A camera’s sensitivity to light is measured in ISO: the higher the number the greater the sensitivity to light. Today’s cameras have a very high ISO range available. Each ISO setting is double the one before, ie, if you increase the ISO from 100 to 200, you double the camera’s sensitivity; and if you increase it from 200 to 400, you double it again. This carries on through the ISO scale.
This is no coincidence that each ISO setting is the double of the one before. This is quite deliberate. The ISO settings are designed specifically to double (or halve) the exposure in the same way that the aperture settings and shutter speed settings are, and this is why the lens aperture, shutter speed and ISO are often described as the ‘Exposure Triangle’.
Each time you double the ISO (for example, from 200 to 400), the camera needs only half as much light for the same exposure. So if you had a shutter speed of 1/250 at ISO 200, going to ISO 400 would let you get the same exposure at 1/500 second (providing the aperture remains unchanged). This is why high ISOs are so often used indoors, especially at sporting events. Needing a fast shutter speed to stop action, photographers regularly choose ISO 1600 or above.
There are some drawbacks to changing the ISO which mean that in practice you tend to change the ISO only when you have to. The problem with this is that all digital images have some background noise. Usually, we don’t see it because it’s very faint as compared to the light falling on the sensor, but when you increase the ISO, you amplify it, and it shows up as a kind of random sparkling. The higher the ISO, the worse the noise. It’s the digital equivalent of grain and results in a sort of “chunky” and “grainy” look to the image. Very early digital cameras had objectionable levels of noise at ISOs as low as 800. Today most digital SLRs can make great quality images at ISOs up to 1600 and above. However, several factors affect the amount of visible noise that shows up in an image apart from the ISO level:
Pixel Size: Size of the pixels used on the sensor. Large pixels result in less noise than small ones. That’s why digital SLRs perform much better at high ISOs than compact cameras. The SLRs have larger sensors and larger pixels.
In-camera Noise Reduction: Since all pixels gather some noise, every digital camera runs processing on every image (although with a NEF, or RAW, file that can be changed later) to minimize that noise. Newer cameras use newer technology to reduce that noise, with the result being less noise at similar ISOs than what earlier cameras could achieve.
Sometimes, this grainy look can be used by photographers for creative purposes so in some circumstances, it is desirable to have a photo with a lot of digital noise.
Here is an at-a-glance guide to ISO settings and which type of situations are they used for while shooting:
ISO 100: Best setting for best quality photos in bright conditions or where no noise is the key requirement.
ISO 200: Ideal for slightly less brightly lit conditions.
ISO 400: Provides fast shutter speeds in overcast conditions or low light. Noise may be just be visible when printing larger sized photos. ( Depends on the camera )
ISO 800: Ideal where faster shutter speeds are the key. Noise might be evident even at normal prints.
ISO 1600 ( or greater ): Provides very fast shutter speed in very low light. Noise level may be very high, depending upon the camera.
Wrap up Points:
- A low ISO value means that the sensor is less sensitive to light.
- Using a lower ISO produces clearer, smoother images.
- Using a higher ISO can result in grainier, ‘noisier’ images.
- Avoid high ISO values except in low light situations, when a long exposure isn’t possible.
So that’s what the term ISO is all about in photography. For any queries regarding this topic, please drop a comment below and we will answer it as soon as possible.