ISO (also known as ASA) is a number (traditionally in range 100 – 800 on most films) that describes the film’s sensitivity to the light. Previously, when using film cameras, ISO was also referred as film speed, and was marked on the film box. Higher number corresponded to higher sensitivity to light, and therefore better suitability for use in low-light conditions.
In digital photography, films are no longer used, but the still rules still apply. This time ISO setting defines the sensitivity of the digital camera’s image sensor. Higher ISO values correspond to higher sensitivity and therefore better low-light performance.
What is relation between ISO sensitivity, aperture and shutter speed ?
We have already talked about shutter speed & aperture in our previous posts. I have read somewhere an excellent comparison between taking a photo and sunbathing, and it also explains to a degree how a photo is created. If you think of your skin as a sensor, the amount you will tan or burn (final look of the photo – in comparison), will depend on the intensity of the sun hitting your skin (aperture), the amount of time spent sunbathing (shutter speed), and your skin’s sensitivity (ISO setting). All 3 factors contribute to the final look of the photo, however in most cases ISO setting is the most overlooked one by beginners.
How can I manipulate ISO setting to my advantage ?
Higher ISO will allow you to take photos in low light (Left ISO 100, Right ISO 1600)
The benefits of ISO sensitivity manipulation become most evident in low-light photography. Lower light means less amount of light hitting the sensor. To maintain the correct exposure a photographer would need to either open the aperture, or lower shutter speed. Open aperture results in less depth of field, meaning possibly blurry background, and slower shutter speed would result in blur due to the camera movement (assuming the photographer is hand-holding the camera). To avoid these issues, the photographer could either use a tripod (which is always the preferred solution), or simply increase the ISO sensitivity. Photos in the example above illustrate the benefit of using higher ISO setting in low-light situations. Both of the photos above were taken using the same aperture and shutter speed, however due to higher sensitivity, the right one was properly exposed. Increasing also works well in areas where a flash is explicitly forbidden, such as museums, art galleries, some indoor sports events etc.
One could ask: “If I can simply increase ISO sensitivity, why do you recommend tripod over ISO increase ?”
Noise – the consequence of using higher ISO
Higher ISO-1600 (left) vs slower shutter speed-with tripod ISO 100 (right). Click for bigger image
Digital noise is a side effect of increasing the ISO sensitivity. It basically occurs due to the fact that sensor is more sensitive to the amount of light, meaning that in some (especially darker areas) it becomes harder do get accurate reading on color and luminosity of the pixel, resulting in higher amount of noise obscuring image detail. Example above illustrates the use of higher ISO vs slower shutter speed. Left image was taken using ISO 1600, while the right one was set to ISO 100 with the help of tipod. Check photos below for the same image at 100% magnification.
Higher ISO (left) vs slower shutter speed (right) at 100% magnification.
It is clearly visible that the left photo exhibits a significant amount of noise when compared to the right one, resulting in much coarser grain. Most digital cameras perform well in lower ISO setting (100-400), but usually only professional DSLRs can successfully cope with higher ISO settings. A general rule of thumb is: if you want noise free image, use lowest ISO possible, and if needed turn noise-reduction feature on.
However, notice that I mentioned that noise is a consequence, not a downside of using higer ISO. If you want to re-create a black/white vintage-style image with your camera, a moderate amount of noise can help recreate the vintage look, and add some extra credibility. Finding the correct amount is the key to success. Happy shooting.
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