ISO is a setting to increase the brightness of a photograph.
Sometimes you have a dilemma when, in low light conditions, you wish to avoid an underexposed image. Let’s say that, even though you have opened your aperture as far as your camera allows in order to retain sharpness, the image is still under-exposed. So then you have set your shutter speed as slow as you can yet still avoid avoid blur, but the image is still under-exposed. If you don’t mind blur – or if you want special effects such as light trails – you could mount your camera on a tripod and take a long exposure. (If you are taking an exposure longer than about 1/50 seconds without a tripod you risk camera shake and undesirable blur.)
In other circumstances, however, in order to increase brightness your only option other than flash is to raise the ISO setting. Unfortunately, if you raise the ISO significantly, you will begin to notice ‘noise’ or ‘graininess’ in your image. You then have to trade off motion blur against graininess – or miss the shot.
I’ve provided here a series of images of the same scene, on the same day, as darkness appears. I took this first one at 20:37 using ISO 100 at f/11 and 1/25 seconds. The focal length was 20 mm for all this series. The image is reasonably sharp and without noise.
This photo was taken 3 minutes later and yet is brighter. I increased the ISO to 200, increased the shutter speed to 1/100 seconds, but retained the aperture of f/11. At ISO 200, there is no noticeable increase in graininess.
By 22:03, I was using ISO 250, a shutter speed of 4 seconds and an aperture on f/3.5 – that’s an increase of 8 aperture stops alone. There is some noise but not much.
By 23:28 I was using ISO 3200, a shutter speed of 25 seconds, and an aperture of f/3.5 – as wide as possible with the lens I was using. There is considerable noise. In the following two images I have cropped section of this image to illustrate the noise or grain.
This is a crop from the middle- left of the picture.
This is a crop from the bottom right.
The shutter speed of 25 seconds was as slow as I could use, with the lens I was using, without causing spot stars to trail as a result of the earth’s rotation relative to the stars.
So, in summary, The ISO setting on a camera can be used to brighten a photo, but start by setting an aperture suitable for your subject. Then set your ISO to its base value – usually 100. Take a test shot and if the preview image indicates motion blur, try a faster shutter speed until you get a sharp test shot. If you still experience blur, increase the ISO but use a faster shutter speed. If after increasing ISO a few times you start getting noise in the image, try opening your aperture.
My featured photo of light trails today illustrates some of the types of trade-off issues. It’s a low light shot for which I wanted sharpness overall ie good front-to-back depth of field for the light trails, and — preferably – a starburst effect from the street lights on the roundabout. My start point was, therefore, a small aperture of f/18. However, I wanted the exposure to be long enough to capture the light trails, so I used a tripod to avoid camera shake and a fairly low ISO of 125. My focal length of 30 mm was wide enough to take in the scene I’d chosen. As the scene became darker I eventually got the shot I wanted with a 30 seconds shutter speed.
I’ve provided a link below to an excellent website that explains ISO in crystal clear terms.