ISO
By Rod Barbee
The ISO rating for film simply tells you how sensitive to the light the film is. The higher the ISO number, the more sensitive to light it is. This means that it takes less light to get the same exposure than it would take for a lower ISO number film.
ISO 200 film is more sensitive to light than ISO 100 film. This means that ISO 200 film needs less light than ISO 100 film. In fact, it takes half the amount of light to get the same exposure. That’s one stop.
ISO is subject to that doubling and halving we’ve already discussed. A 100 ISO film needs two more stops of light for any given exposure setting than an ISO 400 film (100x2x2). An 800 ISO film needs three stops less light than that same ISO 100 film (100x2x2x2).
The main practical effect is that by using higher ISO rated films, the photographer can use faster shutter speeds to get the same exposure. This is important with sports and wildlife photography where shutter speed becomes paramount.
The higher the ISO, the “faster” the film. ISO 400 film is a “faster” film than ISO 100 film, thus you can use faster shutter speeds.
We consider ISO 50 as a “slow” film. Considering ISO 50 and ISO 400 films, you’d need a shutter speed 8 times as long (three stops) for an ISO 50 film to get the same exposure as an ISO 400 film, assuming you’re using the same aperture setting.
There is a trade off however. Higher ISO speed films have more grain, which gets more pronounce as you enlarge the photo.
Slower films use finer grain; this means that they enlarge with greater success. Fuji Velvia 50 is a very fine-grained film preferred by landscape photographers because of its fine grain. I’ve had great results enlarging 35mm Velvia slides to 16x24 and 20x30 inch prints.
All of this just to say that there’s a third component to the exposure equation. Shutter speed and aperture affect the light entering the camera. ISO affects how much light the film needs for an exposure.
In the digital realm, higher ISO translates into more noise. By selecting a higher ISO number, you’re actually boosting the signal amplification of the sensor. Noise is the digital equivalent to grain. Noise appears as random colors and is best described as “splotchiness”, especially visible in darker areas and in areas of less detail, like the sky.
© Rod Barbee. All Rights Reserved