ISO ratings

IN ORDER TO capture a well-exposed image using your DSLR, you need to get a precise amount of light to the sensor. How much light is mainly dependent on the shooting conditions — you need to give more exposure if you’re taking photographs at night than you do in broad daylight, for example. However, there’s another factor you need to consider other than light levels — the ISO your camera is set to. By changing the ISO you can increase or reduce the sensitivity of your camera’s sensor to light and therefore control the exposure required to achieve correct exposure. This has both practical and creative benefits and enables you to produce successful images in all situations — though as with most things in life, there are also drawbacks to using different ISOs.


All DSLRs have a wide range of ISO ratings and as new models launch, that range seems to grow and grow. The default ISO setting is ISO 100 or ISO 200, depending on brand and model. On entry-level models, expect a range of ISO 100/2006400, with maybe a High option that increases it to ISO 12800. More expensive DSLRs may go as low as ISO 50 and as high as ISO 102400! To change the ISO you usually need to depress and hold a button on the top plate or camera back then adjust a dial to increase or reduce the ISO. Settings will be displayed on the top-plate LCD panel or rear monitor, or both, and may be visible on the viewfinder readout.

Because you’re able to change the ISO randomly from shot to shot with a digital SLR (whereas in the days of film you had to change the roll if you wanted to use a different ISO), it can become a more integral part of routine exposure control. Instead of opening up the lens aperture to achieve a faster shutter speed, or setting a slower shutter speed so you can also set a smaller aperture, for instance, you can simply increase the ISO.


The ISO rating effectively changes the sensitivity of the image sensor to light, which in turn changes the amount of light it needs to record a correctly exposed image in any situation. Basically, the smaller the number (the lower the ISO), the less sensitive the sensor is to light and the more exposure it needs, while the bigger the number (the higher the ISO), the more sensitive it is to light and the less exposure that is required.

The rule to remember is that every time you double the ISO, the exposure required is halved and vice-versa, in any situation, which is the same as changing the aperture by one full f/stop or halving/doubling the shutter speed. So, if you take an exposure reading with your camera set to ISO 200 and you get l/60sec at f/8, if you increase the ISO to 400 you can shoot at l/125sec at f/8 or 1/60 sec at f/11 instead. Similarly, if you reduce the ISO to 100, the correct exposure would be l/30secatf/8or l/60sec at f/5.6 in the same situation.

The ISO can usually be adjusted in 1/3 and/or full stops, as follows (the full stops are marked in bold): ISO 100,125,160,200,250, 320,400, 500, 640,800,1000,1250,1600, 2000,2500,3200,4000, 5000,6400,8000, 10000,12800,16000,20000,25600. To change the increments by which ISO is adjusted, go into your camera’s menu. ISO settings higher than 25600 are usually marked at HI and H2 or similar, while settings lower than ISO 100 are marked L.

How to change ISO settings


The importance of ISO ratings means that most digital SLRs have a button dedicated for fast access to ISOs. The most common location is on the rear close to the LCD monitor, although on the top-plate close to the shutter button is a spot that’s becoming increasingly more common. Most cameras allow you to select whether you want the ISO ratings to change in 1/3- or 1/2-stop increments via the Custom Functions tab found on the menu system. We’d suggest you stick to half-stop increments to begin with.


1) Press the ISO button to show the range of available ISOs on the LCD screen.

2) Use the four-way control to select an ISO.

3) More recent models allow fast access to the ISO rating via the Q (Quick) button


1) Press the ISO button to reveal the ISO on the LCD monitor. Change it via the input dial.

2) You can also access the ISO sensitivity settings tab in the Shooting Menu via the MENU button to the left of the LCD monitor.


In situations where you are forced to shoot at high ISO you can set your camera to High ISO NR (Noise Reduction) so that in-camera processing is applied to the image to reduce the appearance of noise. There are two types of noise — Luminance noise and Chrominance noise. The former looks like grain in an image. The latter is pastel-coloured speckles and appears in mid-tones and shadows. High ISO NR tends to attack Chrominance noise when used at lower settings but as you increase the setting from low to high it affects Luminance noise more and more.

