Question: Please explain digital ISO. How does it differ from 35mm film ISO?

Answer: In both film and digital photography, ISO is a standard measurement of the film or sensor response to light—sensitivity. The International Organization for Standardization (based in Geneva, Switzerland) is an organization made up of standardization groups from all over the world.

Film ISO is a fixed sensitivity value resulting from the manufacture of the film. The sensitized material in film consists of microscopic light sensitive crystals whose structure changes when exposed to light, and become images on the film when chemically processed—either as slides or negatives. The ISO varies by the size of the crystals in the film, which are “grown” in manufacture to different sizes to make the different speed film emulsions. Small crystals result in “slow” films. Large crystals result in “fast” films. All film emulsions have a mix of many-sized crystals.
 
The advertised speed of a film shown on the packaging relates to a speed rating in which the film can be expected to perform predictably under standard lighting conditions, set by the ISO organization. Therefore, an ISO 200 film should perform the same in terms of exposure from different manufacturers and with different cameras. This also means that the exposure systems in the cameras should perform predictably in relation to the film speed.

For the average user, the film ISO is a set value with some exposure latitude. Exposure latitude can be employed by users with advanced photographic skills to alter the performance of film through exposure and processing techniques. However, the ISO cannot be changed from frame to frame while the film is in the camera. You are “stuck” with the ISO of the film until the roll is finished and you reload the camera.

Without a doubt, one of the most attractive aspects of digital photography is that you can change your “film type” from shot to shot using different menu settings. A digital camera uses light-sensitive photodiodes (pixels sites or “pixels”) in the sensor instead of the light-sensitive chemical crystals found in film. The sensitivity of the pixels can be increased or decreased by varying the gain (voltage) on the pixels—much like changing the volume on a stereo.

In the early years of digital photography, ISO values shown on digital camera menus emulated the ISOs of the films available at the time—64, 100, 160, 320, and 400. This was so that people could relate to how their digital cameras performed in relation to their film cameras. Today the ISO values are mostly multiples of 50 and 100.
The photographer has the option of selecting preset ISO settings using the camera menus and different shooting modes. However, since digital cameras are capable of automatic exposure, if you look in the metadata of images shot in auto modes, you may find that the camera is “making up” ISOs for specific situations by creating its own gain values.

As you increase the ISO, the image may become grainier. In this respect, digital photography is no different than film photography. In film photography, if you wanted a higher ISO, the trade-off was larger grain. In digital photography the same thing is true; but, unlike film, the camera and imaging software have tools to diminish the graininess.

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