ISO and Grain
The third element that affects the exposure triangle is ISO. ISO refers to the film/sensor's sensitivity to light. The higher the ISO number the more sensitive it is to light. Low ISO values typically begin at 100, though some cameras have lows beginning at ISO 50, while high ISOs can go up to over 10,000.
The higher the ISO, the grainier the image. Grain refers to the optical texture of processed photographic film. It is also referred to as noise in digital photography. Generally I would recommend using the lowest ISO possible in any situation while managing to maintain a sharp image. This means using a fast enough shutter speed, your preferred aperture and the lowest ISO possible to maintain a proper exposure. This example was taken during mid day in Jerusalem with a low ISO of 200 so there is no visible noise in the image.
Many modern photographers don't like grainy images, yet when shooting in darker situations, you may often need to use higher ISO values in order to be able to capture an image at all. This in turn will make your image look grainier. However, I believe a little grain is better than not being able to capture the photo all together. Many old school photographers actually like grainy images because of their vintage feeling. This photo was taken at a late night gathering in Tel Aviv with ISO 4,000. I used an open aperture of F2.5, in order to let in as much light as possible. You can feel the grain in this image. It feels less crisp, but has a vintage look to it.
In recent years, digital camera makers have significantly improved ISO abilities. In the film days anything above ISO 400 was grainy and it was virtually impossible to shoot above ISO 800. Today I've shot usable images at ISO 10,000. The following image was taken at a practice run at the Bascula circus in Tel Aviv at ISO 10,000, an extremely high ISO, yet the picture came out pretty clear. Especially considering that the alternative was to use a lower ISO, but miss out on the photo all together as it would have most likely been underexposed.
I recommend getting to know your camera and figuring out what its highest ISO you can use and get images you're happy with.
By the way, film/sensor's sensitivity to light is referred to as ISO, after the International Standard Organization's (ISO) standard 12232:2006 which deals with "the method for assigning and reporting ISO speed ratings, ISO speed latitude ratings, standard output sensitivity values, and recommended exposure index values, for digital still cameras." You can purchase the standard and read the full document for about USD 110. However, this post covered the important points regarding ISO.
If you found value in this post, don't forget to like and share it.
Keep up the good light!
Yehonathan