Sensitivity

Speed kills. That's a common saying you hear in both defensive driving courses and in drug use prevention. In the world of photography, however, we've been exploring ways to get faster for a very long time. We get more speed with faster shutter speeds, faster flash synchronization speeds, wider apertures, and faster sensitivity. It's the sensitivity variable in exposure that we will explore in this post.

Sensitivity is expressed in ASA or ISO and is a measure of how sensitive film or a DSLR's sensor are. The lower the expressed ISO, the less sensitive the film or sensor. The higher the expressed ISO, the more sensitive the film or sensor. ASA is the original scale that many of us grew up with. We commonly used ASA speeds 64, 100, 200, 400, 800, 1600, etc. Each doubling meant that the film was twice as fast. For example, if the "correct" exposure is 1/125 second at f/8 using ASA 100 film, we could use 1/500 second at f/8 with ASA 400 film.

ISO stands for the International Standards Organization. The ISO set at least a couple of standards for expressing sensitivity. The standard we use today approximates the old ASA we used to use. I guess old habits die hard. So, when you set your DSLR's ISO to 100, 200, 400, 800, etc., it would be the same as if you inserted that speed film.

The huge advantage, of course, of DSLRs is that you don't need to rewind and insert new film as light conditions change. You simply change the ISO. During the course of a day, you can use ISO 200 outside and ISO 400 or more inside. That's fantastic flexibility.

In most presently sold cameras, the higher the ISO, the more noise that is introduced. For example, you will see much more noise at ISO 6400 than at ISO 200. However, manufacturers are building increasingly better sensors. For example, the Nikon D3 produces fantastic images at ISO 6400 -- much better than any older Nikon.

As a starting point, I use ISO 200 outdoors and ISO 400 indoors. When I use my Nikon D3, I don't hesitate in bumping up the ISO, when needed indoors or for sports, to ISO 800 or higher. By increasing the ISO in lower light, my shutter speed and aperture remain within normal working conditions. If I didn't increase the ISO, I may need a shutter speed that is too slow to handhold or an aperture that is either unreasonably wide (e.g., providing too little depth of field -- a topic we will explore later -- or much larger than the lens is capable of). I usually don't let the camera select the ISO because I want to know what's happening with the light.

Play around with different ISO settings in your camera when using it with different lighting conditions so that you understand how ISO affects your shutter speed and aperture.