Taking control of ISO

ISO in analogue

Back in the days of film, a photographer had to choose a specific film sensitivity (ISO speed) to load into her camera, knowing that the entire roll would have to be shot at that ISO. Although the roll might be only 24 or 36 shots, one expected to have the one roll in one’s camera for at least several days. (The wonderful freedom of taking shot after shot in wild experiments, only came after digital released us from concerns about the cost of film and processing.) And the choices to the ordinary consumer were not hugely varied: low ISO like 64 were favoured by some, while color print film was popular in ISO100, 200, and 400. My own preference was for black-and-white print film rated ISO400, but amenable when I developed it differently, to being pushed to ISO800 and, if I really needed it, ISO1600. Of course, the quality of the image could degrade: the grain of the film could become visibile, making it look textured where it should be smooth.

When one had a certain film sensitiviy, one also had neutral density (ND) filters to adjust if the scene was too bright, and tripods and slower exposures to use if the scene were too dark.

Digital ISO speed

Since ISO in a digital camera can be set for each photograph, we have been released from the concern of choosing the right sensitivitiy for the entire roll of film. Even though to me it still feels a bit like cheating, it is also liberating to be able to set an ISO sensitivity for each scene.

And the numbers! My worry about distracting noise in film images shot at ISO800 or 1600, seems almost quaint in comparison to today’s ISO ranges. Many digital cameras today can be set at ISOs from around 200 up to ISO12,800. An entirely different scale, and one which permits photographers to wander casually taking hand-held shots at night without worrying much about unwanted motion blur.

Despite our casualness, there are issues accompanying higher ISO speeds, and these deserve attention before people become too habitatuated to leaving the tripod at home. Image quality tends to degrade with higher speeds, for one. The silicon in the camera sensor has a specific sensitivity, which is usually round ISO200. So, actually, all digital cameras have built-in film which shoots at ISO200. However, just as I could push process my black-and-white film at ISO800 rather than 400, the digital cameras also contain processors which allow greater and greater extension of the effective sensitivity. Thus, although the camera’s sensor silicon is equivalent to ISO200, the camera’s processor can allow the photographer to shoot images at ISO6400, or even ISO12,800 or ISO25,600.

The thing is, the images shot at higher ISO are usually not as rich in real detail as those captured at lower ISO. The camera’s processor could be said to strain, as the input from the scene (the signal) is fainter compared to the background noise. It’s a bit like trying to get a clear signal on a radio when the broadcast is faint.

So, images shot at higher ISO with digital cameras will have some degree of random noise, often showing up as coloured dots making a random pattern. To improve the look of the image, many digital cameras today then smooth the image, to remove the possible noise. Unfortunately, the smoothing can smoothing away parts of the picture, especially fine detail. This, the camera may produce an image with less noise but without some of the original detail.

Take your camera off auto ISO

So, what might you do? Some recommend acquiring equipment with larger sensors, as small sensors are more prone to noise. However, I have a simple first suggestion for you. Take your camera off auto-ISO! It seems that most cameras with auto-ISO functions assume that the user would like a scene exposed as if it were daylight. However, what inspires me – and many others – about low-light photography, is the deep blue sky or many shadows, showing it is night. Selecting the ISO yourself – and keeping it as close to ISO200 as you can – will result in better quality of images.

Try it! You might re-discover the tripod and the joy of long-exposure photography.