ISO Query

Author: * Zoe Xanthippos - 2 Posts on this thread out of 89 Posts sitewide.
Date: May 17, 2006 - 23:50

ISO is the international standard used to denote film speed. It has been carried over to digital imaging and all manufacturers now provide the ISO equivalent for the sensors they employ in their camera. It works on the principle that doubling the sensitivity of the film or sensor results in a doubling of the number. A 200 ISO sensor is twice as sensitive as an ISO 100 sensor. The ISO rating in a digicam limits the exposure range of the camera because it acts as the base from which all exposure combinations are made.
The smaller the ISO number, the slower the sensor. This means that an ISO 100 sensor requires either a slower shutter speed or a wider aperture than the ISO 200 sensor. Most consumer cameras have an ISO rating of 100, but some high end professional models offer electronic enhancement of the sensor’s sensitivity of up to 1600 ISO.
This can be compared to "pushing" of conventional film. When film is pushed, it often causes grain to be clearly visible. With pushing image sensors, pixelation or color shifts may increase and become visible as noise in an image.

http://www.dcviews.com/tutors.htm

First I want to say that making myself post all those specs for my photos here has definitely gotten my mind to working around what I might be doing with my camera. I'm fortunate that the software that comes with my terminal Olympus tells me all this stuff.

Anyway, I got to noticing that the ISO seemed to be defaulting to 64 a lot of the time, sunshine or clouds. I'm familiar with 200 and 400 film and have a fairly good idea of when to use each, but I don't ever recall having seen any 64 ISO film. Anyway, I decided to experiment last week with the four ISO settings on my digital camera by taking a picture of the same building four ways. It was a bright sunshiny day, and as expected the 400 setting produced a clear but washed out photo. The other three settings, 64, 100, 200, were not all that different. I isolated the same part of the pictures in Photoshop and blew them up some, and still no real difference. I also took four photos of a plant in the shade with the camera also in the shade. Again, the 400 wasn't as good as the other three, which were very nearly alike.

Anyone have a simple explanation for me?