be wielding your camera like an expert. This month, how your camera’s ISO setting influences the other exposure variables

Words & pictures by Kingsley Singleton

Primary effects of ISO Like aperture and shutter speed, ISO can be divided into primary and secondary effects. The primary, like that of aperture and shutter speed, is to determine image brightness. In full manual, a longer shutter speed or a wider aperture will make a picture brighter; so will using a higher ISO setting. And while a faster shutter speed or a smaller aperture will make a picture darker, so will a lower ISO. To make this easier to understand, the ISO setting is measured in ‘stops’, just like the other exposure parameters. So, just like the shutter speed a doubling of the ISO indicates that twice the brightness of light is recorded. This means that, with ISO, you can respond to the amount of light in the scene, decreasing or increasing it, to affect the other settings and/or to get a ‘good’ exposure. How ISO affects other settings Let’s put that into practice. Say you’re shooting in aperture-priority mode (where you set the aperture and the camera decides the shutter speed), and that the ISO is set to 200; with those settings let’s imagine you’re getting a shutter speed of 1/60sec. If you thendouble the ISOto400, the shutter speed will also double to 1/125, taking into account the additional ‘sensitivity’. Conversely, if you lowered that ISO setting, halving it from 200 to 100, the shutter speed would also halve, falling to 1/30sec. If you were shooting in shutter-priority mode, and altered the ISO in the same fashion, you’d see the f/number, denoting the aperture, rise or fall, by an equal measure. Why increase or decrease ISO? In the real world, altering the ISO means you can tailor the other settings to suit the light, and your intentions for the exposure. Say, for example you want to use very wide aperture in bright light. This may mean using a very fast shutter speed, possibly one that your camera doesn’t reach. The alternative is to lower the ISO setting, meaning slower speeds can be used. Or say you want to use a fast shutter speed to freeze movement, but the photo is underexposing when you do. Increase the ISO setting and you’ll get a brighter image. ISO range A typical ISO range on a modern DSLR can run from 100 to 25,600. Some, usually pro models extend further to ISO 50 or 64 at the low end, and ISOs of well over 100,000 at the top end. As mentioned, each doubling or halving of the number represents a ‘stop’, which matches the other exposure settings. Taking that typical range of 100-25,600, it provides nine stops of sensitivity, equating to a shutter range of 1sec to 1/250sec, so you can see the versatility that ISO changes provide.

When you take a picture, you expose the camera’s sensor to the light in the scene, and there are three factors that decide how this light is received: aperture, shutter speed and ISO. These three factors form the exposure triangle. As we’ve already looked at shutter speed and aperture in this Camera School series, this month it’s the turn of ISO. The ISO setting is certainly the least glamorous of the three, as it’s not a creative tool as such. But controlling it is no less important. In fact, it has a huge influence on the more creative settings. What is ISO? The best way to think of ISO is that it controls the sensitivity of the camera to light. That’s not entirely accurate from a scientific point of view (which we’ll come back to later), but it’s certainly the easiest way to explain it. Essentially, the higher the ISO, the more sensitive the camera is to light, and the lower the ISO, the less sensitive it is. The ISO can be set in all of the creative shooting modes, like manual, shutter- priority, aperture-priority, and program, but not the Scene modes or full Auto. On DSLRs the ISO setting is usually accessed via a button on the body. On some cameras you’ll need to go into a menu on screen to set it.

Low ISO, slower shutter

100%

Fast and noisy ISO can be used to affect other exposure settings. Shot in aperture-priority, both at f/8, the top pic has an ISO of 100 and a shutter speed of 1/60sec; below, ISO increases to 1600 and the shutter rises to 1/1000sec. But with higher ISO settings comes digital noise.

High ISO, faster shutter

100%

Digital noise Unlike shutter speed and aperture, the secondary effect of ISO doesn’t have a creative use, being more of a by-product than a choice by the photographer. This secondary effect is digital noise; essentially interference caused by increasing the sensitivity. Previously we said that the idea of making the camera more ‘sensitive’ to light wasn’t entirely accurate. That’s because, while ISO settings are analogous with traditional film speed, when you increase ISO you’re actually artificially boosting the signal, which in this case, is light. You can compare it to tuning up the volume on a radio. As you do so, you hear more ‘hiss’. In digital photography, that ‘hiss’ is digital noise, seen as speckles on the image.

The more you increase ISO, the greater the level of noise that will be seen in the image. The amount varies from camera to camera, but ultimately it means you need to be careful howmuch you boost the signal, or you’ll end up compromising image quality. For that reason, photographers often shoot at the lowest ISO settings possible, only increasing the speed when the shutter speed or aperture that they want is unavailable.

NEXTMONTH We’ll take a closer look at digital noise, how to reduce it, and how to use your camera’s Auto ISO to your advantage.

Above The ISO setting on your DSLR or CSC is available in all but the full Auto or Scene modes. It can be accessed from a button on the body, and/or via the shooting menu; it can also be set automatically by the camera.