Understanding ISO / ASA

Use a meter reading as a guideline rather than a dictate for correct exposure. This makes it important that you understand how your particular meter works so you can consistently get good results no matter what the lighting. The place to begin this understanding is the instruction manual that came with your meter or camera. The instructions should familiarize you with the meter's specific features, its flexibility, and its limitations. Most camera and exposure meter instructions provide the basic techniques of light measurement and mention some of the situations that may "fool" the meter. If you can't find the instructions, write to the manufacture for them.

It may be appropriate to understand one standard before actually handling light meters. This standard often referred to as ASA or more correctly as ISO. International Standard ISO 5800:1987 from the International Organization for Standardization (ISO) defines both an arithmetic scale and a logarithmic scale for measuring color-negative film speed. Related standards ISO 6:1993 and ISO 2240:2003 define scales for speeds of black-and-white negative film and color reversal film.

In the ISO arithmetic scale, which corresponds to the older ASA scale, doubling the speed of a film (that is, halving the amount of light that is necessary to expose the film) implies doubling the numeric value that designates the film speed. In the ISO logarithmic scale, which corresponds to the older DIN scale, doubling the speed of a film implies adding 3° to the numeric value that designates the film speed. For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°. Commonly, the logarithmic speed is omitted, and only the arithmetic speed is given (e.g., “ISO 100”). In such cases, the quoted “ISO” speed is essentially the same as the older “ASA” speed.

ISO or ASA (American Standards Association) in the most basic terms is the speed with which your film or digital camera responds to light, so the higher the ISO/ASA rating the more sensitive the film or CCD/CMOS sensor is to light.

In terms of film those with lower sensitivity (lower ISO speed rating like 50 or 100) requires a longer exposure and is thus called a slow film, while stock with higher sensitivity (higher ISO speed rating such as 400 or 800) can shoot the same scene with a shorter exposure and is called a fast film.

The same holds true for digital camera, but you are adjusting the sensitivity of the CCD or CMOS not actually using different film, one of the advantages of using digital format, you can change ISO setting for every second shot without having to physically change film stock!

The basic rule would be a higher ISO gives a higher shutter speed with the same Aperture settings, so less blur. The trade-off is that higher ISO also gives more noise or grain to your images, which can be a bad thing if it’s not a look you appreciate.

Slow shutter speed will give you pictures like the one on the right:

This is so because camera was not held steady during the exposure.

Or

Some what similar problem confronts us when exposure may not permit faster shutter speed to arrest motion of the subject! See the picture below to understand the problem:

ISO or ASA

ISO is the term generally used on Digital Cameras, the standard was ASA and in the later years ISO.

To get a bit more technical it was known as the ISO linear scale, which corresponds to the older ASA scale, doubling the speed of a film (that is, halving the amount of light that is necessary to expose the film) implies doubling the numeric value that designates the film speed so 50, 100, 200, 400, 800 and 1600. My experience with Nikon, their digital ISO ratings tend to be exactly the same as the real film counterparts where the same cannot be said about other manufacturers.