ISO

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. This also applies to the ISO setting of digital cameras. Fortunatly, we no longer need to change out rolls of film just to use a higher or lower ISO setting. I do love the new technology!
As you might guess, ISO is subject to that doubling and halving we’ve already discussed. A 100 ISO setting needs two more stops of light for any given exposure setting than an ISO 400 setting (100x2x2). An 800 ISO setting needs three stops less light than that same ISO 100 setting (100x2x2x2).
The main practical effect is that by using higher ISO setting, 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 “slow”. Considering ISO 50 and ISO 400 films/settings, you’d need a shutter speed 8 times as long (three stops) for an ISO 50 setting to get the same exposure as ISO 400, assuming you’re using the same aperture setting.

There is a trade off however. Higher ISO speed films have more grain, and higher ISO settings on digital cameras result in more noise, which gets more pronounce as you enlarge the photo. 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.

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. Early digital SLR cameras were subject to some pretty nasty noise once you got above ISO 400 or so. Todays' modern digital cameras can perform very well at much higher ISO settings. Be sure to test your camera just to see where noise becomes unacceptable to you.

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 or sensor needs for an exposure.

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