The ISO (or light sensitivity rating) is an algorithmic value that determines the film’s or image sensor’s specific sensitivity for light. These indicators are the ISO values on a roll or the ISO setting on a digital camcorder.
ISO stands for International Standards Organization. The three elements that determine the final exposure are the ISO rating, shutter speed, and aperture setting.
The ISO rating is a range of 25 to 6400, or more, that indicates the specific light sensitivity. The film stock or image sensor that is less sensitive to light will have a lower ISO rating. A higher number, on the other hand, indicates a greater sensitivity to light. This allows for film stock or image sensors to perform better in low light conditions.
ISO Sensitivity & Image Noise
Film stocks had a lower ISO rating which meant that the photosensitive salt grains on film acetate were finer, producing a cleaner, smoother image. Higher ISO ratings had more jagged salt grains, which resulted in “rougher” images.
Digital photography follows the same logic. The lower the ISO rating, and the more sensitive the image sensor, the smoother the image will be.
Higher ISO ratings (more sensitive) mean that the image sensor must be more powerful to create an effective image. This produces more digital noise (multi-colored speckles in shadows and midtones).
What is digital noise? Digital noise is any light signal that doesn’t originate from the subject, and creates random colors in an image. Digital camera engineers designed the image sensor to work best at the lowest ISO, just like film. This is ISO 100 on most digital cameras. However, some DSLRs with high-end features can lower the ISO to 50 or 25.
Another thing about “grain”: In the traditional non-digital image many film photographers found creative and artistic ways to use grain to alter the mood and tone. Digital noise is undesirable due to its nature as random clumps and colored speckles. Some photographers have discovered limited creative ways to make use of digital noise. Perhaps you are able to join this rare club.
ISO Speed vs. motion blur
Low ISO ratings result in smoother, more color-accurate images. This requires the use of ideal lighting conditions. There are certain subjects you will not be able to photograph in low-light conditions. You may also want to photograph fast-moving objects such as a hummingbird or racehorse. Both situations require higher ISOs in order to get images that have an acceptable exposure.
Digital cameras allow you to increase your ISO without the need for film. This flexibility allows you to achieve the image you desire much faster than with film. You can also use higher ISOs to reduce motion blur and/or camera shake. To create motion blur artistically, you can decrease your ISO. The shutter speed can be decreased to less than 1/30s to achieve motion blur, but still produce smooth, noiseless images.
ISO Speed and Image Sensor Size
What ISO setting produces the most digital noise depends on the size of the image sensor in the digital camera. It is important to understand that the size of an image sensor does not necessarily correspond with its pixel count.
The actual dimensions of an image sensor are the size. The image sensor size has always been smaller than a 35mm frame for digital photography. The sensor on point-and-shoot cameras was small. Most DSLR cameras have an image sensor that is the same size as APC film (23x15mm). Higher ISOs like 800 produce more digital noise from smaller image sensors. This is because more pixels are packed into a smaller area.
Many DSLR manufacturers currently produce image sensors that are the same size and shape as 35mm film frames (called Full Frame). The Full Frame sensor is larger and allows for larger pixels to be packed onto it. This results in smoother images, with no grain, at ISO settings of 1600 or higher (in some cases). The larger pixels on Full Frame sensors are more sensitive to light than smaller ones. This means that the electronic energy needed to replicate ISO 800 won’t produce as much digital noise. The Full Frame cameras allow you to capture dynamic images even in low-light conditions.
Image Quality and ISO Sensitivity
It is important that you remember that the ISO rating will determine the image quality.
Although most digital cameras come with an “Auto ISO” default setting, it reduces your control as it can set a higher ISO automatically. This will cause a grainier image (or more noise) than other settings that could have produced a better exposure but less noise.
Two main ways that increasing ISO can affect image quality are: 1) It reduces the distinction between fine details. 2) If you make digital prints or enlarge an image, the inherent digital noise in the image will cause a “muddier image” after conversion.
We’ve already stated that the lower your ISO setting, the better and more natural-looking images you will get. Images with a lower ISO will have more vivid colors and be more appealing to the eye. The best results will be achieved with ISO 100 to 200. You can use ISO 400 depending on your camera’s image sensor and engineering to get sharp, clear images for most enlargements (20×24).
When creating any digital photograph, it is crucial to know the light sensitivity of your image sensor (ISO). A smoother final image is achieved by using a higher ISO rating. This is called the reciprocal scale. How you create your photos depends on the light sensitivity of the image sensor.
You might have to add light sources in order to maintain a low ISO for smoother photos. If you have to shoot in adverse lighting conditions, such as low light or high contrast lighting, or you need to use a faster shutter speed, you will have to sacrifice the smoothness of a low ISO for a shot with a higher ISO.
Also, you can increase ISO but deal with the consequences later. If the alternative (a lower ISO setting), means that you miss the shot then it is better for you to make the trade-off.