We have already covered two ways to control exposure – aperture and shutter speed. In this lesson we will look at a third method, ISO. This way of controlling exposure is a little bit different to the two ways we have looked at so far. Both aperture and shutter speed refer to properties of the lens, and are ways of controlling how much light enters the camera. ISO is related to the film or sensor itself, and controls how sensitive the camera is to light.
The origins of ISO lie with analog cameras, and particularly with analog films. Camera films were given an ISO number which indicated the “speed” of the film, a factor that showed how sensitive the film was to light. A higher ISO number indicated that the film was more sensitive to light. What does this mean in practice? Imagine two films, one with ISO 100 and the other with ISO 200. The ISO 200 film would be twice as sensitive as the ISO 100 film, and so would only need half as much light to produce an image as bright as that produced by the ISO 100 film.
Digital cameras don’t have camera films, so it might seem that ISO is no longer relevant. However, instead of referring to the chemical properties of the film, ISO now refers to the sensitivity of the digital sensor. This sensitivity can be easily adjusted with software and doesn’t require physically changing the sensor. In terms of the numbers it works in the same way – doubling the ISO number doubles the sensitivity to light.
ISO 100 is usually taken as the baseline or reference point. On a typical digital camera you may find ISO numbers between 100 and 3000 or even higher, depending on the quality of your camera. The ISO range is often a key feature of different camera models, with basic cameras offering perhaps only ISO 100, and more professional cameras offering extremely high ISO numbers.
ISO is a powerful tool to add to your abilities. As aperture and shutter speed both affect the way an image looks, it is sometimes the case that we want to fix both of them at a particular value to create a desired effect. Imagine wanting to capture a fast moving object with a shallow depth of field for example – this would require both a fast shutter speed and a wide aperture. Another common example is trying to capture a fast moving object in poor light conditions – a fast shutter speed is needed, but aperture alone may not increase exposure by enough to make the image bright enough. In these cases we may need another way to control the exposure. We can do this by adjusting the ISO to make the film more or less sensitive as needed.
The images above give examples of using ISO under different conditions. Under normal daylight conditions it is a good idea to use a low ISO, typically 100. In more challenging light conditions a higher ISO becomes more useful. However, be cautious of increasing ISO too much as an unfortunate side effect of high ISO is more noise in the photos produced. How much noise is introduced depends on your camera model. It is a good idea to play around with the ISO settings on your camera and try to get an idea of when the introduced noise starts to become a problem. This should give you an idea of the maximum ISO you should use with your camera when shooting.
One key feature of higher end cameras is an increased useful ISO range. For example, the low end Nikon DSLR 3400 has a maximum usable ISO around 3200, whereas a top end Nikon 7500 can take good quality photos up to about 12800, and poor quality photos up to ISO values higher than one million. Of course one big difference in these cameras is the price tag, and it may not be worth paying a significant amount more for higher ISO when first starting out in photography.
To understand a little bit more how higher ISO introduces more noise, compare the two sides of the image below. The image on the left was taken using ISO 100, compared to one on the right, taken at ISO 6400. The difference in quality is clear. Although the exposure is the same in both cases, the side using ISO 100 shows a much sharper and smoother image. Using such a high ISO has introduced noise (clearly visible in the grainy pattern on the yellow wall) and greatly reduced the image quality.
As we have now covered the three main factors determining exposure we can now give an example of how to keep exposure constant despite changing one of the three determining factors. In the second photo the ISO changed from 100 to 6400, which corresponds to an increase of 6 stops (remember that doubling ISO means increasing exposure 1 stop, and you need to double 100 six times to get 6400). To keep the same exposure we need to decrease the shutter speed by 6 stops. To decrease shutter speed one stop we half the exposure time, so six stops means halving the shutter speed six times – so we go from 1/30 to 1/2000.