So maybe you're thinking, “Oh whoa, what's an ISO?” That's ok — that's what we're here for. Check out these thirteen tips to keep you on your ISO A-game:
1. ISO on your digital SLR camera settings refers to the film speed and ultimately how sensitive your image sensor is to light (even though your camera is digital the ISO function is still the same as older film cameras).
2. The most common ISO camera speed settings are: 100, 200, 400 and 800. Depending on your digital camera model you may also have them in the range of 64, 100, 160, 200, 400, 640, 800, 1600.
3. On most cameras you can find the ISO setting by looking at the LCD screen on the top right corner (refer to your manual if you don't see it there).
4. The lower the ISO speed, the slower the speed; the higher, the faster.
5. To change the ISO speed number on your camera, press the ISO button and simply use the up/down/left/right buttons to change the speed. (Refer to your manual if you cannot find this button.)
6. Basically, when you are in sunnier or brighter conditions you want to lower your ISO so that your photo is not overexposed (100 or 200 is usually a good starting point for outdoor sunny conditions).
7. If you are photographing in overcast or evening conditions, you should start your ISO setting within the range of 400 to 800.
8. In low light or night time conditions, you might want to start your ISO at 1600 (you want a slower shutter speed to let more light into your sensor, or else the photo will appear too dark).
9. One thing you should keep in mind: the higher the ISO, the more grainy the photo will appear. So your goal will be to find the lowest possible ISO setting that works for your situation. (However, keep in mind sometimes a grainy photo is better than a photo that is too dark, you will have to make the judgement call in each situation.)
10. A higher ISO is useful for when you want to take photos in dark settings but don't want to use a flash.
11. When your digital SLR camera is in automatic mode, the ISO speed is chosen for you to suit the level of light available sensed by your camera in that particular situation.
12. In some of your manual settings on your DSLR dial, it will chose the ISO for you, such as the P (Program setting).
13. Like we always say, practice makes perfect. Place yourself in variously lit settings and practice setting your ISO from memory. Eventually it will come naturally in every situation!
[Source: SLR Photography Guide]