ISO: International Standards Operations. It’s just a fancy term used to indicate how sensitive your film or digital sensor is to light. The lower your ISO, the less sensitive your camera will be to light. A higher ISO will result in greater sensitivity to light. If you’ve ever worked with film cameras, a standard film canister reads ‘film speed 200’. This is the most common ISO, allowing for a good amount of light and almost no noise in your photos. Sports film would read ‘speed 800’, indicating a more sensitive ISO allowing for a faster shutter speed to capture motion.
Depending on how old your camera is, the level of ‘noise’ will increase dramatically with your ISO. On an older camera, an ISO of 1000 might result in visible grain or noise, but if you have a newer DSLR you might not notice any. You will need to experiment with your camera to become comfortable with how high you will want to set your ISO.
Regardless of my shooting situation, the ISO is always the first AND last thing I adjust before beginning a photo shoot.
As a rule I keep my ISO at 200 when shooting outdoors during the day. This will give me a nice crisp image with no noise, and outside I don’t need the extra sensitivity to light. When shooting indoors near a large window I might dial my ISO up to 800 to compensate for the lack of total ambient light. If I shoot in a dark room or at night, I normally start with an ISO of 1600 and adjust as needed.
Once I have my ISO set to a level I am comfortable with, I can select the aperture setting that will give my images the depth I desire. After that I can adjust the shutter speed while I’m shooting to get the images I want. On some few occasions, I need a faster or slower shutter than what my camera recommends to stay in the properly exposed area of my meter. In those cases I go back to my ISO as a last resort to increase or decrease my camera’s sensitivity to the available light.
Keep in mind while doing this that if you are using an ISO that you are not comfortable with, it is very difficult to remove the corresponding grain from your images in post processing. If you need more light, it might be a better idea to add some light off-camera.
Here’s an example of a shooting scenario:
I’m shooting outside shortly before sunset. I set my ISO to 200, which is my default for shooting in natural light, and I’d like my aperture at 3.5 for a portrait I want to shoot. My subject is a toddler who doesn’t sit still well. Now, with a wide aperture setting I am already letting in as much light as possible, so I dial my shutter speed to 1/200 knowing that my subject will be moving around. I shoot a test image and still have a little more movement than I’d like, but the exposure is perfect. I know I can’t increase my shutter speed without reducing my exposure and making my image too dark, and I can’t change my aperture without losing the ability to focus on my subject. My last option is to increase my ISO to make my camera more sensitive to light, and then I can increase my shutter speed to correct the exposure.
If I am shooting indoor and at night without a flash (normally a concert) I do the opposite. I set my shutter speed so that I know my image won’t be blurry (200-400 depending on the focal length of my lens and the amount of movement), set the aperture where I need it for the desired depth of field, and then I change my ISO in order to make my camera sensitive enough to the light available to be exposed properly.
Try shooting some pictures inside your house during the day. Set your Aperture to 5.0, your shutter speed to 1/200, and then adjust your ISO until your picture looks properly exposed. Use your light meter to help you. Try doing this again at sunset, or even at night with a strong light source.