Читать книгу Collins Complete Photography Course - John Garrett, Graeme Harris - Страница 20

Before you make an exposure, you must choose the ISO setting on your camera. The ISO (International Standards Organization) index is a system for calibrating the sensitivity to light of film emulsions and digital sensors. The ISO settings on most cameras go from 100 to approximately 3200, depending on the make of camera, and the higher the ISO number the more sensitive the film or sensor is to light. The ISO rating of digital sensors is based directly on the film ISO, so the 200 ISO setting on a digital camera is the same sensitivity as 200 ISO film.

ISO button

The practical difference between the ISO setting on a digital camera and the ISO speed of a film is that when using digital the photographer is able to change ISO settings on every frame. In contrast, once you have set the camera’s ISO for a film you must leave it on that setting for the whole film as it will be developed for a specific ISO. However, you can raise the ISO rating of a film from the manufacturer’s given speed (known as ‘pushing’) and have it ‘push processed’ successfully as long as you are consistent with the ISO of that film.

ISO sensitivity

Being able to change the ISO on every frame if you need to has great benefits. If you go from shooting in bright sunlight to low interior light you can increase the ISO from, say, 200 to 1600, thus increasing the light sensitivity of the sensor; the manufacturers call that amplifying the light. With an analogue camera, you would need to change films, which can seem painfully wasteful if you have shot only a few frames of expensive transparency film.

For this comparison of a fine-grained and grainy image, the film grain effect in Photoshop was used. Note that fine detail is lost and the colour is less saturated. The same is true in the case of noise, the digital equivalent of grain.

So why not always use high ISO ratings? The answer is that as a general rule of thumb in both digital and film, the lower the ISO (thus the slower the film) the higher quality the image will be in terms of tonal range, sharpness and colour; the higher the ISO the grainier (in film) or noisier (in digital) the image will be. A digital picture shot on 200 ISO will be sharper and smoother (with no noise) and have more colour saturation. In the case of film, a 50 ISO black and white film will be almost grainless, very sharp and of higher contrast than a 1000 ISO film.

A question of taste

However, we now arrive at an old chestnut – what is good quality? Depending on the subject, a grainy picture may well convey more to the the viewer about the subject than a smoothly fine-grained one. Indeed, you will see that there are a number of deliberately grainy pictures in this book. To some, the smoothness of digital images looks plastic and unreal, and they choose to add film grain effect in Photoshop.

In both film emulsions and digital sensors, the manufacturers have succeeded in pushing the speed of ISO with improved picture quality further and further; some 400 ISO films now have little more grain than the older 100 ISO films, while top of the range digital cameras can go beyond an astonishing 25600 ISO.