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When I’m teaching my photography classes, the students and I are often neck deep into discussions of the exposure triangle; shutter speed, aperture size and sensor sensitivity (ISO). You can find great discussions and explanations of the triangle here on the dPS. I’d like to dissect the ISO corner of the issue, and give you a simple technique you can use to set the ISO to the best value for the situation. Understanding ISO is one thing but setting it correctly on the fly is another.
ISO is an acronym for International Organization for Standardization. It is a group that sets all sorts of standards for science and industry, but the meaning of ISO for photographers with digital cameras, is that it places a numerical value on the sensitivity of the camera’s imaging component, the sensor. Often compared to the film sensitivity rating called ASA (originally developed by the American Standards Association).
ISO is often seen as numbers ranging from 100 to 6400 and higher. The lowest number your camera presents is the lowest sensitivity setting; the highest number is the most sensitive. So, you can use the ISO settings to help you shoot in a variety of situations; bright and dark. Often you are taught to boost the ISO when conditions get dark but the opposite might be true depending upon your intent.
You are also told that high ISO settings degrade the quality of the image in the form of digital noise. Noise is a visual distortion of specks of light. Some is colored, and some just looks grainy. It is a similar look to the grain we would see in film negatives of higher ASA’s. Noise is worse in the shadows and appears more with higher ISO settings. This is true but the degradation of the image is gradual, and you can sometimes use ISO numbers that are quite high and get great, usable images.
You probably came to understand the concept of ISO as part of the exposure triangle, and then immediately asked, “What ISO should I set?” Hmmm, something is missing in the explanation!
First for tripod shooting, set your ISO to the lowest (thus highest quality) setting. There won’t be any camera shake if you practice good technique and your images will have the highest quality. This will hold true unless you need a shorter shutter speed for other reasons. If that is true, keep reading and use the handheld technique.
When shooting handheld, you need to balance the quality you are getting with the ISO setting versus the shutter speed that you can use to get a sharp shot, with no camera movement. For example, if you have your ISO at 100 and you need 1/10th of a second to expose the shot properly, you are going to have some camera movement, and your image will not be sharp.
The focal length of your lens makes a difference as well. The directions above assume you are using a lens that is typical to entry-level DSLR setups. For example the typical kit lens is a 18-55mm zoom. If you are using a longer zoom, say 200mm, then the slowest shutter speed you can hand hold is much shorter. The widely accepted rule states: one over the focal length (so 1/200th of a second in the above mentioned situation) is the longest shutter speed that you can hand hold for a sharp, no-shake image.
However, this is an old rule that was based on 35mm film (and it applies to full frame digital sensor cameras). Many digital cameras have a crop factor, or multiplier effect, for focal length because their sensor is smaller. Yours might be 1.5x, 1.6x or even 2.0x – check your camera’s manual to be sure. Assuming a 1.5x multiplier for a 200mm zoom shot, you would need 1.5 x 200 = 300, so 1/300th of a second is your slowest shutter speed for an image that has no camera shake blur.
At this point you may be thinking that you actually need Shutter or Aperture priority – that is fine. But now you know the ISO range that will work. Perhaps ISO 400 was where you ended up, and the meter was telling you that the shutter speed of 1/200 was going to work. Now you can switch to the mode you want. Let’s say you need Shutter priority to stop some subject movement. Set your camera to Shutter priority, let’s say a shutter speed of 1/400th of a second, and repeat the process of aiming at some possible compositions. If your camera starts blinking the aperture, you will know your setting is out of range. It is telling you the aperture cannot be opened any wider, so you’ll know you need to increase the ISO to use that shutter speed.
For Aperture priority, set the aperture you need for a desired depth of field and use the camera’s meter to evaluate the scenes as described above. If the shutter speed starts goes below your maximum for the lens you’ll know your settings are out of range. Then you will need to bump up the ISO.
Sometimes called Image Stabilization, this is technology that might be built into your lens or camera. It can allow you to shoot at speeds longer than normally recommended, since it counteracts the movement you introduce into the camera. Take the marketing information about how many stops longer you can shoot with a grain of salt. Do your own tests with your equipment, with your best steady technique. I find that most people can learn to get two extra stops of shutter speed latitude. So, in our scenarios above you could shoot at 1/15th and 1/50th of a second respectively.
Try this process for a while with the Program mode to get comfortable, then you can use all of the exposure modes to set your ISO quickly and correctly. As your experience grows you will set your ISO quickly with intelligent estimates. You will never have to guess about your ISO again.