ISO was originally our film speed. Our first wet plates were very very slow. Ten minute exposures were not uncommon. It is one reason that early photographs had a certain look. For example soft water in waterfalls, or the very still stances in portraits. Portrait photographers had to have a special clamp so that the models could push their heads into the clamp so they could hold still for that time. Hence the lack of smiles. We used to call ISO, ASA. These are both acronyms, ISO is the International Organisation for standards while ASA was the American version. For all intents and purposes they are the same. This is trivia unless you remember learning photography in the film days.
Digital sensors have their own quirks, just like film did. So here is a list of things you should know about ISO and how it effects your images. I have included some exercises to help you figure out how it works with you and your camera.
A small ISO number means that the sensor isn't very sensitive to light (50 or 100). A high ISO number means that your sensor is more sensitive to light (3200 or 6400). As the sensor becomes more sensitive it loses its ability to work as effectively, therefore it isn't going to give you as a large exposure latitude as less sensitive setting. It will also be noisier.
- What ISO gives the cleanest photographs?
- What ISO gives you the most colours?
- What ISO gives you the biggest exposure range (sometimes called latitude)
- What happens with the noise as you increase your ISO?
- What ISO is your acceptable upper limit for large prints, small prints and web-sized images?
- Why is the acceptable upper ISO limit different between photographers and camera models?
- How well does your software handle noise reduction?
- What happens to your photographs as you increase the noise reduction?
- Do you need to set your noise reduction in camera while shooting RAW?
- Why does your acceptable upper ISO limit change depending on your subjects? Why is it different for landscapes and portraits?
The best way to figure out your acceptable limit is to experiment. I would start to see what other users are getting from your camera and use that as a starting point. Take some photographs and then look at them at 100% and see how much noise there is. Now apply the noise reduction. I just edge it up slowly and look to see if it is eating into the clarity and sharpness of the photograph.
Adding a little grain is a good way to hide excessive noise reduction.
Now resize your work. Print out a section for a large print, and a small print. Look at the print carefully and decide what your acceptable ISO level is.
Me, I use a mirrorless micro four thirds camera. And I find I can use mine at 3200 ISO and get reasonable A3 prints. For A2 prints I prefer to keep my ISO down to under 800. For portraits I can push this up too 6400 ISO and get a very beautiful A3 print.
Types of sensor noise (for the technical minded readers)