What is ISO?
ISO, which stands for International Standards Organization, is the light sensitivity rating of a digital image sensor. If you have read my article on the Basics of Exposure, then you may recall that alongside shutter speed and aperture, ISO is one of the three pillars that control exposure on your camera. As you increase the ISO, the sensor becomes more sensitive to light, which allows it to capture more light without slowing down the shutter speed or opening up your aperture. Each time you double your ISO number (like from ISO400 to ISO800), the sensor becomes twice as sensitive to light and therefore require only half as much light to attain the correct exposure.
The ISO range for a camera can be as low as ISO35 or ISO50 and as high as ISO240, 800. Majority of DSLRs tend to start at either ISO100 or ISO200 and top off at ISO3200 or ISO6400. Each camera has a “base ISO,” which is the minimum ISO rating that will provide the cleanest image for that camera. This is usually ISO100 or ISO200, but there are some cameras that have a base ISO of ISO50.
Film vs. Digital
For film photography, ISO or ASA (American Standards Associations) speed refers to the film speed of the film roll. Typically, when you are shooting outdoors in a sunny day, you will be using an ISO100 or ISO200 film. If you’re shooting indoors, you would probably switch to an ISO800 film or faster. What’s hard, of course, is if you have to go from an outdoor to indoor location quickly, because that usually means that you would either have to change the roll of film or compensate with your aperture or shutter speed.
Great thing about digital photography is that you can change your ISO speed on the fly, making it easy to transition between exterior and interior shots. On top of that, you can actually view on the LCD screen how your image looks in that particular ISO.
Image Quality, ISO, and Noise
As you go increase your ISO, you start to introduce “digital noise” to your image. Similar to film, which has more and more grain the higher you go in ISO film speed, digital sensor creates more and more noise as you increase your ISO. Noise is the by-product of the increased electric charge needed to make the sensor more sensitive to light and looks like speckles on the image. The consequence of more noise, however, is a rougher-looking image and a decrease in image quality.
There are two types of noise, luminance noise, and chroma noise. Luminance noise retains much of the original color because this type of noise only affects the brightness of the pixels. Chroma noise, on the other hand, looks like colored speckles or grain, and is largely unattractive. This is because the noise is affecting the color of the pixels rather than just the brightness of the pixels. Luckily, post-processing software like Lightroom 4 does a good job in minimizing chroma noise.
Different cameras have different thresholds on when this noise starts to degrade the image quality. This is known as the signal-to-noise ratio. There are several factors that determine signal-to-noise ratio. Aside from the processor of the camera, the megapixel count and the size of the sensor play a role in how well a camera can minimize noise.
Megapixel and Sensor Size
The size of the sensor and the amount pixels on that sensor directly affects the potential amount of noise that can occur when you are shooting at higher ISOs. Imagine that a sensor is like a swimming pool and the pixels are the amount of beach balls that can float in that pool. If you only have 100 balls, you can fit larger size balls in the pool. If you want to fit 1,000 balls, you would either have to have a larger swimming pool or use smaller balls. That is essentially the same relationship with pixel count and sensor size.
A sensor is made up of millions of tiny light-gathering receptors called pixels. One megapixel (MP) consists of one million pixels. If you have two same size sensors and one has 12MP and the other has 24MP, the 12MP sensor can have larger pixels than the 24MP sensor. The larger the pixel size, the better that pixel is in gathering light, just like the larger the beach ball, the more air it can hold. If you want to increase the number of pixels from 12MP to 24MP without decreasing the pixel size, then you would have to increase the physical sensor size. This is like having a larger swimming pool to hold more beach balls without decreasing the size of the balls. The size of the pixel in relation to the sensor size is known as the pixel pitch and is measured in microns.
So as you increase your ISO, you will start to get noise at a lower ISO with a compact camera than with a larger sensor DSLR. A compact camera image can look noisy at ISO800, whereas a full-frame DSLR image can have little to no noise all the way up to ISO3200.
Recommended ISO for Different Scenarios
Here are some recommendations of what ISO to use in different lighting conditions.
• Outdoors with sunny skies: 100-200
• Outdoors with overcast, sunrise and sunset: 200-400
• Well lit interior: 400-800
• Semi-lit interior: 800-1600
• Nightime exterior or dimly lit interior: 1600-6400
• Indoor or nighttime sports: 1600-8000
The other factors that will determine which ISO to use is what shutter speed and aperture combination that you want to use. If you are shooting fast moving subjects that require a fast shutter speed of 1/500th sec or faster, you have to compensate for exposure by either opening up your aperture or increasing your ISO. Using a lens that is “fast” or have a large maximum aperture like f/1.8 allows you to shoot in a lower ISO as opposed to if you are using a lens with a maximum aperture of f/2.8.
Sometimes, you have no choice but to increase the ISO. This is particularly true for shooting events like a wedding reception where you want to have a fast enough shutter speed to make sure your subjects are not blurry.
Additionally, if you want to use a smaller aperture like f/16 to increase the depth of field for landscape photography, you also have to compensate for exposure by either using a slower shutter speed or increasing your ISO.
Now, if you place your camera on a tripod and you’re shooting landscape or the city skyline, then you can shoot during the day or night without having to change your ISO. All you have to do is slow down your shutter speed until you have the correct exposure.
So to recap, the ISO rating refers to the light sensitivity rating of a sensor. The rule of thumb is to shoot at the lowest ISO possible given the lighting condition and shutter speed/aperture combination that you are using. The higher the ISO, the more noise creeps into your images, so if you can get away with using a lower ISO, then do so.
Sensor size and megapixel count also affects how soon noise will start to creep into an image as you increase the ISO. A 24MP compact-camera sensor will be a lot noisier at ISO1000 than a 24MP full-frame sensor. Typically, a higher-end camera does a better job in controlling noise at the higher ISO range than a lower-end camera.
Finally, it’s important that although you may have to use a higher ISO to get a shot in low-light, it is better to use that higher ISO and have more noise than to not get the shot at all or to have too much motion blur from too slow of a shutter speed. At least with noise, you can deal with it in post.