and ISO rating represents how sensitive the film is to light.
for digital, i guess u could interpret it as how sensitive the
sensor is to light.
the higher the rating, the higher then sensitivity. sensitivity
refers to how much light the film/sensor "records" the more
sensitive (higher rating) the more light it records, cetaris
perabus (spelt correctly?)
so as an example, if everything else is the same (aperture,
shutter, shooting conditions) and u shot one photo at ISO200 and
another at ISO400, the one with ISO 400 will be brighter. in terms
of how much brighter, it is referred to "1 stop" brighter. iso
values start from 50 and goes up this way:
50, 100, 200, 400, 800, 1600, 3200, 6400....
there are some films rated at iso 160 etc etc... but those figures
i gave u are all "1 stop" apart.
technically, on digital u could get any ISO u want. as in no need
to stick to the stops. but manufacturers just limit it to the usual
50 - 3200. dun think i've seen any digital cams that can go beyond
that.
the flip side of having a sensitive rating for film or ur sensor
(we call highly sensitive ISOs "fast film") is that u get heaps of
grain.
now, that's all theory, the practical part of it (i assume that u
are shooting digital) is that a digital camera can switch ISOs
between shots, not like film where the film is loaded, it has to be
finished b4 u can load another roll of ur desired ISO.
generally at nite u use higher ISO, day u use lower. but there are
many many exceptions. rules are always broken in photography.
