ISO is a measure of the sensitivity of your camera's sensor to light. (The term ISO is an abbreviation for the International Organization for Standardization.) The higher the number of the ISO, the more light is recorded by your camera's sensor. The trade-off with raising the ISO value is an increase in "noise."
We are familiar with the concept of "grain" from film photography. In that process, the visible spots and speckles on the photograph were actual enlarged grains, a result of the chemical process used to develop high ISO film.
With digital cameras, the noise from higher ISO values is the result of the amplification of the light received by the camera's sensor. Increasing the ISO value increases this amplification, and increased amplification results in an increase in random fluctuations (like the sound of static in radio signals). These random fluctuations show up as the noise or brightly colored pixels visible in a high ISO image. Digital cameras continue to improve at their ability to deal with and reduce such fluctuations, resulting in less visibly noisy images even at higher ISO values.
ISO: a Visual Comparison
This sequence of shots offers a comparison of changing the ISO values from very low (ISO 100) to very high (ISO 12,800). Some cameras offer even higher ISO values. The effects of changing ISO are not always visible when viewing an image at a small size, like in the example above. To truly see and understand how ISO can impact a photograph, you need to look more closely.
This sequence of shots offers a zoomed-in view of the same series of photographs as above. Now the impact of the changing ISO values becomes more obvious. Even as low as ISO 400 or 800, you begin to see the bright, off-color specks that signify digital noise. This noise becomes quite pronounced moving into ISO 3200 and beyond. At ISO 12,800, such noise is everywhere across the image.
Why Raise ISO?
You may be asking, "Why should I ever raise my ISO then, if I do not want noisy photographs?" The answer, of course, is that it depends. Specifically, it depends on the light. In low light situations, you have three options for getting enough light to capture the photograph that you want: open up your aperture to let in more light, use a longer shutter speed to let in more light, or use a higher ISO value to record more light.
There are limitations to all three options. You can only open up your aperture as wide as your lens can go, say f/1.8 or f/3.5. If your aperture is already wide open, then you need another option. (Read more on
Aperture and the F/Stop Conundrum here.)
You can only slow down your shutter speed so much before you need to use a tripod or stable surface, otherwise "camera shake" will become obvious in your photograph. If you are trying to freeze motion with your image or photograph people without blur, then you may want to stick with a quicker shutter speed. (Read more on
Shutter Speed Guidelines here.)
That leaves raising your ISO. Common situations where you may need to consider raising your ISO above 100 include shooting indoors, shooting in dim situations, or shooting at night without a tripod. In these kinds of situations, you need to decide how best to compromise between opening up your aperture, slowing down your shutter speed, and raising your ISO. The final decision comes down to how much noise is acceptable to you in your photograph. And that will depend on your situation, your style, and on how large you plan to print or use the final image ...
ISO and Print Size
An important consideration when raising your ISO when shooting is how you are planning to use those photographs. If you are planning to downsize them and share them online or print 4x6s for personal use, then you have much more latitude to shoot with higher ISO values. If you are planning to order a larger print or a 16x20 canvas, for example, then you should consider sticking to very low ISO values. (A good rule of thumb is to avoid using an ISO above 400 with an introductory-level DSLR if you want to make larger prints.) If you are planning to sell prints or image files, then you will definitely want to use a low ISO to avoid visible noise.
The images above offer a comparison of the impact of ISO at different print sizes. Each square shows the approximate pixels and quality of one inch of the same image if printed as a 4x6 compared to a 16x20. (The original image was 4752 x 3168 pixels, giving the 4x6, above, an approximate resolution of 800 dpi and the 16x20, below, an approximate resolution of 200 dpi.) The larger the image is printed or viewed, the more obvious the noise is from a higher ISO value. Try viewing this image with each square as an actual inch on your screen, and you get an idea for what the final image would look like and for how visible the noise becomes with the larger print.
Controlling ISO: Program vs. Auto Mode
If you generally shoot in full Auto mode on your DSLR or point-and-shoot camera, consider making the switch to Program mode. The main difference between Auto and Program mode is that in Program mode, you set the ISO value for the camera, while in Auto mode, the camera sets the ISO value. In both modes, the camera will still choose the aperture and shutter speed for you.
Taking control of ISO can make a big difference in your final images. A common problem when shooting in Auto is coming home and downloading your photographs onto your computer only to realize that the camera was shooting at an extremely high ISO value. There is very little that can be done in post-processing to "save" an extremely noisy image. By shooting in Program mode, you are guaranteed to keep the ISO value where you want it and to only risk noise if you need to. Your Camera Zero default value for ISO should be 100 or the lowest value available for your camera. (Read more about
Camera Zero and Default Settings here.)
Test Your Own ISO
The best way to understand ISO and your camera is to take your own series of test shots, using the full range of ISO values available. After downloading all the images to your computer, zoom in at 100% and scan around different regions of each photograph to see at which ISO value the photograph appears to be "too noisy" for your purposes. The newer and higher quality your camera, the more likely you will be to be able to "push" your ISO values higher with little or minimal impact on your final photographs. Knowing your own thresholds of acceptability will ensure that you get the image that you want every time.
Boost Your Photography: Learn Your DSLR is now available from Amazon. Get the most out of your camera with practical advice about the technical and creative aspects of DSLR photography that will have you taking beautiful pictures right away.
