Photography News | Issue 46 | absolutephoto.com Technique 47 Camera School PART 11 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 a pro. This month, how to deal with digital noise from increasing ISO sensitivity, and how to use Auto ISO
Words & pictures by Kingsley Singleton
Right These two pictures, taken only moments apart, show the potential difference between using a high and a low ISO: the top picture, for which a low ISOwas selected, displays a high signal-to-noise ratio – the picture is clean and noise free. It’s a high-quality image suitable for every purpose The bottom shot, taken with a very high ISO, displays a low signal-to-noise ratio – and it really shows! The level of noise or grain is high and that affects fine detail and colour saturation. Overall, it is less acceptable.
Along with aperture and shutter speed, ISO is one of the three exposure variables and, as you’ll have seen in last month’s Camera School, it essentially increases or decreases the camera’s sensitivity to light. This allows you to adapt to dim or bright conditions and use the aperture or shutter speed that’s appropriate for the subject, or your intentions. The downside to increasing the ISO is digital noise. Essentially, this is interference caused by boosting the signal from the sensor. It’s a bit like amplifying an audio signal by turning the volume up, except in photography the signal you’re turning up is the light that’s hitting the sensor. When you amplify an audio signal you get ‘hiss’; and when you amplify a light signal you get random, grain-like speckles on the image – digital noise. The random pattern of high ISO noise shouldn’t be confused with fixed-pattern noise and banding, which are linked with very long exposures (we’ll cover this during a future Camera School). Collectively, this relationship between the amount of light and the amount of interference is called the signal-to-noise ratio (SNR). In dim conditions the signal (light) is weak, and in bright conditions the signal is strong. A high SNR, which tends to occur at low ISOs, is good,
and will produce clean images; a low SNR, which tends to occur at high ISOs, features lots of noise and is more problematic. When does digital noise appear? For the most part, digital noise goes unnoticed. It’spresentinalldigitalimages,butonlybecomes visible beyond a certain point in the ISO range – that is, at a certain level of amplification. This point of visibility depends on your camera and how you’re viewing the image, and of course, somemodels performbetter than otherswhen it comes to high ISOdigital noise. Cameras with larger sensors can have better noise performance (DSLRs with full- frame sensors V APS-C chips, for example), though this isn’t always the case. Broadly, each generation of camera tends to deal with noise more efficiently, so you might get the same noise performance at ISO 6400 as you did at ISO 3200 on the previous generation. Noise can also become more apparent when lightening a photo in editing software. An image with apparently no noise can suddenly look quite grainy when excessively brightened, especially in shadow areas where the SNR is lower. Broadly, the higher the ISO, the quicker the noise will appear when brightening a picture in software.
Using Auto ISO
When shooting in your camera’s creative exposure modes – aperture and shutter-priority, manual and program – the ISO can be set manually. This gives an excellent level of control and helps you to adapt to different lighting conditions, as well as achieving creative effects such as long exposures. But for situations where you need to be responsive and act quickly, it’s sometimes an exposure variable that you can do without. That’s when the Auto ISO setting comes in handy. You can normally activate it from the same menu as the manual ISO settings, and it can often be found at one end of the ISO range or the other – for example, if you push the ISO setting beyond its lowest or highest setting you’ll find it (on some cameras it’s a separate setting). On camera bodies with lots of manual inputs, such as Fujifilm’s X-series, it’s marked an A on the ISO dial, just the like automatic aperture and shutter speed settings. Once activated, the Auto ISO will modulate the setting based on the light levels in the scene. If it’s dim, for example, it will set a higher ISO, meaning a faster shutter speed can be used; and if conditions are bright, it will set a lower ISO so that wider apertures can be used. Depending on your camera model, you’ll also be able to limit the upper ISO to be used in Auto mode. So, if you know at what point the ISO setting of your camera gives pictures that are too grainy and desaturated, you can set the top limit in this way. Often a setting of 1600, 3200 or 6400 is a good compromise in terms of performance and avoiding poor quality pictures. On some cameras the minimum shutter speed can also be set; herein you can dictate how low you’ll allow the shutter speed to fall, and the camera will adjust the ISO setting to make sure it doesn’t go any slower.
When does noise become a problem? Digital noise only becomes a problem when it starts to affect fine detail and colour in an image. There are two types of noise which affect the image in different ways: luminosity and colour (or chromatic) noise. As the grain (luminosity noise) becomes more intense, textures and sharp lines get distorted and lose definition. Even when this is occurring it can still look good, in the way that fast film had a certain rough charm. Colour noise can be more of a problem. Images can lose saturation, with false colour appearing, and at very high settings, pictures can start to look blotchy, with smears of red, green or blue.
Know your limits The key to knowing which ISO to use in terms of noise is to test your camera and work out at which point the interference becomes problematic to you. Knowing what looks okay to your eye will mean you can avoid pushing the setting over that level. It’s important to remember, though, some grain is preferable to an image that’s blurred by slow shutter speed camera shake.
NEXTMONTH How to use high ISO noise reduction in camera and in software.
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