ISO (International Standards Organization) speed is a standard method for quantifying film’s—hold on digital’s coming—sensitivity to light. Lower numbers, such as 50 or 100, represent less sensitivity; while higher numbers, such as 800 or 1600, show a film that’s more sensitive to light. ISO numbers are proportional to their sensitivity to light. As you double or halve an ISO number, you double or halve film’s sensitivity to light. Film with an 800 ISO is twice as sensitive to light as 400, and 800 film is half as sensitive to light as 1600.
You may (or maybe not) be surprised to learn that despite what it says on the dial or button, digital cameras don’t have true ISO speeds, which is why you’ll often see the term “ISO Equivalent”, tossed around in camera specifications. But manufacturers have developed technologies to make imaging sensors respond similarly to the way that film responds to light. Digital cameras offer a range of ISO speeds that vary based on the camera’s design but all you really need to know that when you set a digital camera to ISO 400, you can expect a similar response to light than ISO 400 film would produce.
The above photograph made at a winery in Sonoma California. It was shot with a Panasonic Lumix DMC-LX7 with an exposure of 1/50 sec at f/1.5 and ISO 800 but used the camera’s exposure compensation control to slightly overexpose the photograph to open up the shadows in the room and get the shot you see here.
Trivia: Digital cameras are not afflicted with the problem of reciprocity failure as film can be. As film is exposed using slow shutter speeds,t becomes less sensitive to light, shifting color balance. That’s the good news, the bad news is that noise increases during long exposures and high ISO settings. More about noise here.
Along with photographer Barry Staver, Joe is co-author of Better Available Light Digital Photography that’s sadly out-of-print but used copies are available from your friendly neighborhood book or camera store, eBay or Amazon.