Using ISO to Get the Images you Want

By Patty Hankins and Bill Lawrence

In the last two newsletters, we covered two parts of setting the camera that traditionally determine exposure, either by setting the aperture or the shutter speed. In this newsletter, we thought we’d talk about the third consideration in determining exposure, ISO. Also known as ASA (a bit of trivia – ASA stands for American Standards Association, and ISO stands for International Standards Organization – but the ASA number is the same as the ISO number), the ISO is a rating of how sensitive your film or digital sensor is to light. Essentially, doubling your ISO has the equivalent effect on exposure as opening your aperture 1 f-stop (e.g. going from f8 to f 5.6) or doubling your exposure time (e.g. from 1/60th second to 1/30th sec). So, if you are shooting ISO 100 film or digital, and the perfect exposure is 1/100th second at f16, then by switching to ISO 200, you could get the perfect exposure by shooting 1/200th of a second at f16, or by shooting 1/100th of a second at f22.

So why do we care about ISO? Setting the ISO higher lets us use a faster shutter speed, when we need to stop movement, or it lets us shoot with a reasonable shutter speed and a small aperture (large f-stop number) to increase depth of field. For example, when we were out photographing the cheetah cubs at the National Zoo (you can see some of the shots on our website by following the link on the home page at www.hankinslawrenceimages.com), we were shooting at either ISO 400 or ISO 800 to try and get a fast shutter speed to capture the fast-moving cubs.

So why not always set the ISO as high as it will go? Two reasons. First, whether you are shooting film or digital, the higher the ISO, the more noise you get. In digital, this shows up as unpleasant graininess and color variation in your photo; in film, this shows up as larger and more pronounced grain. The good news is that both newer digital cameras and newer professional grain films allow use of pretty high ISO without the interference of excessive noise. So, in general, a lower ISO setting, film or digital, will give a better (less “noisy”) image. The second reason that we might choose a lower ISO than we would need solely for image quality reasons would be if we wanted a slow shutter speed or a very small depth of field (and thus large aperture). For example, Bill will often shoot waterfalls with Velvia transparency film – this film is ISO 50 (though sadly, they are discontinuing it in favor of an ISO 100 version), so will let him shoot a slow shutter speed even in relatively bright light, giving the water flowing over the falls a smoother look (see the last newsletter).

If you have a digital camera, try the same shot using the camera’s lowest and highest ISO, to get an idea of what your camera is capable of. Look at the shots 100% size on your computer monitor. The best places to look for noise are in areas of solid color, e.g. a solid blue sky or a uniformly dark shadow. It’s a good idea to know how much noise your camera produces at different ISOs, so you know what will give you an acceptable image. If your image is too noisy, noise reduction programs such as NeatImage or Noise Ninja can be helpful for digital camera files (or scanned film), but why use them if you can set the ISO lower and still get a good shot. Sometimes, though, you need the shutter speed or need the depth of field that a small aperture gives you. If so, don’t be afraid to turn up the ISO on your digital, or to reach for that high ISO film. Better a noisy image than no image at all.

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