All About ISO

Background
In the early days of film photography, the International Organization for Standardization (ISO) defined a standard to determine how sensitive the film was to light. This was known as the ISO speed. It is still relevent in digital photography today - the ISO equivalent speed is still a measure of how sensitive the CCD inside the camera is - and thus how quickly it will capture the image it "sees" through the lens.

Theory
ISO speeds normally follow the pattern: 50, 100, 200, 400, 800, 1600. A "slow" ISO speed is at the lower end, ISO 25, 50 etc. A "fast" ISO speed would be 800 or 1600. Most digital compact cameras work in the 100-400 range, although some go higher. Most digital SLRs will work in 100-1600, and some can even cope with 50-3200.

Examples
What does this mean for your pictures? Here are some advantages and disadvantages of two extremes of the ISO range:

ISO 100

• Best in sunny conditions outdoors
• Gives good colour saturation
• Best at recording high-contast scenes
• Images have good definition (sharpness)
• Shadows and highlights reproduced in good quality
• File size (for JPGs) will be lower - more images on a memory card
• Leads to slower shutter speeds - can cause camera shake (blur)

[ISO100 - there is good colour saturation and sharpness in the picture]

ISO 1600

• Best in dim conditions - overcast days or indoors without flash
• Gives reduced colour saturation
• Best at recording low-contrast scenes
• Images have reduced definition (sharpness)
• Shadow areas likely to suffer from ISO "noise"
• File size (for JPGs) will be higher - fewer images on a memory card
• Leads to higher shutter speeds - often avoiding blurry pictures

[ISO1600 - there is less colour saturation and sharpness in the picture]

In the above images, I have not done anything to alter the quality apart from cropping and resizing for the web. But this doesn't show the full story - here's a full-size comparision between the photos - showing the centre portion round the doorway:

[Left, ISO100 - good quality, colours and sharpness. Right, ISO1600 showing poor shadow detail and lots of "ISO noise"]

Of course, these effects go gradually from one end to the other as you go through 200, 400 and 800. It's always a trade-off between being able to take (any) picture (in dim conditions) and the technical quality that will result from using a high ISO speed. And the beauty of digital is that you can change the setting from one shot to the next if you need to - in the days of film, you were stuck with it for the whole roll!

So, if your camera will let you set the ISO speed manually, why not have a go at experimenting with different speeds next time you are out shooting? You will then learn what gives you the best results in various lighting conditions.