How To Understand Your Camera’s ISO Ratings
by William Lulow
Most people know these days that the sensitivity of your camera’s CCD is now measured in numbers decided by the International Organization for Standards. In the days of film, the sensitivity to light of a film’s emulsion was decided by the American Standards Association (ASA), hence the ASA rating. You will remember that a “fast” film was rated at around 400ASA and a slow film, like Kodachrome was rated at 25ASA. You could “push” a film’s sensitivity by boosting its ASA. But you would pay for it with extra graininess caused by the underexposure and overdevelopment.
In the digital age, CCDs are much more sensitive to light than emulsions ever were. Now, ISO ratings of 25,000 are not uncommon. The good news is that higher ISO ratings do not have to be accompanied by grainier images. It is true that high ISOs do result in some extra “noise” in the images, but in many cases, that extra noise is tolerable depending on how the photographs will be used. One has to realize that by setting the ISO rating higher, you are still underexposing the image and you may have to alter it somewhat “after capture.” Occasionally, I have had to increase the contrast of these types of images.
So, keeping all these things in mind, you are able to alter an image’s exposure by adjusting the ISO setting, the aperture or the shutter speed. If you leave your camera to its own devices by setting it on AUTO, it will pick the ISO for you based on the amount of light present and your lens’ maximum aperture and shutter speed. If there isn’t enough light for your current settings, it will automatically increase the camera’s ISO setting to accommodate the lighting situation. Remember, the higher the ISO setting, the more noise you are admitting to your image. So, I think you should use the camera’s meter to find out how much light there is and then set your ISO number manually.
Here is an image shot with available light. The ISO setting was 2000. The shutter speed was 1/320th of a second and the aperture was f/2.8.