Understanding ISO

What is ISO? The three letters stand for International Organization for Standardization (ISO). ISO is one of the three pillars of photography referred to as the “Exposure Triangle” (the other two being Aperture and Shutter Speed) and every photographer needs to thoroughly understand it to get the most out of their equipment. It's one of the most prominently featured specifications of any modern digital camera, and it's one single aspect that can make a night-and-day difference in the outcome of your shots. So, what is it? ISO is the measure of your cameras’ digital image sensor’s sensitivity to available light. Unlike the old days when you literally had to change the film to change its sensitivity (formerly expressed as ASA), digital cameras allow you to just set the ISO as desired. Typically, ISO numbers start from 100-200 (Base ISO) and increment in value in geometric progression (power of two), e.g. 100, 200, 400, 800, 1600, 3200, 6400, etc. The important thing to understand here is that each step between the numbers effectively doubles the sensitivity of the image sensor.               The higher the ISO, the more sensitivity and the greater the image sensor's light gathering capacity; meaning the less natural light that is needed to create the desired exposure. Though a higher ISO allows you to capture pictures in low light situations, the downside is that you will end up with increased image noise (more grainy). 100-200 ISO, usually the default setting, is generally accepted as ‘normal’ and will give you lovely crisp shots with very little noise/grain. Even though you should always try to stick to the base ISO in order to get the highest image quality,  there will be times when working in low-light conditions that you will not be able to do so. Though most people tend to keep their digital cameras in ‘Auto Mode’ where the camera selects the appropriate ISO setting depending upon the conditions you’re shooting in, there may be advantages to selecting your own ISO.

Noise: Digital noise in photos taken with digital cameras is random pixels scattered all over the photo that degrades the photo quality. Digital noise usually occurs when you take low light photos (such as night photos or indoor dark scenes) or you use very slow shutter speeds or very high sensitivity modes. Image noise is random (not present in the object imaged) variation of brightness or color information in images, and is usually an aspect of electronic noise. Produced by the image sensor and circuitry of a digital camera, it is the undesirable by-product of image capture that adds spurious and extraneous information. Think of it as analogous to the subtle background hiss you may hear from your audio system at full volume. For digital images, this noise appears as random speckles on an otherwise smooth surface and can significantly degrade image quality. Although noise often detracts from an image, it is sometimes desirable since it can add an old-fashioned, grainy look which is reminiscent of early film. Some noise can also increase the apparent sharpness of an image. Noise increases with the sensitivity setting in the camera, length of the exposure, temperature, and even varies at the same settings on different camera models. A sharp noisy picture is far better than a blurry fine grained one or no picture at all. Digital noise can be dealt with using a combination of optimizing the camera settings and by removing it with professional software.

Noise Reduction: Always a factor is the amount and type of noise reduction being applied in the camera. Because all pixels collect some noise, every digital camera runs processing on every image (except for a RAW file that can be changed later) to minimize that noise. Newer cameras use newer technology to reduce that noise, with the result being less noise at similar ISOs than what earlier cameras could achieve. Improvements in this area have been huge in recent years.The second thing to remember is that many post editing programs offer functions for reducing noise. If it is a matter of choosing between not being able to take a picture and suffering with a noisy image, I'd rather be able to take the picture at a higher ISO and then try to clean up the noise afterwards in a noise reduction software.

ISO Speed & Image Sensor Size: The size of the image sensor determines the ISO speed range that a digital camera can use without suffering from undue noise. One reason for this is because the pixels on the larger image sensor can be larger and therefore receive more light, and thus have a greater signal-to-noise (S/N) ratio. Simply put, if you have two image sensors, each with 4 megapixels resolution, but of different sizes, the 4 megapixels image sensor that is smaller will exhibit more noise at higher ISOs than the larger one. 4 million tiny pixels crammed into a 1/1.8 in. image sensor cannot compete in image quality with 4 million large pixels on an APS-sized image sensor. Most consumer digital cameras use 1/1.8 in. (and smaller) image sensors, so noise at high ISO is more of a problem. A digital SLR (dSLR), on the other hand, uses a large image sensor, usually full frame (24x36 mm) or APS-sized (half-frame). Noise is rarely a problem and the use of a high ISO 400 results in images with barely noticeable noise. Even as manufacturers attempt to bridge the gap between consumer and professional digital cameras by using a slightly larger image sensors, the "megapixels race" has meant that ever more pixels are being crammed into a small area. Where before there were 5 million pixels on a 2/3 in. image sensor, now we see 8 million pixels crammed on the same sized image sensor. It is therefore not surprising that noise remains a problem. And which is why you should not be fooled by the "more megapixels is better" mantra.

Choosing An ISO Setting: The result of the sensor’s increased sensitivity due to higher ISO settings, means that it needs less time to capture an image. When you do decide to override your camera and choose a specific ISO you’ll notice that it impacts the aperture and shutter speed needed for a well exposed shot. For example – if you bumped your ISO up from 100 to 400 you’ll notice that you can shoot at higher shutter speeds and/or smaller apertures. What does this mean? if your camera sensor only needed exactly 1 second to capture a scene at ISO 100, simply by switching to ISO 800, you can capture the same scene at 1/8th of a second or at a shutter speed of 1/125 sec. That can mean a world of difference in photography, since it can help to freeze the motion of a subject that would otherwise end up being blurred when shot at ISO 100, thereby ruining the shot. When should you increase ISO? … Anytime when there is not enough light for the camera to be able to quickly capture an image. If you don’t have a tripod and/or the subject is moving, increasing the ISO as it will enable you to shoot with a faster shutter speed and still expose the shot well. Anytime shooting indoors without a flash, a higher number will be able to freeze motion, but may require the use of a tripod. Whenever you need to get ultra-fast shots, like taking bird pictures. Always ask the question if it is OK for you to introduce noise to the image. Other situations that might require you to push ISO to higher settings might include indoor sporting events, concerts, or the interior of buildings such as art galleries, churches, etc. Auto ISO, which works great in low-light environments, was introduced into digital cameras several years ago to help photographers manage that balance. Turning on this feature allows the camera to push the ISO up when it decides the shutter speed is getting too low for a good picture. The beauty of this setting, is that you can often set the maximum ISO to a certain number, so when the ISO is automatically increased based on the amount of light, it does not cross the set barrier. So, if I want to limit the amount of grain in my pictures, I typically set the maximum ISO to 800.

Summary: You already know it is tough to shoot good digital photos in low light conditions without a flash. The trick is really to get more light into the camera without using that harsh flash – and we can do that by cranking up the ISO setting to the higher part of the range and setting the camera to Aperture Priority (an f-stop (bigger aperture) that allows more light through. It also helps to shoot in RAW mode so that you capture maximum detail in your digital shot (no compression in the captured image). ISO is an important aspect of digital photography to have an understanding of if you want to gain more control of your digital camera. A solid understanding of ISO and experimenting with different settings and how they impact your images will help you make smarter decisions about how to set your camera that, in turn, will lead to better pictures.