what does 200 400 800 ect mean?

what is the diffrence between 200 400 800 etc speed films? all I

understand is that 200 is for bright sunny days and 800 is for fast

action. I tend to get better results with 800, but I want to know

why? can I change the little iso controler on my camera to 200 or

800 and get diffrent/better results while useing 400? what is iso

anyway? this stuff has never made sence to me and I just want to

understand....

As you mention, this refers to a film's "speed." A faster film "collects" light more rapidly than a slow film. this means that you need less light for a faster film than a slow one. A 400 speed film needs half the light as a 200 speed film. An 800 speed film needs half the light of a 400 speed film.

The benefits is that, with a faster film, you can use a faster shutter speed or a smaller aperture to get a correct exposure. For instance, if you had 1/50 at f8 with a 200 speed film, you could do 1/100 at f8 with a 400 speed film. If the difference between a sharp image and a blurry one (from camera shake) is the difference between 1/50 and 1/100 shutter speeds, then the 400 speed film has obvious advantages.

The downside is that faster films generally have more grain than slower ones.

"ISO" is a standard speed rating generated under specific testing conditions. This is a "nominal" speed determined through this testing by the manufacturer. Others can speak with much greater detail here than I can.

allan

Tash,

In short, ISO is a measure of the films sensitivity to light. The larger the number, the more sensitive it is to light.

Changing this on your camera could result in either overexposed or underexposed film. Changing it does /not/ alter the films sensitivity to light only how your camera's meter reacts to light.

These are the film "speeds". I'm sure someone can explain better than me some of the really technical aspects, but here is all you should need to know:

The higher the number, the less the amount of light is needed, at a constant aperture (focal length of the lens divided by the diameter of the opening), to make a correct exposure.

The numbers are relative, as in 200 requires more light than 400, which requires more light than 800.

When you double the speed of film, you half the amount of light that is needed for a proper exposure. And when you half the speed of your film, you double the amount of light that is needed for a proper exposure.

In general, faster films (higher ISO numbers) have more grain than slower films (lower ISO numbers).

The goal is generally to use the slowest possible film you can, while still being able to get the pictures you need to. So, on a bright day, if you will be shooting a lot of action shots, you may want to use ISO 800 so that your shutter speeds will be fast enough to freeze the action. Or, if you are shooting posed portraits on the same sunny day, you only want to use a film just fast enough to give you a shutter speed fast enough to prevent blur from camera movement or subject movement. Generally, 1/the focal length of the lens should be fast enough to avoid blur caused by camera shake when you hand-hold the camera. If you are using a tripod, you can go much slower.

You can just change the film speed setting on your camera, but if you haven't changed the film as well, you will have to compensate for this when you have the film developed. If you are using ISO 200 film and your shutter speeds are half as fast as you would like them to be, you can change the ISO setting on your camera to fool the camera in to thinking that you are shooting with ISO 400 speed film (which requires half the amount of light, or, one stop less exposure), to get the shutter speed you need while holding aperture constant. But, that will mean that your film has been exposed with only half the amount of light needed. You can then tell the lab to "push" the film one stop to make up for the underexposed film. But that is not so simple as it sounds. When you push a film, you wind up with grainier, more contrasty images. You lose details in the highlights and in the shadows and the grain is more than a properly exposed negative of the same film type. The more you push a film, the worse the side-effects from pushing become.

Check out the learn section here as well as read a book or two. I've covered some of the basics of ISO without really covering shutter speed and aperture too deeply, but to know what to expect from your shots, you need an integral understanding of all of these, and more.