What is ISO ?
Back in the day before digital cameras, there was a such thing as film speed. Film commonly came in speeds of 100, 400, and 800. In the digital era that we are in (although film is not dead!) the speed of film is the equivalent of ISO. The ISO determines how fast the sensor of the camera responds to light. If the ISO is 100 (the lowest full stop) the sensor is less responsive to light.
How Do I Choose the Correct ISO ?
On bright sunny days in full sun, an ISO of 100 is an easy choice. Shade on a bright sunny day, depending on how deep your shade, you may be using an ISO of 400 or more. Bright indoor light near a window can also be dialed in around ISO 400. As the available light decreases, the ISO must be increased in order to keep your shutter at a speed that doesn’t record camera shake. Evening outdoors and farther from window light indoors will require a raise in the ISO to 800 to 1600. And as the daylight disappears ISO can be increased up to 3200 on most cameras, and up to a crazy ISO >12800 on some cameras.
A little rehash:
- Bright Full Sun-100
- Bright Window light Indoors- 400
- Sunny Outdoor Shade- 400-800
- Overcast Day Outdoors- 400-800
- Indoors Indirect Light Source 800-1600 (and up)
- Evening hours toward sunset- 400-800
- After sunset outdoors- 1600- 3200
- Low light situations- 3200-6400
- Expect some noise over 800 ISO
How Does It Affect My Photos?
Same as aperture and shutter speed, there is always a draw back as you change your ISO. With each increase of ISO, grain is introduced. It starts to become noticeable around 800 on consumer cameras and above 1600 on pro cameras depending on how the photographer shoots. The higher you increase ISO (larger number), more and more grain or noise is added to the image. So, unless a noisy or grainy image is part of your artistic vision, the ideal is to keep your ISO as low as possible without introducing some other nasty side effect like camera shake or too wide an aperture for your taste.
Grain is most noticeable in the shadow areas of your images. Grain is also introduced when underexposure is a problem. An underexposed image at a high ISO is a double whammy. The best way to beat grain is good exposure and even overexposure if possible without losing data in other areas.
Try using your camera to set different ISOs. At what ISO do you start to notice noise/grain? In consumer cameras, its will be somewhere around 800-1600 or so. Zoom into your images at 100% where you can really see the patterns of the noise. Look at the shadows. Color will start to be introduced at really high ISOs. While, my D750 does pretty well at higher ISOs, my D80 (my first camera) was terrible. Green and magenta dots will become visible as well as some banding across the image, which can absolutely ruin an image.