ISO determines how sensitive your sensor is. By increasing your ISO, you will essentially be adding gain/power to your sensor and allowing it to capture images with less exposure time.
ISO’s generally range from 100-12800 (depending on camera), although more expensive cameras allow you to extend this from 50-102400.
There are pro’s and con’s to using different ISO’s which I will go through in more detail below. First I will explain what image noise is.
What is Noise
Image noise is speckles of different brightness and colour in pixels in an image. It is more noticeable when you zoom in close to an image. Often when you print (in small sizes), the noise won’t be noticeable.
Noise is generally an unwanted aspect in a picture and it’s always best to try and shoot so that you get as little noise as possible.
Example of noise
Low ISO is the best way to ensure your pictures contain minimal noise. When shooting in low light it can be hard to keep the ISO low, as well as ensuring images are sharp. Sometimes the only way to keep the ISO low will be to use a Tripod and have long Shutter Speeds. See my City at Nights Tutorial.
Having a high ISO will be useful for shooting in low light when you can’t use or or don’t have a tripod. Examples such as bars and busy streets at night are examples. Also on a cloudy dark day you may also need to increase the ISO to a higher number.
By having a high ISO you can shoot at higher Shutter Speeds, but the negatives are that you may get image noise in your image, which may be OK if you’re not printing your images in large sizes or selling your images.
Sometimes even if it’s a bright day you may want to increase your ISO so that you can get even faster Shutter Speeds for shooting things such as flying birds, vehicles etc.
I will generally always keep my ISO at 100, especially when shooting at night on a tripod. If I need a faster Shutter Speed and I don’t want to increase the aperture, or there just isn’t enough light for a faster Shutter Speed I will also increase the ISO. Sometimes I also set the ISO to auto but with a limit on the camera so that it doesn’t go too high, such as a max of ISO 1600. I shoot with the Canon 6D which has this option. Not all cameras will have this feature though.
One thing to keep in mind if you’re taking taking pictures for yourself (ie not for clients), is that if you can’t get a sharp picture without increasing the ISO, then it’s best to increase the ISO and have a noisy picture than not get an image at all.