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Camera School: ISO and sensitivity

Lesson four: Master ISO

ISO is the measurement of a camera sensor’s sensitivity to light. Based on the speed ratings of conventional 35mm film (denoted as ASAs), most compacts, superzooms and DSLRs will allow photographers to alter the ISO setting while in Program, Aperture Priority, Shutter Priority or Manual modes. But why would you alter the ISO and what effect does it have on the image?

In its most simplistic terms, photos taken at lower sensitivities (ISO 100, for instance) will be less noisy than photos taken at higher settings (ISO 1600). This is due to the way the sensor gathers light. In brighter conditions, there’s lots of light around and the sensor doesn’t need to be as sensitive to light. When the light levels start to drop, the sensor needs to amplify its sensitivity. While this does help gather more light, it also gathers more noise and this is shown by a graininess within the photo at high ISOs. Noise is the term used to describe the grain and artefacts that appear in photos at higher sensitivities.

Similar to 35mm film, different ISO settings are suitable for different shooting scenarios. For instance, most cameras are set to ISO 100 by default, as normal lighting conditions should be around the 100-200 mark. Photographers can then utilise a low sensitivity of 50-64 (if available) for bright conditions and 800 and above for low-light conditions. Much like choosing the right film speed, choosing the right ISO can make the difference between a successful and unsuccessful shot.

In the last two Camera School lessons, we’ve talked about how aperture and shutter speeds work together to create perfect exposures, and the third element to this equation is ISO. If you don’t have enough light – when you’re shooting a wedding in a church, for instance – you can up the ISO to allow you to keep a faster shutter speed, but still achieve a good exposure.

Camera sensors work by collecting light into each individual pixel on its surface. How big the pixels are varies depending on how big the sensor is. For instance, a 14MP sensor in a compact will be a lot smaller than a 14MP full-frame or APS-C-sized sensor. This is why compacts typically suffer from more noise than DSLRs. The smaller sensors with more megapixels crammed onto a smaller surface mean smaller pixels, and these generate more noise.

It is important to understand how and why ISO works in order to manipulate it to ensure you achieve the best possible results. In situations like sunsets and sunrises, or interior shots of museums or churches, you’ll have little choice but to shoot in lower-light conditions. You’ll also be without the use of your flash, meaning your camera is your only weapon. By choosing a long shutter speed (1/2sec or over), a wide-open aperture (f2) and a high ISO (ISO 800 or more), you should be able to collect enough light to capture a successful shot.

If you do choose to turn your ISO off Auto, always check it before your next shoot. A whole day of clear blue skies and treasured memories can be quickly ruined by noise, which you may not notice until you load your images onto the computer at home.
 

ISO 100
The lowest ISO setting will vary depending on your camera model, but at this level there should be no sign of noise

ISO 400
ISO 200-400 is considered the best speed for outdoor, well-lit situations and no noise should be immediately visible
 

ISO 800
At ISO 800, some noise will creep into compact shots. This level is good for action images, indoor shots or lower light

ISO 1600
Most cameras will have noise when you zoom in on shadow and contrast areas by ISO 1600

ISO 3200
Into the realm of DSLRs, this is unusual on lower-end cameras and only the best-quality sensors will achieve usable results
 

ISO 6400
Few cameras will go above this setting, and all cameras will suffer from noise and grain all over the image

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