In the last several posts, we have been talking about creative exposure, the exposure controls–aperture, shutter speed, and ISO–and how learning to work with these different settings can help you take charge of the look of your photos.
Today, we’ll look at the last of these controls–ISO.
ISO–which stand for International Standards Organization–is the third point on the exposure triangle and it represents the sensitivity of the camera’s image sensor to light.
If you’ve ever shot with film, ISO probably sounds familiar. In the days of film photography, you would choose a film based on its ASA rating. Film ASA ratings were (and still are) an indication of how sensitive film is to light. These ratings are also sometimes referred to as the speed of a roll of film and are measured in whole numbers typically starting at 50. So there's ASA 50, ASA 100, ASA 125, ASA 200, ASA 400, etc. The higher the ASA rating, the more sensitive the film is to light. As the sensitivity goes up, less light is required for image exposure. And, since film sensitivity impacts the amount of noise or grain in an image, as the ASA film speed increases, so does the graininess of your photos.
ISO is the digital equivalent of the ASA ratings. Both terms are used to describe the sensitivity of the recording medium. So whereas ASA indicates the sensitivity of film, the ISO indicates the sensitivity of the camera’s image sensor. Just as in film, ISO increases make the image sensor more sensitive to light. A higher ISO setting means that less light in required to properly expose an image. And, same as film, the higher the ISO, the more noise and graininess you’ll see in your photos.
The lowest ISO setting on most cameras is ISO 100.
There’s one big practical difference between ASA and ISO. ASA is a feature of each individual roll of film. So once you load that film into y our camera, you’re stuck with that ASA value for all of the shots taken with that roll of film.
ISO, on the other hand, can be changed on the fly, for each individual shot. So you can use a low ISO for one shot and then a high ISO for the next shot. In fact, most modern cameras allow you to choose from a sensitivity range of about ISO 100 to ISO 6400 and beyond.
The choice of ISO affects the look of an image in two ways.
As ISO changes, so does the sensitivity of a camera’s image sensor. And with that, the amount of light required to exposure an image changes.
The sensitivity of the sensor doubles when the ISO doubles. This means that changing from ISO 100 to ISO 200 reduces the amount of light needed to properly expose an image by half.
So a change in ISO can change the brightness or darkness of your photos. If you increase or decrease the ISO setting (without changing either of the other exposure parameters), the image will get lighter (with an ISO increase) or darker (with an ISO decrease).
As ISO increases, so does the graininess captured in your image. The added digital noise is caused by the increase in electrical charge across the image sensor needed to boost the sensitivity of the sensor’s photo receptors.
This graininess shows as speckles, or digital noise on your photos. So an image shot at a low ISO value will show a fine texture with little or no visible grain. On the other hand, an image shot at a higher ISO will be coarser-looking with visible grain.
You can see this effect in the series of images below, taken at various ISO settings. Each image includes an area of 100% zoom. You can click on any of them to enlarge.
As you look through these examples, keep in mind that the amount of grain at any particular ISO setting depends on the camera. Higher-end cameras tend to show less grain when shooting with larger ISO speeds. And, generally speaking, full frame cameras show less digital noise than those with cropped sensors.
We’ve seen that ISO affects how much grain shows in an image. And, so, you can control the amount of grain in your image by controlling the ISO.
But the real power in knowing how to work with the ISO setting is the fact that it gives you additional flexibility in setting the other exposure settings–the aperture and the shutter speed. Knowing how the different ISO settings affect your images lets you know how high you can go and that provides some latitude, giving you options in setting the aperture and the shutter speed that you might not otherwise have.
For example, setting a higher ISO can allow you to shoot without a tripod without worrying about camera shake ruining your image.
On the other hand, setting a lower ISO means that you can use a slower shutter speed, letting you capture motion as a blur–the soft, silky look of moving water–even in bright light.
Here are some very general guidelines/suggestions for setting ISO based on lighting conditions.
But there are other factors that you will want to consider when setting the ISO:
When you are hand holding your camera, it’s important to use a shutter speed fast enough to avoid camera shake. The typical rule of thumb is that the shutter needs to be faster than the focal length (in mm) of the lens. So if you are using a 100mm lens length, your shutter speed needs to be at least 1/100th of a second to avoid camera shake.
I personally like to add some wiggle room to this rule because I just don’t seem to have especially steady hands. So I usually double the hand-holding rule and try to use a shutter speed that’s twice as fast as the focal length of the lens. So this means that if I am shooting with a 100 mm lens, I use a shutter speed that is at least 1/200th of a second.
By the way, note that I used the word try above because sometimes rules need to be bent….
But either way, raising the ISO can be helpful when shooting handheld because lighting conditions can sometimes make a fast enough shutter speed impossible otherwise.
If your subject is moving and you want to capture an image that freezes that motion, you will need a fast shutter speed. Depending on lighting conditions, you may need to raise the ISO to get there.
Again, if your subject is moving and you want to show that motion as a blur, you will need a slow shutter speed. Depending on lighting conditions, you may need to lower the ISO to get there.
This is another situation where you might need to use a higher ISO. In order to capture a deep depth of field, you need to use a small aperture opening and that restricts the amount of light getting to the image sensor. An increase in the ISO will shorten the shutter speed and allow you to capture a crisp image.
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We've seen how managing the ISO setting allows you to control the amount of grain that shows in your photos. But it does more than that. Understanding and working with the ISO setting gives us added flexibility in terms of setting the other two exposure settings–aperture and shutter speed.
The bottom line is that ISO is an important and useful tool, and one that you will want to be comfortable with if you are looking to take creative control of your exposure.