ISO ( Sensitivity )

17/11/09

Conventional film comes in different sensitivities (ASAs) for different purposes. The lower the sensitivity, the finer the grain, but more light is needed. This is excellent for outdoor photography, but for low-light conditions or action photography (where fast shutterspeeds are needed), more sensitive or "fast" film is used which is more "grainy".

Likewise, digital cameras have an ISO rating indicating their level of sensitivity to light. ISO 100 is the "normal" setting for most cameras, although some go as low as ISO 50. The sensitivities can be increased to 200, 400, 800, 1,600, 3,200 and even more on high-end digital SLRs. When increasing the sensitivity, the output of the sensor is amplified, so less light is needed. Unfortunately that also amplifies the undesired noise. Incidentally, this creates more grainy pictures, just like in conventional photography, but because of different reasons. It is similar to turning up the volume of a radio with poor reception. Doing so will not only amplify the (desired) music but also the (undesired) hiss and crackle or "noise". Improvements in sensor technology are steadily reducing the noise levels at higher ISOs, especially on higher-end cameras. Unlike conventional film cameras which require a change of film roll or the use of multiple bodies, digital cameras allow you to instantly and conveniently change the sensitivity depending on the circumstances.















The two images above indicate levels of noise at different ISO levels. First image is shot at 100 ISO, the second at 800 ISO.

So in what sort of situation would you find yourself needing to increase the ISO on your camera. As mentioned above, low light would be one such situation. We'll say that you find yourself indoors, wishing to take a photograph of whatever is going on, but don't wish to use flash. You normally have your ISO set at 100, so you dal in a figure of 800, whilst at the same time knowing that you may have to do work on the image later to reduce noise. The following image is one that l took at a recent club night when the workshop subject was table top photograph. This meant that all the main lighting had been turned off, reliance for illumination being left to the individual lighting on each table top :





























































This image has already been put through a noise ruction piece of software, but you can see that certain areas of the image are blown.































































Using a technique called " dodge and burn " (very simple and one to be dealt with in a later Tutorial ) you can see that the blown areas have been reduced, to the extent that you now have a very workable image. I have deliberately overdone the " dodge and burn " to emphasise what can be done. The image may not be as sharp as one would like, but that can sometimes be a problem when shooting hand held in low light.