ISO sensivity

Classical films come with different sensitivity which defines their purpose. With lower sensitivity the grain is finer, but more light is needed. Those films with low sensitivity are excelent for bright conditions, but aren't the greatest solution for photography in closed evironments or sport happenings where short exposure times are needed. Here are, of course, used higher sensitivity films that besides shorter exposure times bring inevitable increased noise. Digital cameras also have ISO value that indicates sensor's sensitivity to light. ISO 100 is accepted as basic value, aldo there are cameras that offer ISO 64 or even ISO 50.

Photography taked with ISO 100

Sensitivity can be increased to ISO 200 and ISO 400 on almost all cameras, while more advanced offer ISO 800 and DSLR cameras have ISO 1600 and even ISO 3200. It is accomplished the following way: sensor recieves always the same amount of energetic charge, but amplifier afterwards in correlation with set ISO value amplifies that impulse. For explanation sake, let's take that boost factor is x2 at ISO 100. That means that this factor is x4 at ISO 200, and x8 at ISO 400 etc. Of course, all of this would be ideal and could go to infinity if amplifier besides correct informations of scene itself wouldn't also amplify electronic noise which is inevitable on every electronic device.
So with amplifying ISO value we get shorter exposures, but also more noise on photography. This especialy protrudes on more compact digital cameras that have smaller dimension sesors, with relatively large number of photocells where noise is inevitable. It is logical that sensor with 8x6mm dimensions, that gives 5MP photography must have more more noise than sensor that has 36x24mm dimensions and gives photography with same number of 5MP. Everything mentioned above brings us to conclusion that it is always best to use the smallest possible ISO value which enshures sufficient exposure time. This specially applies to compact digital cameras, that are much more sensitive to noise appearing problem then DSLR's owing to sensor size.

Same scene on ISO 400 with compact digital camera