Your ISO Choices Affect Exposure Too

Today’s Post by Joe Farace

The ISO (International Standards Organization) standard is a method for quantifying film’s sensitivity to light. Hold on, digital is coming in the next paragraph. 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. Conveniently, ISO numbers are proportional to their sensitivity to light. As you double or halve an ISO number, you double or halve the 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 your camera dial, button or LCD screen, digital cameras don’t really have true ISO settings, which is why you’ll sometimes see the term “ISO Equivalent,” tossed around in camera specifications. But manufacturers developed technologies to let their imaging sensors respond similarly to the way that film responds to light. DSLRs and mirrorless cameras offer a range of ISO speeds 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.

These days camera manufacturers are offering higher and higher ISO settings with their newer cameras. The full-frame Pentax K-1 Mark II, for example, has a maximum ISO setting of 819,200. You can read my review of the camera here that includes some shots made at that setting.

How I made this shot: This image was made near the entrance to the Corkscrew turn at Mazda Speedway at Laguna Seca during the nighttime portion of a 12-hour ALMS race. For some images I cautiously used a speedlight; you can some examples here. But for others it was just what was left of the available light. Camera was an EOS 50D with a now discontinued EF 22–55mm f/4–5.6 USM lens (at 55mm.) For some thoughts on this camera/lens combination, read my post Love The One (Camera) You’re With. Image was shot with an exposure of 1/20 sec at f/8 and ISO 800, panning during exposure.

Back to film for a final comparison: When film is exposed using slow shutter speeds, it becomes less sensitive to light and, in turn, shifts color balance that’s caused by reciprocity failure. For most photographic materials, reciprocity is accurate over a specific range of values of exposure duration but becomes inaccurate when departing from this range. Fortunately, digital cameras are not afflicted by this problem. That’s the good news, the bad news is that during long exposures and high ISO settings there is a corresponding increase in digital noise. You can read more about noise here.



Along with photographer Barry Staver, Joe is co-author of Better Available Light Digital Photography that’s out-of-print but new copies are available for $21.88 or used copies for giveaway prices—less than three bucks—from Amazon, as I write this. Kindle version, for some reason, is really expensive.