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In very simple terms: ISO is a camera setting that allows you to control the brightness of your photos.
It is very closely tied to the camera’s shutter speed and aperture settings. The combination of all three is what gives your photo it’s final brightness value.
Confused? Stay with me!
When you press the shutter button on your camera, your chosen aperture and shutter speed allows a certain amount of light to enter your camera and hit the sensor. The sensor then turns that light into a digital signal (your photo).
If the amount of light hitting the sensor is too low, the signal will be weak, and the resulting photo will be dark. This is where ISO comes in.
Choosing a higher ISO will ‘turn up’ the signal, much like turning up the volume on your radio (do people still use radios?). The amplified signal will then create a brighter photo.
You can see the result of ‘turning up’ the ISO in the series of photos below. The shutter speed and aperture are the same in all of these shots. Only the ISO setting has been changed.
Technically, ISO doesn’t affect the exposure at all, only the aperture and shutter speed settings do that, as they control the physical amount of light that enters the camera.
From a practical viewpoint though, doubling the ISO value will double the brightness of the resulting photo. This has the same effect as increasing the exposure by 1 stop.
Before we move on to figuring out which ISO we should use at any given time, first we need to discuss a very important topic.
Unfortunately, choosing higher ISO Values comes at a cost to image quality. There are three main tradeoffs that come with increasing the ISO of a photo.
All photos have noise.
The process of turning light into a digital image is not a perfect conversion. Any imperfections in the process of converting light into a digital image manifest as noise, or grain in an image. When the light is strong, and the ISO is low, the noise can’t be seen unless you’re looking very closely.
Going back to the ‘turning up the radio’ analogy I used earlier:
If you turn up the radio loud enough, you will hear distortion and the sound quality will go down. A similar thing happens when you ‘turn up’ the ISO. Not only does the photo signal get brighter, so does the noise, making it more visible.
Dark areas and areas with smooth tones (sky, skin, etc) will show the noise most noticeably, but as the ISO increases it can also degrade the lighter areas and fine details in an image as well.
The example below shows the effects of high ISO noise on the sky and trees in a photo.
Noise can be somewhat reduced in post processing. We use and recommend Topaz DeNoise as our go-to solution. You can download a free trial at the Topaz Labs Website.
Dynamic range is a little out of scope for this beginner article, but it is important to mention.
As ISO values go up, the dynamic range available to you goes down. This means you need to be more careful about getting your exposure correct as you will have less recovery options in post-processing.
Higher dynamic range affects photos that have very bright and very dark tones in the same image.
At high ISO settings, colours may become less saturated and may not be captured as faithfully as at low ISO. Modern cameras generally control this very well and it will only affect the highest of ISOs.
This is probably not something you will notice in many photos, but is something you should be aware of if you’re shooting at very high ISO settings.
Now that you know about the issues affecting higher ISO photography, we can talk about which ISO you should use.
First the bad news:
I can’t give you a number.
And now the good news:
The answer is: Set the ISO as low as possible that will give you the shutter speed and aperture needed for the shot.
I can give you some general guides to help you make the correct choice. The information below is very general, and you should experiment to find what works for you and the scene in front of you.
Click on a title below to expand the explanation
It depends on the camera you use.
Because ISO is such an important function in photography it is usually done via a button press and a dial movement.
Consult your camera’s manual for exact instructions, or type ‘How to change ISO on <insert your camera make and model>’ into Google. Alternatively, you can ask us in the comment section at the end of this article if you’re still having issues.
Most modern cameras include a feature called Auto-ISO.
When turned on, Auto-ISO will adjust the ISO setting to automatically set the correct brightness of your photo. You generally set up the highest ISO that you are comfortable with and the camera will do the rest, not going above that value.
With auto-ISO enabled:
Turn auto-ISO off for landscape photography on a tripod, or any other situation where you want to keep the camera at the lowest possible setting.
For all other situations, auto-ISO usually does a pretty good job. Just make sure that you set the maximum ISO value to something that you are comfortable with the noise levels generated.
Remember: A noisy, sharp photo is almost always better than a blurry, clean one!