1. What is ISO? It's a Red Bull for your camera!
I've mentioned before that ISO is the camera's sensitivity to light. Or as I like to call it, a Red Bull for your camera. It's the boost you give your camera when you need more light. Let's say you're at the museum and using a flash / tripod is not allowed. You're already shooting wide open (the biggest aperture hole you've got) and it's still not enough light. The light meter says you need to use a crazy slow shutter and it's going to be stupid blurry. What can you do?
Bump up your ISO!
2. Use a higher ISO setting anytime you need more light - but be aware of noise
If you need more light, bumping up your ISO is a great option. But just like downing a case of Red Bull, there are consequences. A really high ISO setting will show digital noise or "grain" in your picture. The less light in the scene - the more grain is noticeble.
Each camera handles ISO differently... some get too grainy at ISO 800, others at ISO 3200. The more expensive the camera, the better it can handle ISO. Just play with your settings to see what's the highest ISO you feel comfortable using. Since high ISO causes grain, you'll want to shoot in the lowest ISO possible if you can. If you want to be artistic and have grain in an image then crank up that ISO. And if given the choice between grain in the image and NO IMAGE AT ALL by all means bump up that ISO. It's better to have a photo, than nothing.
3. What you need to know:
- The choice of ISO setting is up to you - the person taking the picture and what you want from the photo
- Don't forget to check your ISO when setting your light - be sure to do this with every new location or change in light
- You can leave your ISO in auto but I recommend controlling it manually for the best results
- If you don't have a tripod or can't use flash - bump up that ISO to get the shot. A grainy shot is better than no shot
Here are some photo examples to give you an idea what ISO setting to choose in certain situations:
On bright sunny days your ISO can pretty much be as low as possible. If you have the ISO too high and it's really bright, you could have an overexposed photo no matter how small your aperture (or fast your shutter speed.) This is why it's important to keep an eye on your ISO. Don't forget to change your ISO back to low if you go from a darker location (like inside a museum) to a brighter location.
If it's a cloudy day, I'll typically choose an ISO around 400. This is also a great starting point for shaded areas (like the candy apples below.)
Sometimes I still want more ISO - in this example I wanted to use a smaller aperture so I could get more of the ferris wheel in focus. Since I didn't have a tripod, my choice was to bump up my ISO.
Here's one of my favorite images from the Seattle Aquarium. I needed to use a lot more ISO for the shot so I bumped it up to 1600! It allowed me to take advantage of the ambient lighting while still having enough depth of field (aperture) and shutter speed to get decent focus.
I love this photo of Sleeping Beauty's castle @ Disneyland. It was pretty much pitch black, the last bit of twilight almost gone. I didn't have a tripod and I really wanted to get a picture of the ambient light (so no flash!) This is a great example of when you might want to use a high ISO to get the shot. Otherwise I would have had to use a slow shutter that would have been too blurry to see much of anything or used flash. Because there was very little light, the grain in the photo shows up even more. That was my creative choice.
ISO is a pretty simple concept and once you get an idea of the range to pick your ISO settings. Don't be afraid to change your ISO and see how it helps in your photography. Just remember to change the ISO settings back to a lower setting - because once you set it manually it stays that way until you change it.