The lower the film speed the more light that is required to get a proper exposure.
Since we photographers are always struggling to get enough light, the first thought that comes to
mind is to use the highest ISO rating we can find!
Weeell – not so fast...
The trade-off is that - while slower films DO need more light, they produce better photos. They
are much less grainy and more saturated. The higher speed films may need less light, but they
introduce more grain and less saturation.
ISO speeds halve and double. By that I mean that if you have a film with a speed of 200, it
would need half as much light as a film speed of 100. 400 speed needs half as much light as 200,
and so on.
Typically, shooting with a film speed of 100 is normal and you would want to have a very good
reason to go faster or slower.
On a side note - the additional grain, etc., isn’t necessarily a bad thing. It can be a fun way to
experiment and to add a bit of creativity.
Here is a shot I did with film rated at 1600 if I’m remembering correctly. The enlarged section
clearly shows the grain that was introduced by using a high speed film.
“So what?” you say. “I don’t shoot with film!”
Good news... The ISO speed rating was such a useful and popular concept that digital camera
designers included it in the photo sensor! You can shoot at 100 ISO, 200, 1600 or whatever and
it works the same way the film ISOs did.