Photography Basics: ISO


Nikon D3s – ISO 3200 – f/2.8 – 1/1000s

One of the three components of exposure of photographs is the sensitivity of your image sensor or film inside your camera.  This measure of sensitivity is expressed as ISO speed – and it’s kind of a big deal when your light is less than ideal.

What is ISO?

The ISO speed of film or digital camera sensors is derived from standards adopted by the International Organization for Standardization, which is made up of representatives of several national standardization entities.  Their goal is to create a universal method of standardization for a number of products and industries.  Film speed or digital camera sensor sensitivity is one of the things for which this organization has developed international standards.

In order for a camera manufacture to rate camera settings with each ISO number, it must meet the respective ISO standards for signal to noise and brightness measurements.  In theory, ISO 100 on one digital camera should be the same sensitivity as ISO 100 on any other digital camera.

That’s enough of a history lesson.  So, what does ISO mean to you and me?

When we talk about ISO, we are talking about how sensitive to light is film or and digital camera sensor.  ISO helps us answer questions about how much light we need for how long in order to have a properly exposed photograph.

It’s Still About the Light

Light is the overarching concern with any element of exposure. We are bound by what light we have available (either naturally or artificially).

If we use a slow shutter speed, the light will “hit the sensor” for a longer time.  If we use a larger aperture, “more” light will pour through the lens and to the sensor.

ISO 100 is about the lowest level most digital cameras can be set, although you will see some cameras drop to ISO 80, ISO 64, or even ISO 50.

It’s common to see digital cameras with high sensitivity settings at ISO 1600, ISO 3200 and beyond.  The Nikon D3s and Canon 1D Mark IV (both are $5000 cameras) offers sensitivity settings up to ISO 102,400.  These cameras can see in the dark better than I can with my eyes.

If we use a low ISO, the film or sensor is less sensitive and we may either need to use the lens aperture to get more light to the sensor, or we need to use the shutter speed to deliver light over a longer period of time.

If we use a high ISO, we can live with less light over a shorter period of time.

How to Use ISO on Your Camera

If you have a digital camera, you likely have the ability to manually change the ISO setting at any time prior to making an exposure.  Some basic point and shoot cameras, however, are fully automated and do not allow you to modify the ISO settings.

Generally speaking, a lower ISO setting is preferred over a higher ISO setting.  This is because lower ISO settings generally offer better overall image quality and do not produce as much image noise as higher ISO settings.

At times, you will want to use your own creative inspiration to make an image look a certain way by choosing an appropriate shutter speed and aperture combination.  In such cases, you may need to rely on your ISO settings to get a proper exposure.  Often, this may lead to setting a higher ISO setting than you would normally consider.

High ISO Can Mean Noisy Images

One of the side effects of an extremely sensitive image sensor (or film) can be noise.  Image noise often looks like grain on the image itself, but it can also be seen as color splotches and banding.  It can be a rather plain, and not too distracting grain, which is commonly referred to as “luminance noise.”  This form of noise seems to add a bit of texture to the image, and can sometimes work to improve the aesthetics of a photo.

Noise can also be an ugly “chroma noise,” which looks like color splotches throughout the image.  This is a very unattractive form of noise, and is quite noticeable when present in photos.

Camera manufacturers are continuously finding better ways of dealing with noise and making images appear less “noisy” in both high-end DSLRs and compact point and shoot cameras.  Additionally, the ability to shoot in RAW image formats and process photos in powerful programs like Noise Ninja or Adobe’s recent Lightroom 3 software is further pushing the boundaries of high ISO settings.

Wrapping Up ISO

If you have previously struggled with understanding what ISO means, I hope this sheds some light on the subject.

If you have not read the other topics in the Photography Basics series, consider taking a look at them now:

Combined with an understanding of shutter speed and aperture, you should now be able use these components of exposure to create the image you are after.  Likewise, you will be able to take your understanding of each of these exposure concepts and recognize and solve exposure-related problems in your images on the fly.

If you have any questions, please feel free to ask in the comments below.  Likewise, if you have additional thoughts or advice on this topic, feel free to join in the discussion.

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