ISO, film speed
ISO or film speed is one thing that is actually a bit confusing. First we will take a step back in time and look at where it all comes from. ISO is a standard issued by ISO to replace an older ASA standard, it also brought the Russian GOST standard in, combining it all into one standard. ISO is very closely matched to the old ASA scale, and those of us who remember film, remember you could buy ASA-200 or ASA-400 film. Higher numbers exist as well. What is indicated is normally called the film speed. With this we mean how much time does the film need to fully expose in a given amount of light. The higher the number the more sensitive the film is. With more sensitive film it needs less time, and thus is 'faster'. All this comes at a price of course. Film has grains of silver halide that reacts when it get exposed to light. The bigger the grain the more sensitive it is. So with a higher ISO we get a grainier photo.
Now we have a digital camera so why do we care about film, and grain? Easy, the CCD has something similar. The CCD in the digital camera is the film. It converts lights to an electric value. This converter has something called 'signal gain', essentially what it is is to indicate how much noise it should filter out. Any electric device has noise in it, this is kept at a manageable level by various filters and other clever solutions. In your camera you can adjust the 'signal gain', effectively rising or lowering the threshold for what it lets through. This roughly translates to the grain seen in film. So with a higher ISO you make the CCD more sensitive to light, but at the cost of introducing noise. But hold your horses, this is not all, there are other ways to adjust the sensitivity of the CCD in the camera, most cameras lets you adjust the signal gain before the conversion to digital, other do it after it has been converted. Yet another way is that you can adjust how to interpret the lightness value of the RGB value that comes out of the CCD. This means that we can not know which way your camera manufacturer chose to use for their ISO. At the end of the day, we do not really care, we just need to know that the higher the ISO the more noise, or grain, you have.
Relation between film ISO and digital ISO
We have now learned what ISO is, and we have also seen that they are not the same for a digital camera as for a film camera. What our friends at ISO did was to set up rules for matching this up. So digital ISO is matched to film ISO according to the rules set by ISO. As always with standards they may not be followed, or they may be interpreted differently by different manufacturers. I have so far found that they are actually quite good at this, so changing the ISO from 100 to 200 will yield pretty much the same result regardless of the camera.
There are several rules of thumb for ISO settings they are as follows
|Clear sunny day||100|
|Candle light||800 or higher|
In general you want as little noise as possible in your photo so go for lower ISO when you can. Remember previous lessons about shutter speed and hand held camera. With all this you can make a good well informed decision on what ISO to use, what aperture, what shutter speed to use and lastly if you need to get the tripod out or not.