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Treating ISO as the foundation of the exposure triangle and only adjusting it when you really need to will help you produce more consistently creative photographs.
ISO stands for International Standards Organization, which does not really help you understand what it is. But it does indicate the standard is international and it is constant across all brands and types of cameras.
The ISO is the measurement of how responsive your camera’s sensor is to light. The lower the numeric value, the less responsive, the higher the value, the more responsive.
Editor’s note: ISO is actually much more complicated than that but for purposes of this article, this is generally considered the easiest explanation of ISO to understand, especially for beginners.
Choosing a low ISO setting, say less than 400, is best when there’s a lot of light or when you have a tripod and the style of photograph you want to make allows you to use a long exposure. When the ISO setting is low, the sensor is less responsive to light, so, therefore, it requires more light to create a well-exposed photograph.
Using a low ISO setting will result in better technical quality photos generally. There will be little or no digital noise, the colors and contrast in your images will be better.
Choosing a higher ISO setting is best when the light is low or you are not able to make a long exposure. Higher ISO setting means your camera’s sensor is more responsive to light, so it needs less light to reach the sensor to create a well-exposed photograph.
It also means the technical quality of your images may be affected by digital noise, colors may be less vibrant and overall image contrast is flatter. How much, depends on how high you have your ISO setting and your camera model.
Sensor technology is rapidly changing and, if you have a newer, higher-end model of camera you can more confidently choose to make photos at higher ISO settings than with older, lower-quality cameras.
Unlike shutter speed and aperture settings, the ISO setting has no direct creative impact on your photographs. If you think the inclusion of obvious digital noise in an image mimics a creative value similar to film grain I suggest you do some more serious study on the matter.
Adjusting the ISO can assist you to achieve the shutter speed and/or aperture settings you desire to create the style of photograph you have in mind.
If you are wanting to blur a background using a wide aperture setting when the light is bright, you will need to adjust your ISO to one of the lowest settings to accomplish this. If you were to use a high ISO setting you may not be able to obtain a good exposure with a wide enough aperture setting, so your background will not be as soft looking as you want it to be.
Alternatively, if you want to create an image where everything in your composition is in sharp focus, it is best to choose a higher ISO, especially when the light is not so bright. By choosing a higher ISO you will be able to set your aperture to a higher f-stop number and achieve a greater depth of field than if your camera were set to a lower ISO value.
Choosing a low ISO can assist you in achieving a slow shutter speed when you want to create a photograph incorporating some motion blur. If you are photographing a moving subject, like a waterfall, and wish capture a lovely silky effect in the water, you will need to use a slow shutter speed.
This is easier to do when your ISO setting is low.
Freezing action by using a fast shutter speed will often require you to choose a higher ISO setting, especially if the light is not so bright. Being able to adjust your shutter speed so that is will render a fast moving subject as though it’s frozen in time will often mean balancing your exposure with a higher ISO.
If you are comfortable with having your camera in control of your exposure, then Auto ISO is a good option to consider. If you set your ISO to Auto as you adjust your aperture and/or shutter speed settings, the ISO will modify itself to make an exposure the camera finds appropriate.
If you do choose to work with Auto ISO, I recommend you do some testing first to discover what maximum ISO setting you are comfortable with for your camera.
To do this, take a series of photos of the same subject in the same lighting conditions and double your ISO setting each time. Then compare all the photos (look at them close-up and full image) and find the ISO setting for the image you are comfortable with, the one just before you see too much digital noise.
Editor’s note: Try not to overly pixel-peep. By looking at your images at 100% on your computer screen you will not get a true feeling for the amount of noise which will be visible at a normal viewing distance.
Many cameras have a means to set a maximum when using Auto ISO. So you can now set this to the number you determined with the test above.
Adjusting your ISO setting is generally only necessary when you want to achieve a specific effect or when the light conditions change.
When we do our photography workshops we always make sure to choose some locations which are outdoors and some which are indoors. This gives us the opportunity to demonstrate when it’s good to make an adjustment to the ISO setting.
If you are photographing outdoors on a bright day, your ISO setting will most likely be between ISO 100 and ISO 400. If you go inside, especially to a dimly lit building with few windows, you may find yourself struggling to obtain a good exposure with a fast enough shutter speed if you are only adjusting your aperture and shutter speed settings.
By adjusting your ISO so your camera’s sensor becomes more responsive in the low light you will be more flexible and capable of being more creative with your camera.