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ISO – Meaning and Myth

ISO is a measure of the sensitivity of a film or sensor to light. But unlike film, which has a fixed sensitivity determined by the amount of silver present within the emulsion, the sensitivity of a digital camera is variable.

Setting the camera to ISO 400 effectively makes the sensor twice as sensitive to light as it would have been at ISO 200. By doubling the ISO you’re effectively doubling the sensitivity of the sensor. As a consequence you only need half as much light to record an image. This feature enables you to shoot at a higher Shutter Speed than you otherwise would, enabling you to reduce the possibility of either camera or subject movement. One of the great advantages of a digital camera is the ability to change ISO, from frame to frame. When shooting film you need 2 or more cameras to have anything like the same flexibility. Otherwise you have to finish your film, prior to replacing it with a roll of different sensitivity/film speed.

Digital cameras increase sensitivity by generating more current (electricity) through the sensor. But as sensitivity increases, above your camera’s default ISO, the introduction of random, artificially created data (artefacts) is generated by background signals from the sensor’s pixels. This phenomena, referred to as Noise, will appear as a seemingly random pattern of dots, either white (Luminance Noise) or one of a variety of different colors (Color Noise).

You will obtain the best image quality at your camera’s default ISO setting (ISO 100 for Canon and ISO 200 for Nikon). However, by changing your ISO to a higher setting you may be able to minimise movement and, as a result, make a seemingly sharper picture.

The Canon 5D Mark II, Canon 7D and Nikon D3X cameras feature higher ISO capabilities with less loss of quality than was the case with previous models. Most folks would be unlikely to see a discernable increase in noise, assuming you’ve produced an appropriately exposed image from a well-lit scene, up to and including ISO 800.

High ISO settings can be utilised to better freeze subject movement in action photography, or to achieve a narrower aperture and, thereby, extend depth of field (DOF). The actual Shutter Speed you shoot at is dependant on a number of factors including lens aperture, ISO, amount of available light and the effect you are looking to achieve (e.g. freezing action or creating blur).

See if you can complete the following exercise?

ISO                                    Shutter Speed                        Aperture

400                                    1/500 second                                    f 8

200                                    second                                       f 8

100                                    second                                       f 8

ISO                                    Shutter Speed                        Aperture

400                                    1/125 second                                    f 16

200                                    1/125 second                                    f

100                                    1/125 second                                    f

It’s very important to understand that changing the ISO does not make the image either lighter or darker and, therefore, has no effect on exposure. It is a Myth! Increasing ISO simply provides you with a faster Shutter Speed, helping you to better freeze action; or a narrower Aperture, to achieve extended DOF than would be the case at a slower ISO.

Canon 5D camera with Canon 90mm f1.2 L series USM lens

Canon 5D camera with Canon 90mm f1.2 L series USM lens

The above photograph was made along the Great Ocean Road in Victoria, Australia. Despite the fact that the image was made at the very end of the day, shooting at a high ISO would not have created the powerful effect I’ve achieved by marrying the quality of ISO 100 with a long exposure to show the movement of water (and time) through the still photograph. The image was made with a Canon 5D camera and Canon 85mm f1.2 L series USM lens.

By the way the answers to the quiz, from top to bottom, are as follows:

1/250 second

1/125 second

f 11

f 8

Copyright All Rights Reserved

Glenn Guy, Blue Sky Photography


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