Exposure and Noise – 1st digital meeting 6th Oct 2009

President Roger Dye welcomed Barry Pearson as the speaker for this opening digital session of the 2009/2010 season. Roger noted that a speaker’s basic problem now was getting the balance right between being too simple and too advanced for the audience.

Barry stated that his talk was really a “back to basics” one. Photographers through the ages have had the problem of choosing the correct exposure for a scene. Given their particular film with its stated sensitivity (in terms of “Film speed”) the problem was to choose the correct aperture and shutter speed which would allow the correct amount of light onto the sensitive film. With the advent of the digital electronic sensor the photographer had a range of “film speeds” which could be set, thereby introducing a new variable to the process.

The aperture determined the “depth of field” of the process. The shutter speed either “froze the motion” or introduced the desired amount of blurring (for instance to suggest “motion” within the image). In the past there was a trade-off between the desirable settings of these two parameters. Barry suggested that with advent of digital SLR cameras one could more easily choose the optimum aperture (f-number) and optimum shutter speed, and vary the film speed (defined by the ISO number) to accommodate the resultant light input. This was a great freedom offered by the digital sensor. Barry often set his default ISO value at 800 as a starting point in his photography. This was a revelation to many in his audience who tended to stick to lower values, and who thought it daring to set a 400 ISO value.

But nothing is perfect, and Barry noted that electrical noise is always present in electrical systems, including cameras. If small apertures and fast shutter speeds are used, then the amount of light falling on the sensor pixels is small, and setting a high ISO value is basically increasing the gain of the amplifier which has to measure the amount of light. Increasing the gain magnifies the noise in the total signal recorded. Thus, as the light signal gets smaller, (but the noise component doesn’t) eventually the signal is lost in the noise. However way before this situation is reached there comes a stage when the noise component is unacceptable in the image. There are several programs available to reduce this noise effect and Barry had purchased Noise Ninja which he is pleased with. Most images are improved by a degree of sharpening, but sharpening can emphasize the noise, unless the noise reduction is attempted first.

Barry used a variety of images to emphasize the various points he was making and promised to put them and his notes on the NCPS web site for later perusal. [See links below].

In proposing a vote of thanks Bill Chadband thanked Barry for a most enjoyable evening. He noted that Barry was always enthusiastic and also very thorough in his research before making his pronouncements. And tonight he had also been humorous and carried his audience with him as he explored the various aspects of exposure, noise and ISO numbers. He had pitched the level just right for his audience.

Bill Chadband