Have you ever connected your earphones to an amplifier? Try this once. Connect you earphone to an amplifier rather than directly connecting it to your laptop or MP3 player. Now, keeping the volume low on the source (laptop or MP3 player), gradually increase the volume from the controls on the amplifier. You would notice a slight hiss kind of sound. This sound goes on increasing as the volume is increased.
Very similar to this is the relation between ISO setting of camera and image noise. But before moving on to it, just recollect the old cameras/SLRs. Those days, films were used in the cameras on which image negative was developed. Depending on the requirements of the photographer, he used to choose films of different sensitivity. After the arrival of digital sensor in the cameras, this concept was termed as ISO. The ISO setting controls the sensitivity of the image sensor towards the falling light. Hence we choose proper ISO setting under any given lighting condition in combination with the aperture and shutter speed to get proper effect. But, increasing ISO also produce image noise. This noise can be seen pretty easily after the ISO settings of 400. After 1600, depending on the camera sensor, either image noise and/or color noise becomes too much to recover a sharp image.
If ISO gives noise in the image, why do we use high ISO at all?
The answer is very simple. There are situations when you cannot reduce shutter speed beyond a certain limit else you get motion blur. Neither can aperture be reduced too much every time, because then depth of field is reduced significantly. In such cases, the only option is increasing the sensitivity of the digital sensor. Or even in cases where a part of the subject is under shadow, you have to use higher ISO so that entire image seems evenly exposed.
So, the question: When to use a low ISO?
One should always try to use the lowest possible ISO to retain the details in the image after post-processing attempts. In cases where there is plenty of light, use ISO as low as 100-200. This will result into images with nearly zero noise in most of the dSLRs. Even in low light conditions, try to shoot with a tripod, so that you can afford slower shutter speeds and hence can reduce ISO. Also, try to use reflectors to even out the lighting in the areas where shadows are more prominent.
When to use a high ISO then?
Most beginners don’t buy a monopod/tripod for beginning. It is one of the most essential tools which one should always have. Otherwise, you can go for higher ISO settings under low light to compensate for the under-exposed conditions. A few times, using higher ISO becomes necessary:
- While shooting star trails.
- A relatively higher ISO would be needed when you want to freeze motion.
- While shooting Indoor sports
and so on so forth…
Technical Note – Why higher ISO deteriorates the image quality: A camera sensor stores data as pixels. When using higher ISO settings, each of these pixels are more sensitive towards the incoming light. This works fine in increasing the overall exposure, but the higher light sensitivity causes problems. Higher the sensitivity, more is the tendency to gather the reflected light from atmosphere. This includes even the light being reflected from the dust particles hanging around in the atmosphere. That is why we encounter Gaussian as well as Salt-pepper noise in images shot at higher ISO.