It’s tempting to always use the highest Noise Reduction setting, but this can lead to disappointing results because in order to reduce Luminance noise the image tends to be smoothed out and detail is lost. Your best bet is to experiment with the different levels of High ISO NR and see how they affect the image.

If you shoot in JPEG mode the effects of High ISO NR will be carried over when you download the images to your computer. But if you shoot in Raw, they won’t be recognised by third-party raw processors such as Adobe Camera Raw (ACR) in Photoshop and Lightroom so you either have to use the camera-maker’s own Raw processor or apply Noise Reduction using tools in the third-party Raw processing software you prefer.


As well as having a range of fixed ISOs, the majority of DSLRs also give you an Auto ISO option. When selected, this setting allows the camera to adjust the ISO automatically as required to prevent the shutter speed from dropping below a minimum that you can preset. You can also preset the minimum and maximum ISO ratings that Auto ISO can use so that you keep some control over image quality. You could set it from ISO 100-800, say, or ISO 400-1600. This is a really useful setting if you’re shooting handheld in changeable light, but only if you set sensible parameters — otherwise you could end up shooting at silly ISOs when a better solution would be to mount your camera on a tripod to keep the camera steady or set a wider lens aperture to increase the shutter speed.

Using ISO settings

The main reason why you’re likely to change the ISO setting on your DSLR is so you can use a faster shutter speed and/or a smaller aperture — to avoid camera shake when handholding in failing light, or to freeze a moving subject. This is achieved by setting a higher ISO. In some situations you may also wish to use a slower shutter speed, to record movement or intentionally blur an image. This can be achieved by setting a lower ISO so your camera’s sensor requires more light.

What you need to remember is that as ISO increases, image quality is affected. Shadow density is reduced so areas of shadow can appear muddy rather than black, noise increases — which makes the images more grainy — and colour saturation drops. That said, the latest generation of DSLRs are capable of amazing image quality at much higher ISOs compared to film.

UNDER ISO 100 Some DSLRs have L (Low) settings that allow you to reduce the ISO below the nominal ISO so the exposure needs to be increased. While useful to record motion or blur your subject, they’re not true ISO settings — your camera merely overexposes then corrects it in-camera. You’re better off sticking to the default ISO and using an ND filter to reduce the light.

ISO 100-200 For general use and to achieve optimum image quality, stick to the default ISO as image quality has been optimised at that setting. The optimum ISO is usually 100, but on some models it’s ISO 200. In sun, exposures are easy to manage, but if light levels drop and you’re struggling to hold the camera steady at a low ISO, use a tripod rather than increase the ISO.

ISO 320-640 This intermediate range is ideal to increase the shutter speed to avoid camera shake in lower light or freeze a fast-moving subject. Up to ISO 400, image quality is amazing with modern DSLRs. If you can’t be bothered to carry a tripod with you, this ISO range will keep you out of trouble without compromising image quality too much.

ISO 800-1600 Most digital SLRs start to show a drop in image quality from ISO 800 upwards, but to be honest, it’s small in many cases so you shouldn’t ever be afraid to shoot at ISO 800-1600 if you need to do so. This range is the ideal choice for shooting handheld in low light — raise the ISO rating to within this range if you’re shooting portraits by window light, or when shooting sport and action in poor weather or indoors.

ISO 2000-6400 Once you get to ISO

2000 and above there will be a noticeable increase in noise/grain and shadow density starts to tail off, but in situations where you need to shoot at such high ISO ratings to get successful shots, any drop in image quality is a small price to pay — though if you have one of the latest generation of digital SLRs, even at ISO 6400 you’ll be amazed at the image quality.

ISO6400+ Beyond ISO 6400 images become noisy, colours look dull and shadows go muddy. These ISOs are for emergencies, though they’re worth trying for stark, gritty black & white images. Such high ISOs also make it possible to take handheld shots in situations that were once off-limits, such as outdoors at night and even ISO 102400 can produce usable images when paired with the right subject.

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