How to Calculate the ISO in Photography

Stopping action with low light may require a faster ISO setting.

In film photography, ISO was determined by your choice of film. You read it and set it on your camera or meter. A low-ISO, or slow, film — 100 or lower — was for when your subject wasn't moving, your camera was steady and there was plenty of light available. Faster films — as high as 1,000 even for amateurs — were for lower light and faster subjects. The risk was coarser grain, or visible roughness around the bits of light-sensitive material, in faster films. Your digital camera comes with a single light sensor, perhaps at a stated ISO equivalent. You may be able to enhance the signal between the sensor and the memory, but electronic amplification can create electronic noise.

Things You'll Need

• Manual for your digital camera
• Guide number (GN) for your flash unit
• Tripod

Instructions

1. Locate the controls for changing the ISO setting on your particular camera. Most models use either a button plus a dial or a setting in one of the displayed menus. Some automated settings, such as for sports photography, may also include changes to the ISO setting.

2. Check the guide numbers on any accessory flash unit you consider pairing with your camera for studio or journalistic photography. The guide number indicates the power of the flash and may be based on a specific light sensitivity. The guide number, multiplied by the distance from the flash to the subject at the specified ISO setting, yields the appropriate aperture for proper exposure.

3. 

   A fast ISO setting allows better control of focus on small subjects.

   Stabilize your camera on a tripod or wall to enable a lower ISO setting in low-light conditions. The lowest ISO setting appropriate to any particular shot allows you to capture the sharpest details and best colors. Combine stabilization with a fast setting to achieve better focus control in extreme close-up macro shots of plants, insects or other natural elements.

4. 

   Fireworks and lightning can be captured at a low ISO and long exposure.

   Capture desirable changes over time, such as in fireworks or lightning, by using a low ISO setting and a long exposure. These images have high contrast, but the photons are actually produced fairly slowly. Your camera needs to look for a while to blend them together as your eye does.

5. 

   Changing ISO may reduce the glare of street lights.

   Bracket your shots of immobile subjects, such as buildings, by taking the same shot at several different ISO settings. Two of the great advantages of digital photography are the ability to change "film speed" as often as every shot and the low cost of "wasting" shots. Make note of the settings you've used for future reference, and evaluate the shots on your computer screen before discarding the less successful ones.

Tips & Warnings

• In the same situations that produced graininess in fast film, pushing your ISO setting too hard can produce electronic "noise" — stray pixels, especially in large solid fields of color. Noise is more likely to take a noticeable pattern than is film grain, which tends to only look a bit blurry. Bracketing will also help you evaluate noise and what you may have gained in other factors of the photo.

References

• The New Manual of Photography; John Hedgecoe
• Kodak Guide to Digital Photography; Rob Sheppard
• Digital Photography FAQ; Ronald Parr