PBase

Hi
i am very confused,How to Decide How Much iso ? pls help me out

Refering to analogic fotografie the "standard" is 400ASA, if you take more or less it's becouse you have something in mind.
Basecly for black and white 100asa is more contrast and the grain is lower, then 400 have a richest variety of grey so it's softer.
For color the grain is the same and the less you have sensitivity, the more the colors appears to be pure.
I don't now the initial sensitivity of Ccd or Cmos but I think it must be around 320ASa.
It's a question of choice, depend of what you whant to achieve. Taking photos at night by 3200asa at 1/60s will not be the same efect that taking at 320 at 1/7s...

sorry for my bad english

For digital, most will use the lowest ISO possible to get a desired combination of shutter speed and F-stop. The trade off is digital noise. At high ISO often around 800, sometimes lower, many sensors begin to register data that isn't actually there, or data that is incorrectly rendered. This creates noise, which degrades image quality. There is a totally different set of decision making factors for shooting film.

I second both r_karck and prinothcat: it's a matter of choise.
When you need faster shutter speeds, choose a higher ISO/ASA number. If you want the best image quality [clean images, less grain(film) or noise (digital)], choose the lowest ISO number as possible.

ISO or ASA (if you have an older camera you may even see a DIN number) define the sensitivity of the film. The higher the number the more sensitive the film or sensor is. The pay-off is grain of digital noise and less contrast. For a given exposure level you need a certain combination of ISO/ASA, shutter speed and aperture. If you don't have or can't use a tripod, you'll opt for faster shutter speeds. For a given aperture, the only option is to increade the ISO.

I hope this was an answer to your question.