Film ISO explained please

Joined: 19 Mar 2005 Posts: 4

I was just wondering what the different ISO's on film mean and which ISO's are best for what kind of jobs? TY in advance

-Unla

Posted: Mon Mar 21, 2005 6:12 pm

This is a rating as to the film's sensitivity to light.
100 ISO film needs twice as much light as ISO 200 film to take a photo (all other things being equal) ISO 50 needs 4 time as much light as ISO 200.
Common steps are ISO 50, 100, 200, 400, 800 - but there are others.
Generally as the ISO number goes up the grain of the film goes up. You would be concerned with grain if you are enlarging your photos. I have also heard that you get richer colours form lower ISO (speed) film but I cannot confirm this.
The difference from one level to the next (i.e. 200 to 400) is equal to one stop of light.
On a camera that is like moving from an aperture of f4 to f2.8 , or a shutter speed of 1/125 to 1/60.
You would generally use faster film for low light situations, or to stop faster action.

Posted: Mon Mar 21, 2005 9:50 pm

areinders gave a pretty good explaination.

One way to look at it that helped me was to compare them to a glass of water. The light being the water.

50 ISO film is like a large glass of water it (say a quart). It takes so much time to fill it.

100 ISO would be half a quart so half the amount of time to fill

200 ISO would be half again smaller or 1/4 the amount of time to fill compared to the 50 ISO one. And so on.

To continue the analogy...

The apature is like the the size of the faucet. A small faucet (small apature say f22) takes longer to fill the glass than a large faucet (an apature of say f1.4) will fill it much more quickly.

In either case, when the glass is full that equates to when the film has recieved enough light for a proper exposure.

I hope that makes sense to you.

Joined: 19 Mar 2005 Posts: 4

Cool,

Thank you both very much for your explanations, I think I'm getting it.
:)

-Unla

Posted: Wed Mar 23, 2005 4:06 pm

The "sticky" in this forum "Metering and Bracketing" gives a very detailed explaination of this--well worth reading. By the time you've read the entire thread you'll be an expert.

Packard