What is ISO and how it affects the photograph?
ISO is sensor sensitivity against light, to keep it simple remember a simple rule, low ISO (ISO 100 or ISO 80) for daylight or day time and higher ISO (ISO 800, ISO 1600) for night time. ISO speed affects the shutter speed and aperture combinations so you can obtain correct exposure.
While taking pictures on auto camera automatically selects ISO depending upon the available light. You can also manually select it if you think that you are unable to capture a bright image in dark.
Sometimes while hand holding the camera you cannot use shutter speed more than 1/5th of the second and you do not want to use flash or capture the wider area with low available light. Bump up the ISO to 400 or 800 and you will be able to capture an image that is visible or sometimes good however it can’t be great with higher ISO.
BEST ISO for shooting in manual mode (Manual ISO)
Best ISO to choose is the lowest ISO in your camera, mostly 100. I use manual ISO at 100, use other settings to get what I need. When I get failed to get what I want I bring in the lights but do not change my ISO. I only use higher ISO where I do have only dim light and can’t bring in my lights because higher ISO will result in noisy pictures that looks pathetic. Even with the semi professional DSLR Canon 40D when I tried to shot at 1600 highest ISO the pictures were too grainy. The maximum ISO I will use on my DSLR might be 800. An image sensor is usually calibrated so that it gives the best image quality (greatest S/N ratio) at its lowest possible ISO speed. For most consumer digital cameras, this value will be expressed as ISO 50, ISO 64 or ISO 100. A few digital cameras use ISO 200 as their lowest ISO speed. Like me all other professional photographers use ISO-100 or minimum ISO your camera have.