ISO and Noise
ISO (often called ‘sensitivity’) is very different from every other variable in photography. The best way to think about it is that it is merely a technical means to achieve an end: to increase the brightness of images that would otherwise be very, very dark.
Shutter speed and aperture are real variables with both a technical and a creative aspect. By adjusting them you’re using physical mechanisms to make your image brighter or darker. At the same time, you’re making a creative, aesthetic choice. When you change your shutter speed, you enter the realm of motion blur. (Intended or not.) When you change aperture, you enter the realm of depth-of-field (blur in space).
When you change the ISO value, you enter the realm of noise. There is no reward, only the technical price you pay for digitally amplifying the brightness of your image: noise. Because what happens to your image is exactly what happens to a soundtrack that you’re listening to as you start turning up the volume.
ISO is not a real variable. Increasing your ISO will degrade the physical quality of your photo. It reduces the amount of fine detail, it corrodes your image.
When it comes to setting your ISO there’s only one rule of thumb: keep it as low as possible.
That being said recent digital cameras are getting incredibly good at reducing noise at higher ISO settings. A few years ago the highest ISO setting on digital cameras was 1600 and those images were rendered technically rather inferior by noise. Today most cameras can go up to 12800 or 25600 (many of them even higher) and you can take very fine images with them at 1600 or 3200.
Whether you have a brand new camera or you inherited one from a friend, my advice is: test your camera. Take a series of images going through your ISO range, starting from 100 all the way to your highest setting. Compare your pictures on a computer monitor and you’ll find out how your camera reacts to boosting ISO, up to what point noise is acceptable to you and when it gets too much for your taste.
Different cameras will, naturally, produce different amounts of noise at different ISO settings. The images below show you just how great this loss of quality might be. The first set of pictures were taken with a Canon EOS 550D, an entry-level DSLR. Being a model from 2010 it’s almost an ‘ancient’ camera by today’s standards. Following the images from this camera are comparison shots from a Nikon D800, a professional, full-frame camera from 2012.
Canon 550D gallery
1. Side-by-side comparisons. On the left, the photo was taken at ISO 100. On the right, you have images from 200 all the way to 6400.
2. These are crops from the above photo to make the presence of noise even more obvious.
3. Another set of crops.
4. As a bonus, a set of images shot with a different Canon camera, a full-frame, Canon 5D Mark III. Full-frame cameras inherently perform a lot better at higher ISOs.
Nikon D800 gallery
1. Side-by-side comparisons. On the left, the photo was taken at ISO 100. On the right, you have images from 200 all the way to 25600.
2. These are 100% crops from the above photo to make the presence of noise even more obvious.
3. A set of 200% crops.
4. These images demonstrate the difference between RAW and JPEG files. JPEGs are compressed and as ISO increases, the compression algorithm performs worse and worse as it cannot ‘tell apart’ the noise from the fine details in the image. At lower ISOs and especially if the JPEG is the final image (i.e. you are certain that you will not need to perform much editing on the file) the visual difference between the two files are minimal.
Nikon D800 comparison video
This video is a screencast showing you the above comparison between RAW and JPEG files in Lightroom. Pause it at your convenience to see how various areas of fine detail change at ISO settings of 100, 400, 1600, 6400, 12800 and 25600. Please take into account that you are watching a compressed file which by its very nature reduces the amount of fine detail you can see :).